2024 |
Charrondiere, C., Hopfinger, E., Brun, C., Cohard, J., & Sicart, J. (2024). Evidence Of Strong Wave Turbulence And Of Bolgiano Temperature Spectra In Katabatic Winds On Steep Slopes. Physics Of Fluids, 363(2).
Abstract: The Katabatic Winds On Steep Slopes Investigated In The Present Study Reveal A Novel Spectral Behavior, Observed In The Outer Part Of The Jet. At Low Wavenumbers, The One-Dimensional (1D) Velocity Spectra Show Evidence Of A K(X)(-1) Range For The Three Components Of The Velocity Vector: E-U(K(X)), E-V(K(X)), E-W(K(X)) Proportional To K(X)(-1) [As Well As For The 1D Temperature Spectrum E-Theta(K(X)) Proportional To K(X)(-1)]. This Suggests The Existence Of Strong Wave Turbulence. A Necessary Condition For Strong Wave Turbulence To Be Manifest Is That The Flow Direction Wavenumber, K(X), Extends To Much Lower Values Than The Slope Normal One, K(Z). This Is Satisfied In The Present Field Experiment Where Wave Energy Is Injected At Wavenumber K(X) = K(N) = (N-A Sin Alpha)/(U(J)) Over Bar, While K(Z) Similar To 1/Delta(Z), With N-A The Ambient Stratification, Alpha The Slope Angle, (U(J)) Over Bar The Maximum Wind Velocity, And Delta(Z) The Shear Layer Thickness Of The Jet. In The Inertial Range, The Velocity Spectra Exhibit A Power Law K(X)(-5/3) Over Two Decades, Whereas The Temperature-Buoyancy Spectra Show Evidence Of A – 7/5 Slope In The Buoyancy Sub-Range, Followed By A – 5/3 Slope. The Change In Spectral Slopes Occurs At The Bolgiano Scale L-B That Is Close To The Dougherty-Ozmidov Scale L-Oz. The High Reynolds Number Based On The Taylor Micro-Scale, Re-Lambda Similar To 10(3), Allows Clear Identification Of The Spectral Laws.
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Diedhiou, C., Panthou, G., Diatta, S., Sané, Y., Vischel, T., & Camara, M. (2024). Simple Scaling Of Extreme Precipitation Regime In Senegal. Scientific African, 232.
Abstract: Extreme Precipitation Exhibits High Temporal And Spatial Variability, And Understanding This Variability Is Crucial For Designing Hydraulic Infrastructures And Assessing The Impacts Of Natural Risks Such As Floods And Droughts. The Simple Scaling (Ss) Model, Which Describes The Dependence Of Extreme Rainfall Statistics On Timescales, Is Used To Estimate Intensity Duration Frequency (Idf) Curves. This Study Aims To Evaluate The Validity Of The Simple Scaling (Ss) Hypothesis For 14 Rain Gauges In Senegal And Explore The Existence Of Breaks In Temporal Scaling Regimes (Transition Regimes). For The Analysis Of Extreme Characteristics, We Considered The Annual Maximum Series (Ams) Of Precipitation In Sub-Hourly And Supra-Daily Durations Ranging From 10 Min To 7 Days. The Empirical Validation Of The Ss Models Was Confirmed For The Majority Of The Scaling Intervals. We Identified Two Scaling Breaks, Indicating The Presence Of Three Scaling Regimes: Short Durations (Sd), Intermediate Durations (Id), And Long Durations (Ld). The Spatial Variation Of Scaling Exponents Reveals The Existence Of Different Scaling Regimes Across Senegal. The Results Are Valuable For Modeling The Spatial Distribution Of Scaling Exponents, Which Can Aid In Characterizing Idf Curves At Ungauged Locations.
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Valois, R., Rivière, A., Vouillamoz, J., & Rau, G. (2024). Technical Note: Analytical Solution For Well Water Response To Earth Tides In Leaky Aquifers With Storage And Compressibility In The Aquitard. Hydrology And Earth System Sciences, 282(4), 1041–1054.
Abstract: In Recent Years, There Has Been A Growing Interest In Utilizing The Groundwater Response To Earth Tides As A Means Of Estimating Subsurface Properties. However, Existing Analytical Models Have Been Insufficient In Accurately Capturing Realistic Physical Conditions. This Study Presents A New Analytical Solution To Calculate The Groundwater Response To Earth Tide Strains, Including Storage And Compressibility Of The Aquitard, Borehole Storage, And Skin Effects. We Investigate The Effects Of Aquifer And Aquitard Parameters On The Well Water Response To Earth Tides At Two Dominant Frequencies ( O-1 And M-2 ) And Compare Our Results With Hydraulic Parameters Obtained From A Pumping Test. Inversion Of The Six Hydro-Geomechanical Parameters From Amplitude Response And Phase Shift In Both Semi-Diurnal And Diurnal Tides Provides Relevant Information About Aquifer Transmissivity, Storativity, Well Skin Effect, Aquitard Hydraulic Conductivity, And Diffusivity. The New Model Is Able To Reproduce Previously Unexplained Observations Of The Amplitude And Frequency Responses. We Emphasize The Usefulness In Developing A Relevant Methodology To Use The Groundwater Response To Natural Drivers In Order To Characterize Hydrogeological Systems.
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2023 |
Bodjrenou, R., Cohard, J., Hector, B., Lawin, E., Chagnaud, G., Danso, D., et al. (2023). Evaluation Of Reanalysis Estimates Of Precipitation, Radiation, And Temperature Over Benin (West Africa). Journal Of Applied Meteorology And Climatology, 626(8), 1005–1022.
Abstract: In West Africa, Climatic Data Issues, Especially Availability And Quality, Remain A Significant Constraint To The Development And Application Of Distributed Hydrological Modeling. As Alternatives To Ground-Based Observations, Reanaly-Sis Products Have Received Increasing Attention In Recent Years. This Study Aims To Evaluate Three Reanalysis Products, Namely, Era5, Water And Global Change (Watch) Forcing Data (Wfd) Era5 (Wfde5), And Merra-2, From 1981 To 2019 To Determine Their Ability To Represent Four Hydrological Climates Variables Over A Range Of Space And Time Scales In Benin. The Variables From The Reanalysis Products Are Compared With Point Station Databased Metrics Kling-Gupta Effi- Ciency (Kge), Mean Absolute Error (Mae), Correlation, And Relative Error In Precipitation Annual (Repa). The Results Show That Era5 Presents A Better Correlation For Annual Mean Temperature (Between 0.74 And 0.90) Than Do Wfde5 (0.63-0.78) And Merra-2 (0.25-0.65). Both Era5 And Wfde5 Are Able To Reproduce The Observed Upward Trend Of Tem-Perature (0.2 & Deg;C Decade -1) In The Region. We Noted A Systematic Cold Bias Of -1.3 & Deg;C In All Reanalyses Except Wfde5 (-0.1 & Deg;C). On The Monthly Time Scale, The Temperature Of The Region Is Better Reproduced By Era5 And Wfde5 (Kge & Ge; 0.80) Than By Merra-2 (Kge , 0.5). At All Time Scales, Wfde5 Produces The Best Mae Scores For Longwave (Lw) And Shortwave (Sw) Radiation, Followed By Era5. Wfde5 Also Provides The Best Estimates For The Annual Precipitation (Repa 2 ]-25, 25[ And Kge & Ge; 50% At Most Stations). Era5 Produces Similar Results, But Merra-2 Performs Poorly In All The Metrics. In Addition, Era5 And Wfde5 Reproduce The Bimodal Rainfall Regime In Southern Benin, Unlike Merra-2, But All Products Have Too Many Small Rainfall Events.
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Camberlin, P., Togbedji, C., Pergaud, J., Berger, A., Aellig, R., Fink, A., et al. (2023). The Representation Of Dry-Season Low-Level Clouds Over Western Equatorial Africa In Reanalyses And Historical Cmip6 Simulations. Climate Dynamics, .
Abstract: Within The Equatorial Zone, Western Equatorial Africa (Wea) Has A Record Low Sunshine Duration During The June-September Dry Season Due To The Persistence Of Low Clouds. This Study Examines The Ability Of Two Reanalysis Products (Era5 And Merra-2) And Eight Cmip6 Models (Both Coupled And Atmosphere-Only Historical Simulations) To Reproduce The Climatology Of These Low Clouds, By Comparing It With Ground Observations And A Satellite Product. All Datasets Show A Reasonable Representation Of The Regional Distribution Of Low Clouds Over The Tropical Atlantic And The Neighbouring African Continent. However, Cmip6 Models Tend To Underestimate The Low Cloud Fraction, Especially Over Wea In The Coupled Simulations. This Underestimation Is Partly Due To An Insufficient Seasonal Sea-Surface Temperature (Sst) Cooling Over The Eastern Equatorial Atlantic From April To July In Most Models, Which Reduces The Lower-Tropospheric Stability (Lts). However, The Inability To Reproduce The Jjas Low Cloud Fraction Does Not Necessarily Scale With The Sst Biases Of The Cmip6 Models. Observed Interannual Variations Of Wea Low-Cloud Fraction Are Strongly Controlled By Lts, Itself Mostly Related To Atlantic Sst. The Strong Dependence Of Low Clouds On Interannual Sst Variations Is Captured By Most, But Not All The Cmip6 Models. Additional Drivers Of Interannual Variations Identified In This Study, Such As Mid-Tropospheric Temperatures Over Wea And Bight Of Bonny Surface Winds, Emerge Inconsistently In Cmip6. Further Analyses Are Needed To Disentangle The Roles Played By Sst And Independent Atmospheric Forcings On Wea Low Cloud Formation.
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Chabi, B., Alassane, A., Kpegli, K., Lawson, F., Zakari, A., Koukpohounsi, B., et al. (2023). Characterising Groundwater And Surface-Water Interconnections Using Hydrogeology, Hydrochemistry And Stable Isotopes In The Oueme Delta, Southern Benin. Hydrogeology Journal, .
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Champagne, O., Aellig, R., Fink, A., Philippon, N., Camberlin, P., Moron, V., et al. (2023). Climatology Of Low-Level Clouds Over Western Equatorial Africa Based On Ground Observations And Satellites. Journal Of Climate, , 428944–430644.
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Champagne, O., Arain, A., Wang, S., & Leduc, M. (2023). Future Change In Amplitude And Timing Of High-Flow Events In A Canadian Subarctic Watershed. Cold Regions Science And Technology, 2092.
Abstract: The Hudson Bay Basin Is A Large Contributor Of Freshwater Input In The Arctic Ocean And Is Also An Area Affected By Destructive Spring Floods. In This Study, The Hydrological Model Mesh (Modelisation Environmentale Com-Munautaire -Surface And Hydrology) Was Set Up For The Groundhog River Watershed Situated In The Hudson Bay Basin, To Simulate The Future Evolution Of Streamflow And Annual Maximum Streamflow. Mesh Was Forced By Meteorological Data From Era5 Reanalyses In The Historical Period (1979-2018) And 12 Models Of The Coupled Model Intercomparison Project Phase 5 (Cmip5) Downscaled With The Canadian Regional Climate Model Version 5 (Crcm5) In Historical (1979-2005) And Scenario Period (2006-2098). The Projections Consistently Indicate An Earlier Spring Flow And A Reduction In The Amount Of Annual Maximum Streamflow By The End Of The 21St Century. Under The Rcp8.5 Scenario, The Annual Maximum Streamflow Occurring In The Spring Is Expected To Be Advanced By 2 Weeks And Reduced On Average From 852 M3/S (+/- 265) In The Historical Period (1979-2018) To 717M3/S (+/- 250) By The End Of The 21St Century (2059-2098). Because The Seasonal Projection Of Streamflow Was Not Investigated In Previous Studies, This Work Is An Important First Step To Assess The Seasonal Change Of Streamflow In The Hudson Bay Region Under Climate Change.
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Dang, D., Ha, Q., Némery, J., & Strady, E. (2023). The Seasonal Variations In The Interactions Between Rare Earth Elements And Organic Matter In Tropical Rivers. Chemical Geology, 6386.
Abstract: The Escalation Of Global Demands For Critical Minerals To Facilitate The Green Energy Transition Creates Emerging Needs To Better Understand Their Environmental Behaviours. Several Nations Recognize Rare Earth Elements (Rees) As Priority Critical Minerals And Emphasize The Necessity To Evaluate Their Environmental Mobility And Potential Effects On Natural Ecosystems And Human Health. Here, We Investigated The Seasonal Variations In Dissolved Concentrations Of Rees And Their Leachable Fractions From Suspended Particles In Samples Collected Bi-Weekly From The Sai Gon And Dong Nai Rivers In Southern Vietnam. The Investigation Period Extended Over Sixteen Months, Including A Transition From Wet To Dry Seasons. We Reported Significant Mobility Of Rees In The River Waters During The Wet Season Due To Watershed Runoff, Especially In Sai Gon River With The Occurrence Of Acid Sulphate Soils. Dissolved Ree Concentrations In The Dry Season Were Significantly Lower Because Of The Interactions With Organic-Rich Particles (Up To 13% Of Particulate Organic Carbon, Poc) Mainly Derived From Phytoplankton Materials. The Conditional Distribution Coefficients (Logkd From 5 To 7 For Pr) Were Proportional To Poc Content. The Scavenging Capacity Of Particulate Organic Matter Also Led To Affect The Fractionation Between Light Rees And Heavy Rees In Solution, And The Characteristic Middle Ree-Enrichment Patterns For The Leachable Rees Fraction. We Also Reported Significant Eu Anomalies (Eu/Eu* Up To 7) In The Dong Nai River During The Dry Season And Associated This Observation With The Decomposition Of Organic Matter That Previously Accumulated Eu. However, Further Studies Are Required To Confirm This Hypothesis Because Of Small Reservoir Effects; Elevated Eu Anomalies Were Observed In Water Samples With Low Dissolved Eu Concentrations.
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Deen, T., Arain, M., Champagne, O., Chow-Fraser, P., & Martin-Hill, D. (2023). Impacts Of Climate Change On Streamflow In The Mckenzie Creek Watershed In The Great Lakes Region. Frontiers In Environmental Science, .
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Do Amaral, F., Trung, T., Pellarin, T., & Gratiot, N. (2023). Datasets Of High-Resolution Water Level And Discharge From The Saigon-Dong Nai Estuary System Impacted By A Developing Megacity, Ho Chi Minh City-ViETNAm. Data In Brief, .
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Gupta, A., Reverdy, A., Cohard, J., Hector, B., Descloitres, M., Vandervaere, J., et al. (2023). Impact Of Distributed Meteorological Forcing On Simulated Snow Cover And Hydrological Fluxes Over A Mid-Elevation Alpine Micro-Scale Catchment. Hydrology And Earth System Sciences, 272(1), 191–212.
Abstract: From The Micro- To The Mesoscale, Water And Energy Budgets Of Mountainous Catchments Are Largely Driven By Topographic Features Such As Terrain Orientation, Slope, Steepness, And Elevation, Together With Associated Meteorological Forcings Such As Precipitation, Solar Radiation, And Wind Speed. Those Topographic Features Govern The Snow Deposition, Melting, And Transport, Which Further Impacts The Overall Water Cycle. However, This Microscale Variability Is Not Well Represented In Earth System Models Due To Coarse Resolutions. This Study Explores The Impact Of Precipitation, Shortwave Radiation, And Wind Speed On The Water Budget Distribution Over A 15.28 Ha Small, Mid-Elevation (2000-2200 M) Alpine Catchment At Col Du Lautaret (France). The Grass-Dominated Catchment Remains Covered With Snow For 5 To 6 Months Per Year. The Surface-Subsurface Coupled Distributed Hydrological Model Parflow-Clm Is Used At A Very High Resolution (10 M) To Simulate The Impacts On The Water Cycle Of Meteorological Variability At Very Small Spatial And Temporal Scales. These Include 3D Simulations Of Hydrological Fluxes With Spatially Distributed Forcing Of Precipitation, Shortwave Radiation, And Wind Speed Compared To 3D Simulations Of Hydrological Fluxes With Non-Distributed Forcing. Our Precipitation Distribution Method Encapsulates The Spatial Snow Distribution Along With Snow Transport. The Model Simulates The Dynamics And Spatial Variability Of Snow Cover Using The Common Land Model (Clm) Energy Balance Module And Under Different Combinations Of Distributed Forcing. The Resulting Subsurface And Surface Water Transfers Are Computed By The Parflow Module. Distributed Forcing Leads To Spatially Heterogeneous Snow Cover Simulation, Which Becomes Patchy At The End Of The Melt Season And Shows A Good Agreement With The Remote Sensing Images (Mean Bias Error (Mbe) = 0.22). This Asynchronous Melting Results In A Longer Melting Period Compared To The Non-Distributed Forcing, Which Does Not Generate Any Patchiness. Among The Distributed Meteorological Forcings Tested, Precipitation Distribution, Including Snow Transport, Has The Greatest Impact On Spatial Snow Cover (Mbe = 0.06) And Runoff. Shortwave Radiation Distribution Has An Important Impact, Reducing Evapotranspiration As A Function Of The Slope Orientation (Decreasing The Slope Between Observed And Simulated Evapotranspiration From 1.55 To 1.18). For The Primarily East-Facing Catchment Studied Here, Distributing Shortwave Radiation Helps Generate Realistic Timing And Spatial Heterogeneity In The Snowmelt At The Expense Of An Increase In The Mean Bias Error (From 0.06 To 0.22) For All Distributed Forcing Simulations Compared To The Simulation With Only Distributed Precipitation. Distributing Wind Speed In The Energy Balance Calculation Has A More Complex Impact On Our Catchment, As It Accelerates Snowmelt When Meteorological Conditions Are Favorable But Does Not Generate Snow Patches At The End Of Our Test Case. This Shows That Slope- And Aspect-Based Meteorological Distribution Can Improve The Spatio-Temporal Representation Of Snow Cover And Evapotranspiration In Complex Mountain Terrain.
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Houngnibo, M., Ali, A., Agali, A., Waongo, M., Lawin, A., & Cohard, J. (2023). Stochastic Disaggregation Of Seasonal Precipitation Forecasts Of The West African Regional Climate Outlook Forum. International Journal Of Climatology, .
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Houphlet, S., Dusseux, P., Adiko, A., Konan-Waidhet, A., Munoz, F., Bigot, S., et al. (2023). Object-Based Characterization Of Vegetation Heterogeneity With Sentinel Images Proves Efficient In A Highly Human-Influenced National Park Of Cote D'Ivoire. Environmental Monitoring And Assessment, 1951(1).
Abstract: Forest Monitoring Requires More Automated Systems To Analyze High Ecosystem Heterogeneity. The Traditional Pixel-Based Detection Method Has Proven To Be Less And Less Effective. A Novel Change Detection Method Is Therefore Proposed To Detect Changes In Forest Cover Using Satellite Images At Very High Spatial Resolution. This Is Object-Oriented Classification, Which Groups Pixels Into Interpreted Objects, Based On Their Spectral Values, Spatial, And Textural Properties. Using Sentinel And Lansat Images, We Tested For The First Time In The West African Rainforest Zone The Effectiveness Of This Method For Better Detection, Delineation, And Analysis Of Land Use And Occupation Types. The Mean Shift Algorithm Was Used In Both The Segmentation And Classification Processes. Next, We Compared The Proposed Object-Oriented Method With A Pixel-Based Image Classification Detection Method By Implementing Both Methods Under The Same Conditions. High Detection Accuracy (> 90%) And An Overall Kappa Greater Than 0.90 Were Obtained By The Object-Oriented Method, Which Is About 20% Higher Than The Pixel-Based Method. The Object-Based Method Was Free Of Salt And Pepper Effects And Was Less Prone To Image Misregistration In Terms Of Change Detection Accuracy And Mapping Results. This Study Demonstrates That The Object-Based Classifier Is A Much Better Approach Than The Classical Pixel-Based Classifier. In Addition, It Shows The Problems Of Detecting Heterogeneous Landscapes And Explains The Observed Confusions Between The Types Of Vegetation Formations Specific To Tropical Wetlands. The Results Obtained Are Encouraging And The Contribution Of High-Resolution Images And The Object-Based Method To Better Discrimination Of Tropical Wetland Vegetation Is Discussed.
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Mahaman, R., Nazoumou, Y., Favreau, G., Ousmane, B., Boucher, M., Babaye, M., et al. (2023). Paleochannel Groundwater Discharge To The River Niger In The Iullemmeden Basin Estimated By Near- Surface Geophysics And Piezometry. Environmental Earth Sciences, .
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Moron, V., Camberlin, P., Aellig, R., Champagne, O., Fink, A., Knippertz, P., et al. (2023). Diurnal To Interannual Variability Of Low-Level Cloud Cover Over Western Equatorial Africa In May-October. International Journal Of Climatology, .
Abstract: This Study Examines The Diurnal To Interannual Variations Of The Stratiform Cloud Cover In May-October (1971-2019) From A 3-Hourly Station Database And From Era5 Reanalyses Over Western Equatorial Africa (Wea). The Main Diurnal Variations Of The Local-Scale Fraction And Genus Of Stratiform Clouds Are Synthesized Into Three Canonical Diurnal Types (I.E., “Clear,” “Clear Afternoon,” “Cloudy” Days). The Interannual Variations Of Frequencies Of The Three Diurnal Types During The Cloudiest Months (Jjas) Are Mostly Associated With Two Main Mechanisms: A Meridional Shallow Overturning Cell Associating More “Cloudy” And Less “Clear” And “Clear Afternoon” Days To Anomalous Southerlies Below 900 Hpa Over And Around Wea, Anomalous Ascent Around 5 Degrees-7 Degrees N, Anomalous Northerlies Between 875 And 700 Hpa, And Anomalous Subsidence Over The Equatorial Atlantic. This Circulation Is Strongly Related To Interannual Variations Of The Equatorial Atlantic Upwelling (I.E., More Clouds When The Upwelling Is Strong) Associated With A Meridional Shift Of The Intertropical Convergence Zone Over The Tropical Atlantic And Adjacent Continents. The Second Mechanism Operates Mostly In The Zonal Direction And Involves Again The Coupled Ocean-Atmosphere System Over The Equatorial Atlantic, But Also The Remote El Nino-Southern Oscillation (Enso). An Anomalously Cold Equatorial Atlantic Drives Increased Low-Level Westerlies Toward The Congo Basin. Warm Enso Events Promote Broad Warm And Easterly Anomalies In The Middle And Upper Troposphere, Which Increase The Local Static Stability, And Thus The Local Stratiform Cloud Cover Over Wea. The Present Study Suggests New Mechanisms Responsible For Interannual Variations Of Stratiform Clouds In Wea, Thus Providing Avenues Of Future Research Regarding The Stability Of The Stratiform Cloud Deck Under The Ongoing Differential Warming Of Tropical Ocean And Land Masses.
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Ouhechou, A., Philippon, N., Morel, B., Trentmann, J., Graillet, A., Mariscal, A., et al. (2023). Inter-Comparison And Validation Against In-Situ Measurements Of Satellite Estimates Of Incoming Solar Radiation For Central Africa: From The Annual Means To The Diurnal Cycles. Atmospheric Research, 2872.
Abstract: This Study Pictures For The First Time Incoming Solar Radiation Mean Evolution In Central Africa, Intercomparing 8 Gridded Products (Namely Ceres-Ebaf, Ceres-Syn1Deg, Tpdc, Cmsaf Sarah-2, Cmsaf Clara-A2, Cams -Jade Satellite Products, As Well As Era5 Reanalysis And Worldclim 2 Interpolated Measurements) And Station -Based Estimations (Faoclim 2) Or Measurements. At The Mean Annual Scale, All Products Picture Low Levels Of Global Horizontal Irradiance (Ghi) To The West (Sw Cameroon To Sw Republic Of Congo) And Higher Levels To-Wards The North And South Margins Of The Region. However, Ghi Levels In The Cmsaf Products Are Much Higher Than In Ceres And Tpdc. The Mean Annual Cycles Of Ghi Extracted For 6 Sub-Regions Are Bimodal, With Two Maxima During The Two Rainy Seasons (March-May And September-November) And Two Minima During The Two Dry Seasons (December-February And June-August). These Seasonal Cycles Are Well Reproduced By Most Products Except Their Amplitude Which Is Dampened In Tpdc. At The Daily And Sub-Daily Time-Scales, Products Were Compared With In-Situ Measurements From Ten Meteorological Stations Located In The Western Part Of Central Africa. The Products' Performance Is Assessed Through Scores As Bias And Rmse But Also By Considering The Diurnal Cycles' Shape, Amplitude And Frequency Of Occurrence Along The Annual Cycle. The Products Properly Reproduce The Shape Of The Four Types Of Diurnal Cycles With Nonetheless Noticeable Differences In The Cycle'S Frequencies Of Occurrence.
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Ousmane, B., Nazoumou, Y., Favreau, G., Babaye, M., Mahaman, R., Boucher, M., et al. (2023). Groundwater Quality And Its Implications For Domestic And Agricultural Water Supplies In A Semi-Arid River Basin Of Niger. Environmental Earth Sciences, .
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Paolini, G., Escorihuela, M., Merlin, O., Laluet, P., Bellvert, J., & Pellarin, T. (2023). Estimating Multi-Scale Irrigation Amounts Using Multi-Resolution Soil Moisture Data: A Data-Driven Approach Using Prism. Agricultural Water Management, 2902.
Abstract: Irrigated Agriculture Is The Primary Driver Of Freshwater Use And Is Continuously Expanding. Precise Knowledge Of Irrigation Amounts Is Critical For Optimizing Water Management, Especially In Semi-Arid Regions Where Water Is A Limited Resource. This Study Proposed To Adapt The Prism (Precipitation Inferred From Soil Moisture) Method-Ology To Detect And Estimate Irrigation Events From Soil Moisture Remotely Sensed Data. Prism Was Originally Conceived To Correct Precipitation Products, Assimilating Soil Moisture (Sm) Observations Into An Antecedent Precipitation Index (Api) Formula, Using A Particle Filter Scheme. This Novel Application Of Prism Uses Initial Precipitation And Sm Observations To Detect Instances Of Water Excess In The Soil (Not Caused By Precipitation) And Estimates The Amount Of Irrigation, Along With Its Uncertainty. This Newly Proposed Approach Does Not Require Extensive Calibration And Is Adaptable To Different Spatial And Temporal Scales. The Objective Of This Study Was To Analyze The Performance Of Prism For Irrigation Amount Estimation And Compare It With Current State-Of-The-Art Approaches. To Develop And Test This Methodology, A Synthetic Study Was Conducted Using Sm Observations With Various Noise Levels To Simulate Uncertainties And Different Spatial And Temporal Resolutions. The Results Indicated That A High Temporal Resolution (Less Than 3 Days) Is Crucial To Avoid Underestimating Irrigation Amounts Due To Missing Events. However, Including A Constraint On The Frequency Of Irrigation Events, Deduced From The System Of Irrigation Used At The Field Level, Could Overcome The Limitation Of Low Temporal Resolution And Significantly Reduce Underestimation Of Irrigation Amounts. Subsequently, The Developed Methodology Was Applied To Actual Satellite Sm Products At Different Spatial Scales (1 Km And 100 M) Over The Same Area. Validation Was Performed Using In Situ Data At The District Level Of Algerri-Balaguer In Catalunya, Spain, Where In Situ Irrigation Amounts Were Available For Various Years. The Validation Resulted In A Total Pearson'S Correlation Coefficient (R) Of 0.80 And A Total Root Mean Square Error (Rmse) Of 7.19 Mm/Week For The Years From 2017 To 2021. Additional Vali-Dation Was Conducted At The Field Level In The Segarra-Garrigues Irrigation District Using In Situ Data From A Field Where Sm Profiles And Irrigation Amounts Were Continuously Monitored. This Validation Yielded A Total Bi-Weekly R Of 0.81 And A Total Rmse Of-9.34 Mm/14-Days For The Years From 2017 To 2021. Overall, The Results Suggested That Prism Can Effectively Estimate Irrigation From Sm Remote Sensing Data, And The Methodology Has The Po-Tential To Be Applied On A Large Scale Without Requiring Extensive Calibration Or Site-Specific Knowledge.
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Valois, R., Derode, B., Vouillamoz, J., Kotchoni, D., Lawson, M., & Rau, G. (2023). Use Of Atmospheric Tides To Estimate The Hydraulic Conductivity Of Confined And Semi-Confined Aquifers. Hydrogeology Journal, .
Abstract: Characterizing Groundwater Responses To Natural Drivers Is Cost Effective And Offers Great Potential In Hydrogeological Inves-Tigations. However, There Is A Lack Of Method Development And Evaluation, For Example By Comparing Results With Those Derived From Using Conventional Methods. This Paper Presents A Modified Method To Calculate The Hydraulic Conductivity (K) Of Confined Aquifers Using The Well Water Response To Atmospheric Tides. The Approach Separates The Earth And Atmospheric Tide Influences On Filtered Well Water-Level Records In The Time Domain. The Resulting Ill-Posed Regression Deconvolution Problem Can Be Overcome By Constraining The Well Water Response To Atmospheric Tides In Order To Follow A Physically Real-Istic Semi-Diurnal Barometric Response Function (S-2-Brf), Or To Follow Directly A Modified Hydraulic Model (Be-Hvorslev) Similar To A Slug Test Evaluation. An Analysis With Synthetic Data Shows That K Up To 10(-4) M/S Can Be Estimated When Pressure Records With Short Sampling Intervals Are Available. Application To A Field Dataset From Cambodia And Benin, With 20-Minute To 60-Minute Sampling Intervals, Respectively, Results In K Values Of 5.82 Center Dot 10(-7) M/S And 2.9<Middle Dot>10(-7) M/S. This Agrees With Results Independently Derived From Pumping Tests For Both Confined Sediments And Semi-Confined Hard-Rock Conditions. This Method Offers A Promising And Low-Cost Approach To Derive K Solely From Monitoring Datasets In Confined Aquifers. This Is Especially Advantageous For Low-Conductivity Formations Where Hydraulic Testing Takes Time.
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2022 |
Chagnaud, G., Panthou, G., Vischel, T., & Lebel, T. (2022). A synthetic view of rainfall intensification in the West African Sahel. Environmental Research Letters, 171(4).
Abstract: The West African Sahel has been facing for more than 30 years an increase in extreme rainfall with strong socio-economic impacts. This situation challenges decision-makers to define adaptation strategies in a rapidly changing climate. The present study proposes (i) a quantitative characterization of the trends in extreme rainfall at the regional scale, (ii) the translation of the trends into metrics that can be used by hydrological risk managers, (iii) elements for understanding the link between the climatology of extreme and mean rainfall. Based on a regional non-stationary statistical model applied to in-situ daily rainfall data over the period 1983-2015, we show that the region-wide increasing trend in extreme rainfall is highly significant. The change in extreme value distribution reflects an increase in both the mean and variability, producing a 5%/decade increase in extreme rainfall intensity whatever the return period. The statistical framework provides operational elements for revising the design methods of hydraulic structures which most often assume a stationary climate. Finally, the study shows that the increase in annual maxima of daily rainfall is more attributable to stronger storm intensities (80%) than to more frequent storm occurrences (20%), reflecting a major rainfall regime shift in comparison to those observed in the region since 1950.
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Charrondiere, C., Brun, C., Hopfinger, E. J., Cohard, J. M., & Sicart, J. E. (2022). Mean Flow Structure Of Katabatic Winds And Turbulent Mixing Properties. Journal Of Fluid Mechanics, 9419.
Abstract: The recent field measurements of katabatic winds on steep alpine slopes provide a unique database for theoretical analysis of the mean flow development and the determination of mixing properties. The theory is based on the depth-integrated momentum and heat equations, and demonstrates an increase in mean velocity U with downstream distance x according to x(n) (n <= 1/2). An equation for the mean wind velocity is established, expressing its dependency on the buoyancy flux, related to the heat flux to the ground, entrainment and bottom friction. No ambient stratification, and ambient wind and constant ground surface temperature, lead to U similar to x(1/2), while constant heat flux to the ground leads to U similar to x(1/3) and requires that the reduced gravity decreases as x(-1/3). Stable ambient stratification N causes, in addition to small-amplitude mean flow oscillations, a decrease in reduced gravity with x, in which case the assumption of constant surface heat flux along x is only an approximation. The turbulent fluxes are a function of gradient Richardson number Ri with the ratio of turbulent diffusivity to viscosity K-h/K-m changing from nearly 1.4 to approximately 0.5 at Ri approximate to 0.5. A new mixing efficiency is introduced that includes turbulence kinetic energy production or consumption by along-slope turbulent buoyancy flux. It increases with Ri up to 0.25 at Ri approximate to 0.5 and then remains nearly constant. The measurements allowed us to determine the bottom drag coefficients and interfacial entrainment, with the ground surface heat flux being determined from the mean buoyancy flux.
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Colliander, A., Reichle, R., Crow, W., Cosh, M., Chen, F., Chan, S., et al. (2022). Validation of Soil Moisture Data Products From the NASA SMAP Mission. Ieee Journal Of Selected Topics In Applied Earth Observations And Remote Sensing, 15, 364–392.
Abstract: The National Aeronautics and Space Administration Soil Moisture Active Passive (SMAP) mission has been validating its soil moisture (SM) products since the start of data production on March 31, 2015. Prior to launch, the mission defined a set of criteria for core validation sites (CVS) that enable the testing of the key mission SM accuracy requirement (unbiased root-mean-square error <0.04 m(3)/m(3)). The validation approach also includes other (“sparse network”) in situ SM measurements, satellite SM products, model-based SM products, and field experiments. Over the past six years, the SMAP SM products have been analyzed with respect to these reference data, and the analysis approaches themselves have been scrutinized in an effort to best understand the products' performance. Validation of the most recent SMAP Level 2 and 3 SM retrieval products (R17000) shows that the L-band (1.4 GHz) radiometer-based SM record continues to meet mission requirements. The products are generally consistent with SM retrievals from the European Space Agency Soil Moisture Ocean Salinity mission, although there are differences in some regions. The high-resolution (3-km) SM retrieval product, generated by combining Copernicus Sentinel-1 data with SMAP observations, performs within expectations. Currently, however, there is limited availability of 3-km CVS data to support extensive validation at this spatial scale. The most recent (version 5) SMAP Level 4 SM data assimilation product providing surface and root-zone SM with complete spatio-temporal coverage at 9-km resolution also meets performance requirements. The SMAP SM validation program will continue throughout the mission life; future plans include expanding it to forested and high-latitude regions.
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Dommo, A., Vondou, D., Philippon, N., Eastman, R., Moron, V., & Aloysius, N. (2022). The Era5'S Diurnal Cycle Of Low-Level Clouds Over Western Central Africa During June-September: Dynamic And Thermodynamic Processes. Atmospheric Research, 2802.
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Hachgenei, N., Nord, G., Spadini, L., Mora, H., Courvoisier, F., Sutra, J. F., et al. (2022). A device to simulate contaminant transfer and surface and subsurface flow through intact soil monoliths. Vadose Zone Journal, .
Abstract: Many contaminants of agricultural origin are released into rural environments, particularly at the soil surface. Their fate has been extensively investigated in repacked soils, but only few studies have addressed their transport in structurally preserved natural soils. Much remains unknown about their fate and transfer within and between environmental compartments, while the susceptibility of these compartments to the contaminants adverse effects can vary considerably. The lack of studies regarding surface and subsurface transfer of contaminants through intact soil compared with studies on repacked soil led us to propose a device and protocol for sampling intact soil monoliths (60 x 30 x 22 cm(3), length, width, depth [LWD]) without heavy machinery. This is achieved by a modular design with removable top and bottom lid and a protocol of cutting the soil and replacing the affected bottom soil with a drainage layer of glass beads. The device allows the application of artificial rainfall events with simultaneous highly resolved quantification of infiltration excess overland flow and drainage discharge. It is designed to facilitate the collection of samples for physical, biological, and chemical analyses that fulfill cleanliness standards for organic contaminant analysis at trace levels using only poorly reactive stainless steel and glass materials. Testing of the device was performed by measuring the transfer of the antiparasitic drug ivermectin (IVM) through and over a silt-loam pasture soil. This test case illustrates how the device can be used to gain valuable information on the transfer of trace organic contaminants through topsoils.
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Hounsinou, M., Mamadou, O., Wudba, M., Kounouhewa, B., & Cohard, J. M. (2022). Integral turbulence characteristics over a clear woodland forest in northern Benin (West Africa). Atmospheric Research, 2682.
Abstract: This study aims at investigating whether the Integral Turbulence Characteristics (ITC) obey the Monin-Obukhov Similarity Theory (MOST) above a forest site in a Sudanian climate, and at identifying the appropriate ITC models for this ecosystem. Data were collected from a 18 m tower equipped with an Eddy Covariance system, above the clear forest close to Bellefoungou's village, Northwest of Benin, West Africa. The turbulence intensity parameters calculated for five years and half, were analyzed according to wind speed, stability conditions and seasons. From their relationships with the stability parameter, data driven models were then obtained by the nonlinear least squares. The results showed that, all similarity functions follow MOST with a 1/3 power law whatever the stratification of the atmosphere during all the seasons excepted the temperature which had a parabolic shape in near neutral condition (-0.05 < zeta < 0.1). A seasonal dependence of all ITCs was evidenced under stable conditions. We also showed that the heat transfer is relatively more efficient than H2O transfer under both stability conditions. The established temperature and CO2 similarity models are found to be closer, and for some given stratification conditions, to those already existing in literature. But a noteworthy finding is that the models often used to assign a quality criterion to turbulent fluxes showed an overestimation relatively to those established 'locally' for u and w through all atmospheric stratification.
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Nguyen, A., Dao, T., Strady, E., Nguyen, T., Aime, J., Gratiot, N., et al. (2022). Phytoplankton characterization in a tropical tidal river impacted by a megacity: the case of the Saigon River (Southern Vietnam). Environmental Science And Pollution Research, .
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Nsangou, D., Kpoumie, A., Mfonka, Z., Ngouh, A. N., Fossi, D. H., Jourdan, C., et al. (2022). Urban Flood Susceptibility Modelling Using Ahp And Gis Approach: Case Of the Mfoundi watershed at Yaounde in the South-Cameroon plateau. Scientific African, 151.
Abstract: Floods are considered as the natural hazards that affect the world's major metropolises the most. Thus, the present study aimed at evaluating the sensitivity to flood risks of the Mfoundi watershed (96.5 km(2)) located in the heart of the Cameroonian political capital in a tropical humid forest zone, more precisely in the South Cameroon plateau. The methodological approach adopted was to identify the factors that most favor the risk of flooding in the area from intense literature review and field investigations; the analysis of these factors and the calculation of the Flood Harzard Index (FHI) using the Analytical Hierarchy Process (AHP) approach coupled with the Geographical Information System (GIS) environment. The results reveal that among the ten parameters of the natural environment (elevation, drainage density, rainfall, slope, distance from the river, topographic humidity, hydraulic conductivity, groundwater level, geology and land cover) selected, the land cover, elevation and the geology are the factors that most influences the flooding phenomenon in the area. The value of Flood Hazard Index (FHI) varied from 4.16 to 9.16, the higher the value, the more sensitive the area is to the risk of flooding. Five main classes of flood susceptibility are highlighted: very low, low, moderate, high and very high, representing 9.50, 26, 23, 22 and 19.5%, respectively of the study area. To validate the efficiency of the obtained flood susceptibility map, the adopted Area Under the Curve (AUC) method shows a very good accuracy (0.84 or 84%). The results of this study constitute a basic tool for decision-making for environmental management by public authorities and decentralised territorial authorities with territorial jurisdiction. (C) 2021 Published by Elsevier B.V. on behalf of African Institute of Mathematical Sciences / Next Einstein Initiative.
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Pellarin, T., Zoppis, A., Roman-Cascon, C., Kerr, Y. H., Rodriguez-Fernandez, N., Panthou, G., et al. (2022). From SMOS Soil Moisture to 3-hour Precipitation Estimates at 0.1 degrees Resolution in Africa. Remote Sensing, 141(3).
Abstract: Several recent studies have shown that knowledge of the spatiotemporal dynamics of soil moisture intrinsically contains information on precipitation. In this study, we show how SMOS measurements can be used to generate a near-real-time precipitation product with a spatial resolution of 0.1 degrees and a temporal resolution of 3 h. The principle consists of assimilating the SMOS data into a model that simulates the evolution of soil moisture, which is forced by a satellite precipitation product. The assimilation of SMOS soil moisture leads to an adjustment of the satellite precipitation rates. Using data from more than 200 rain gauges set up in Africa between 2010 and 2021, we show that the PrISM algorithm (for Precipitation Inferred from Soil Moisture) almost systematically improves the initial precipitation product. One of the original features of this study is that we used the IMERG-Early satellite precipitation product, which has a finer spatial resolution (0.1 degrees) than SMOS (~0.25 degrees). Despite this, the methodology reduces both the RMSE and bias of IMERG-Early. The RMSE is reduced from 8.0 to 6.3 mm/day, and the absolute bias is reduced from 0.81 to 0.63 mm/day on average over the 200 rain gauges. PrISM performs even slightly better on average than IMERG-Final in terms of RMSE (6.8 mm/day for IMERG-Final) but better scores are obtained by IMERG-Final in terms of absolute bias (0.35 mm/day), which utilizes a network of field measurements to correct the biases of the IMERG-Early product with a 2.5-month delay. Therefore, the use of SMOS soil moisture measurements for Africa can be an advantageous alternative to the use of gauge measurements for debiasing rainfall satellite products in real time.
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Philippon, N., Ouhechou, A., Camberlin, P., Trentmann, J., Fink, A. H., Maloba, J. D., et al. (2022). Characterization Of Sunshine Duration In Western Equatorial Africa: In Situ Measurements versus SARAH-2 Satellite Estimates. Journal Of Applied Meteorology And Climatology, 616(2), 185–201.
Abstract: Western Equatorial Africa is one of the least sunny areas in the world. Yet, this has attracted little research so far. As in many other parts of Africa, light availability is mainly estimated using in situ measurements of sunshine duration (SDU). Therefore, this study conducts the first characterization of SDU evolution during the annual cycle for the region. It also evaluates the skill of satellite-based estimates of SDU from the Surface Solar Radiation Data Set-Heliosat, edition 2.1 (SARAH-2.1). Mean annual SDU levels are low: less than 5 h day21 at the regional scale, with the sunniest stations in the northeast (Cameroon and Central African Republic) and the least sunny in an similar to 150-km-wide coastal strip in Gabon and Republic of the Congo (RoC). For most of the stations except the southeast ones in the Democratic Republic of Congo, the lowest SDU levels are recorded in July-September, during the main dry season, with persistent overcast conditions. They are as low as 2.5 h day21, especially on the windward slopes of the Massifs du Chaillu and du Mayombe, nd of the Bateke Plateaus in Gabon and RoC. Although the mean annual and monthly spatial patterns are well reproduced in SARAH-2.1, SDU levels are systematically overestimated by 1-2 h day21. The largest positive biases are recorded during the December-February dry season, especially at the northernmost stations. Analyses at the daily time scale show that SARAH-2.1 biases arise from a twofold problem: the number of dark days (SDU, 1 h day21) is 50% lower than observed whereas that of sunny days (SDU. 9 h day21) is 50% higher than observed.
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Segura, H., Espinoza, J. C., Junquas, C., Lebel, T., Vuille, M., & Condom, T. (2022). Extreme austral winter precipitation events over the South-American Altiplano: regional atmospheric features. Climate Dynamics, .
Abstract: The South American Altiplano has a marked dry season during the austral winter (June to August, JJA). However, during this season synoptic meteorological conditions triggering heavy precipitation can damage socioeconomic activities, often causing the loss of human lives. Using daily in-situ precipitation data from 39 rain-gauge stations over the northern Altiplano (18 degrees S -15 degrees S; > 3000 m.a.s.l.) for the JJA season, we computed the historical percentile 90 (p90) and we identified extreme rainy days with precipitation higher than p90 in the 1980-2010 period. We identified 100 winter extreme precipitation events (WEPEs) over this region that can last between one to 16 days. The K-means analysis was applied to anomalies of geopotential height at 500 hPa from ERA-Interim data during the initial day or Day(0) of WEPEs lasting 1 day (42 cases), 2 days (19) and more than 2 days (39). We found 59 WEPEs characterized by an upper-level trough over the Peruvian-Chilean coast. At 850 hPa, these 59 WEPEs are also associated with cold surges along the eastern Central Andes, indicating an association between the upper-level trough and the cold surge in developing deep convection over the northern Altiplano. A lead-lag composite analysis further showed a significant lower- and mid-tropospheric moistening over the western Amazon 2 days before the onset of these 59 WEPEs, due to low-level northerly wind anomalies originating over equatorial South America. The other 41 WEPEs are associated with a low-level southerly wind regime crossing the equator and a mid-and upper-level low-pressure system over the Peruvian-Chilean coast. While the low-level southerly regime enhances mid-tropospheric moisture transport from the equator towards the Altiplano due to the developed shallow meridional circulation when propagating equatorward, a low-pressure system promotes intensification of upward motion, boosting the upslope moisture transport from the lowlands to the east of the Central Andes towards the Altiplano.
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2021 |
Abdou, M., Vandervaere, J., Descroix, L., & Moussa, I. (2021). Comparative hydrodynamic study of granitic and sedimentary catchments in Western Niger. Hydrological Sciences Journal-Journal Des Sciences Hydrologiques, .
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Barree, M., Mialon, A., Pellarin, T., Parrens, M., Biron, R., Lemaitre, F., et al. (2021). Soil moisture and vegetation optical depth retrievals over heterogeneous scenes using LEWIS L-band radiometer. International Journal Of Applied Earth Observation And Geoinformation, 102.
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Chagnaud, G., Panthou, G., Vischel, T., Blanchet, J., & Lebel, T. (2021). A unified statistical framework for detecting trends in multi-timescale precipitation extremes: application to non-stationary intensity-duration-frequency curves. Theoretical And Applied Climatology, .
Abstract: There is a large agreement that global warming induces changes of precipitation regimes of different nature and amplitude depending on the timescale considered. This question is of special concern regarding extreme rainfall that might have critical socio-environmental consequences. A unified framework is proposed here for detecting trends in extreme rainfall. It is based on the GEV distribution, whose parameters depend both on a simple scaling formulation to account for multiple time durations of rainfall and on time to account for the non-stationarity deriving from climatic trends. The implementation of the model is illustrated in the Sahel region by analyzing 30 in situ rainfall series of 28 years measured at time-steps from 2 to 24 h. While the separate analysis of the point series proves inconclusive for detecting trends at any of the time-steps considered, the inclusion of all the series and time-steps into the proposed unified model allows trends to be detected at a high level of confidence (p-value < 1%). This trend essentially appears in the scale parameter of the regional GEV distribution, involving a 15 to 20% increase of the 10-year rainfall in 28 years, and a 23 to 30% increase of the 100-year rainfall. The main advantages of the proposed framework are (i) its parsimony, allowing for reducing the uncertainty associated with the model inference; (ii) its capacity for detecting trends either in the mean and/or in the variability of the extreme events; and (iii) its ability for producing non-stationary Intensity-Duration-Frequency curves that are coherent over a range of durations of accumulation.
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Charrondiere, C., Brun, C., Cohard, J., Sicart, J., Obligado, M., Biron, R., et al. (2021). Katabatic Winds over Steep Slopes: Overview of a Field Experiment Designed to Investigate Slope-Normal Velocity and Near-Surface Turbulence. Boundary-Layer Meteorology, .
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Fall, C., Lavaysse, C., Drame, M., Panthou, G., & Gaye, A. (2021). Wet and dry spells in Senegal: comparison of detection based on satellite products, reanalysis, and in situ estimates. Natural Hazards And Earth System Sciences, 21(3), 1051–1069.
Abstract: In this study, the detection and characteristics of dry/wet spells (defined as episodes when precipitation is abnormally low or high compared to usual climatology) drawn from several datasets are compared for Senegal. Here, four datasets are based on satellite data (TRMM-3B42 V7, CMORPH V1.0, TAMSAT V3, and CHIRPS V2. 0), two on reanalysis products (NCEP-CFSR and ERAS), and three on rain gauge observations (CPC Unified V1.0/RT and a 65-rain-gauge network regridded by using two kriging methods, namely ordinary kriging, OK, and block kriging, BK). All datasets were converted to the same spatio-temporal resolution: daily cumulative rainfall on a regular 0.25 degrees grid. The BK dataset was used as a reference. Despite strong agreement between the datasets on the spatial variability in cumulative seasonal rainfall (correlations ranging from 0.94 to 0.99), there were significant disparities in dry/wet spells. The occurrence of dry spells is less in products using infrared measurement techniques than in products coupling infrared and microwave, pointing to more frequent dry spell events. All datasets show that dry spells appear to be more frequent at the start and end of rainy seasons. Thus, dry spell occurrences have a major influence on the duration of the rainy season, in particular through the “false onset” or “early cessation” of seasons. The amplitude of wet spells shows the greatest variation between datasets. Indeed, these major wet spells appear more intense in the OK and Tropical Rainfall Measuring Mission (TRMM) datasets than in the others. Lastly, the products indicate a similar wet spell frequency occurring at the height of the West African monsoon. Our findings provide guidance in choosing the most suitable datasets for implementing early warning systems (EWSs) using a multi-risk approach and integrating effective dry/wet spell indicators for monitoring and detecting extreme events.
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Fayjaloun, R., Dabaghi, M., Cornou, C., Causse, M., Lu, Y., Stehly, L., et al. (2021). Hybrid Simulation of Near-Fault Ground Motion for a Potential M-w 7 Earthquake in Lebanon. Bulletin Of The Seismological Society Of America, 111(5), 2441–2462.
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Fayjaloun, R., Dabaghi, M., Cornou, C., Causse, M., Lu, Y., Stehly, L., et al. (2021). Hybrid Simulation of Near-Fault Ground Motion for a Potential M-w 7 Earthquake in Lebanon (vol 111, pg 2441, 2021). Bulletin Of The Seismological Society Of America, 111(6), 3498–3499.
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Fovet, O., Belemtougri, A., Boithias, L., Braud, I., Charlier, J., Cottet, M., et al. (2021). Intermittent rivers and ephemeral streams: Perspectives for critical zone science and research on socio-ecosystems. Wiley Interdisciplinary Reviews-Water, .
Abstract: Intermittent rivers and ephemeral streams (IRES) are now recognized to support specific freshwater biodiversity and ecosystem services and represent approximately half of the global river network, a fraction that is likely to increase in the context of global changes. Despite large research efforts on IRES during the past few decades, there is a need for developing a systemic approach to IRES that considers their hydrological, hydrogeological, hydraulic, ecological, and biogeochemical properties and processes, as well as their interactions with human societies. Thus, we assert that the interdisciplinary approach to ecosystem research promoted by critical zone sciences and socio-ecology is relevant. These approaches rely on infrastructure-Critical Zone Observatories (CZO) and Long-Term Socio-Ecological Research (LTSER) platforms-that are representative of the diversity of IRES (e.g., among climates or types of geology. We illustrate this within the French CZO and LTSER, including their diversity as socio-ecosystems, and detail human interactions with IRES. These networks are also specialized in the long-term observations required to detect and measure ecosystem responses of IRES to climate and human forcings despite the delay and buffering effects within ecosystems. The CZO and LTSER platforms also support development of innovative techniques and data analysis methods that can improve characterization of IRES, in particular for monitoring flow regimes, groundwater-surface water flow, or water biogeochemistry during rewetting. We provide scientific and methodological perspectives for which this interdisciplinary approach and its associated infrastructure would provide relevant and original insights that would help fill knowledge gaps about IRES. This article is categorized under: Water and Life > Conservation, Management, and Awareness
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Goni, I., Taylor, R., Favreau, G., Shamsudduha, M., Nazoumou, Y., & Ngatcha, B. (2021). Groundwater recharge from heavy rainfall in the southwestern Lake Chad Basin: evidence from isotopic observations. Hydrological Sciences Journal-Journal Des Sciences Hydrologiques, 66(8), 1359–1371.
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Herzog, A., Hector, B., Cohard, J., Vouillamoz, J., Lawson, F., Peugeot, C., et al. (2021). A parametric sensitivity analysis for prioritizing regolith knowledge needs for modeling water transfers in the West African critical zone. Vadose Zone Journal, 20(6).
Abstract: Hard rock aquifers (HRAs) in West Africa (WA) are located within a thick regolith layer. The representation of thick tropical regolith in integrated hydrological models lacks consensus on aquifer geometries and parameter ranges. Our main objective was to determine the knowledge requirements on saturated hydraulic conductivity (K-s) to model the critical zone (CZ) of HRAs in WA. A parametric sensitivity analysis with a focus on the representation of the K-s heterogeneity of the regolith was conducted with a critical zone model (Parflow-CLM [Community Land Model]) of the Upper Oueme catchment in Benin (14,000 km(2)) at a 1- x 1-km(2) resolution. The impact of parameter changes in the near subsurface (0.3-to-5-m depth) and in the deeper regolith aquifer (24- and 48-m maximum depth) was assessed in five modeling experiments. Streamflow was largely dependent on K-s and on clay distribution in the near subsurface and less on the properties of the deeper subsurface. Groundwater table depths and amplitudes were controlled by vegetation and topography as observed on instrumented hillslopes and for K-s within the literature range. Experiments with higher K-s suggested a K-s threshold where dynamics become less determined by one-dimensional vertical and more determined by lateral processes. Such heterogeneity impacts from smaller scales need to be accounted for when hydrological models are upscaled to larger domains (1- x 1-km(2) resolution or coarser). Our findings highlight the need for a new conceptual approach to represent clay distribution in order to develop catchment-scale CZ models of HRAs in WA that capture the observed processes.
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Klein, C., Jackson, L., Parker, D., Marsham, J., Taylor, C., Rowell, D., et al. (2021). Combining CMIP data with a regional convection-permitting model and observations to project extreme rainfall under climate change. Environmental Research Letters, 16(10).
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Macdonald, A., Lark, R., Taylor, R., Abiye, T., Fallas, H., Favreau, G., et al. (2021). Mapping groundwater recharge in Africa from ground observations and implications for water security. Environmental Research Letters, 16(3).
Abstract: Groundwater forms the basis of water supplies across much of Africa and its development is rising as demand for secure water increases. Recharge rates are a key component for assessing groundwater development potential, but have not been mapped across Africa, other than from global models. Here we quantify long-term average (LTA) distributed groundwater recharge rates across Africa for the period 1970-2019 from 134 ground-based estimates and upscaled statistically. Natural diffuse and local focussed recharge, where this mechanism is widespread, are included but discrete leakage from large rivers, lakes or from irrigation are excluded. We find that measurable LTA recharge is found in most environments with average decadal recharge depths in arid and semi-arid areas of 60 mm (30-140 mm) and 200 mm (90-430 mm) respectively. A linear mixed model shows that at the scale of the African continent only LTA rainfall is related to LTA recharge-the inclusion of other climate and terrestrial factors do not improve the model. Kriging methods indicate spatial dependency to 900 km suggesting that factors other than LTA rainfall are important at local scales. We estimate that average decadal recharge in Africa is 15 000 km(3) (4900-45 000 km(3)), approximately 2% of estimated groundwater storage across the continent, but is characterised by stark variability between high-storage/low-recharge sedimentary aquifers in North Africa, and low-storage/high-recharge weathered crystalline-rock aquifers across much of tropical Africa. African water security is greatly enhanced by this distribution, as many countries with low recharge possess substantial groundwater storage, whereas countries with low storage experience high, regular recharge. The dataset provides a first, ground-based approximation of the renewability of groundwater storage in Africa and can be used to refine and validate global and continental hydrological models while also providing a baseline against future change.
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Massazza, G., Bacci, M., Descroix, L., Ibrahim, M., Fiorillo, E., Katiellou, G., et al. (2021). Recent Changes in Hydroclimatic Patterns over Medium Niger River Basins at the Origin of the 2020 Flood in Niamey (Niger). Water, 13(12).
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Nagaraj, P., Subbarayappa, M., Vouillamoz, J., & Hoareau, J. (2021). Estimation of anisotropic hydraulic conductivity using geophysical data in a coastal aquifer of Karnataka, India. Hydrological Processes, 35(10).
Abstract: Estimation of hydraulic parameters is essential to understand the interaction between groundwater flow and seawater intrusion. Though several studies have addressed hydraulic parameter estimation, based on pumping tests as well as geophysical methods, not many studies have addressed the problem with clayey formations being present. In this study, a methodology is proposed to estimate anisotropic hydraulic conductivity and porosity values for the coastal aquifer with unconsolidated formations. For this purpose, the one-dimensional resistivity of the aquifer and the groundwater conductivity data are used to estimate porosity at discrete points. The hydraulic conductivity values are estimated by its mutual dependence with porosity and petrophysical parameters. From these estimated values, the bilinear relationship between hydraulic conductivity and aquifer resistivity is established based on the clay content of the sampled formation. The methodology is applied on a coastal aquifer along with the coastal Karnataka, India, which has significant clayey formations embedded in unconsolidated rock. The estimation of hydraulic conductivity values from the established correlations has a correlation coefficient of 0.83 with pumping test data, indicating good reliability of the methodology. The established correlations also enable the estimation of horizontal hydraulic conductivity on two-dimensional resistivity sections, which was not addressed by earlier studies. The inventive approach of using the established bilinear correlations at one-dimensional to two-dimensional resistivity sections is verified by the comparison method. The horizontal hydraulic conductivity agrees with previous findings from inverse modelling. Additionally, this study provides critical insights into the estimation of vertical hydraulic conductivity and an equation is formulated which relates vertical hydraulic conductivity with horizontal. Based on the approach presented, the anisotropic hydraulic conductivity of any type aquifer with embedded clayey formations can be estimated. The anisotropic hydraulic conductivity has the potential to be used as an important input to the groundwater models.
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Rahimi, J., Ago, E. E., Ayantunde, A., Berger, S., Bogaert, J., Butterbach-Bahl, K., et al. (2021). Modeling gas exchange and biomass production in West African Sahelian and Sudanian ecological zones. Geoscientific Model Development, 14(6), 3789–3812.
Abstract: West African Sahelian and Sudanian ecosystems provide essential services to people and also play a significant role within the global carbon cycle. However, climate and land use are dynamically changing, and uncertainty remains with respect to how these changes will affect the potential of these regions to provide food and fodder resources or how they will affect the biosphere–atmosphere exchange of CO2. In this study, we investigate the capacity of a process-based biogeochemical model, LandscapeDNDC, to simulate net ecosystem exchange (NEE) and aboveground biomass of typical managed and natural Sahelian and Sudanian savanna ecosystems. In order to improve the simulation of phenology, we introduced soil-water availability as a common driver of foliage development and productivity for all of these systems. The new approach was tested by using a sample of sites (calibration sites) that provided NEE from flux tower observations as well as leaf area index data from satellite images (MODIS, MODerate resolution Imaging Spectroradiometer). For assessing the simulation accuracy, we applied the calibrated model to 42 additional sites (validation sites) across West Africa for which measured aboveground biomass data were available. The model showed good performance regarding biomass of crops, grass, or trees, yielding correlation coefficients of 0.82, 0.94, and 0.77 and root-mean-square errors of 0.15, 0.22, and 0.12 kg m−2, respectively. The simulations indicate aboveground carbon stocks of up to 0.17, 0.33, and 0.54 kg C ha−1 m−2 for agricultural, savanna grasslands, and savanna mixed tree–grassland sites, respectively. Carbon stocks and exchange rates were particularly correlated with the abundance of trees, and grass biomass and crop yields were higher under more humid climatic conditions. Our study shows the capability of LandscapeDNDC to accurately simulate carbon balances in natural and agricultural ecosystems in semiarid West Africa under a wide range of conditions; thus, the model could be used to assess the impact of land-use and climate change on the regional biomass productivity.
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Senior, C., Marsham, J., Berthou, S., Burgin, L., Folwell, S., Kendon, E., et al. (2021). Convection-Permitting Regional Climate Change Simulations for Understanding Future Climate and Informing Decision-Making in Africa. Bulletin Of The American Meteorological Society, 102(6), E1206–E1223.
Abstract: Pan-Africa convection-permitting regional climate model simulations have been performed to study the impact of high resolution and the explicit representation of atmospheric moist convection on the present and future climate of Africa. These unique simulations have allowed European and African climate scientists to understand the critical role that the representation of convection plays in the ability of a contemporary climate model to capture climate and climate change, including many impact-relevant aspects such as rainfall variability and extremes. There are significant improvements in not only the small-scale characteristics of rainfall such as its intensity and diurnal cycle, but also in the large-scale circulation. Similarly, effects of explicit convection affect not only projected changes in rainfall extremes, dry spells, and high winds, but also continental-scale circulation and regional rainfall accumulations. The physics underlying such differences are in many cases expected to be relevant to all models that use parameterized convection. In some cases physical understanding of small-scale change means that we can provide regional decision-makers with new scales of information across a range of sectors. We demonstrate the potential value of these simulations both as scientific tools to increase climate process understanding and, when used with other models, for direct user applications. We describe how these ground-breaking simulations have been achieved under the U.K. Government's Future Climate for Africa Programme. We anticipate a growing number of such simulations, which we advocate should become a routine component of climate projection, and encourage international coordination of such computationally and human-resource expensive simulations as effectively as possible.
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Wilcox, C., Aly, C., Vischel, T., Panthou, G., Blanchet, J., Quantin, G., et al. (2021). Stochastorm: A Stochastic Rainfall Simulator for Convective Storms. Journal Of Hydrometeorology, 22(2), 387–404.
Abstract: Stochastic rainfall generators aim to reproduce the main statistical features of rainfall at small spatial and temporal scales. The simulated synthetic rainfall series are recognized as suitable for use with impact analysis in water, agricultural, and ecological management. Convection-driven precipitation, dominant in certain regions of the world such as the intertropical belt regions, presents properties that require specific consideration when modeling: (i) strong rainfall intermittency, (ii) high variability of intensities within storms, (iii) strong spatiotemporal correlation of intensities, and (iv) marked seasonality of storm properties. In this article, improvements for an existing stochastic generator of rainfall fields that models convective storms are presented. Notable novelties include (i) the ability to model precipitation event timing, (ii) an improved temporal disaggregation scheme representing the rainfall distribution at subevent scales, and (iii) using covariates to reflect seasonal changes in precipitation occurrence and marginal distribution parameters. Extreme values are explicitly considered in the distribution of storm event intensities. The simulator is calibrated and validated using 28 years of 5-min precipitation data from the 30-rain-gauge AMMA-CATCH network in the Sahelian region of southwest Niger. Both large propagative systems and smaller local convective precipitation are generated. Results show that simulator improvements coherently represent the local climatology. The simulator can generate scenarios for impact studies with accurate representation of convective precipitation characteristics.
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Wongchuig, S., Espinoza, J., Condom, T., Segura, H., Ronchail, J., Arias, P., et al. (2021). A regional view of the linkages between hydro-climatic changes and deforestation in the Southern Amazon. International Journal Of Climatology, .
Abstract: In the last four decades, the Southern Amazon (south of 8 degrees S) has shown changes in the spatial and temporal patterns of its hydro-climatic components, leading to drier conditions. Due to climate and land-use changes, this region is considered as a zone under biophysical transition processes. Previous studies have documented a complex interaction between climate and deforestation either on a large-scale or based on limited in situ data, typically covering the Brazilian Amazon. In this study, we analyse the relationships between hydro-climate, the surface water-energy partitioning and an index of regional forest cover change for the period 1981-2018. Additionally, we discretized three regions covering the Bolivian Amazon and the southern portions of the Peruvian and Brazilian Amazon due to their differences in the evolution of land use. In the Bolivian region, a high ratio of forest cover change, exceeding 40-50%, is related to a significant tendency to become water-limited. This change is associated with decreased rainfall, increased potential evapotranspiration and decreased actual evapotranspiration. Regardless of the region analysed, those that are characterized by a high ratio of forest cover change (>40-50%) show growing imbalance between increasing potential and decreasing actual evapotranspiration. However, in the Peruvian and Brazilian regions, hydro-climatic conditions remain energy-limited due to minor rainfall changes. The observed differences in surface water-energy partitioning behaviour evidence a complex dependence of both sub-regional (i.e., land cover changes) and large-scale (i.e., strengthening of the Walker and Hadley circulations) conditions. Our findings indicate a clear link between hydro-climatic changes and deforestation, providing a new perspective on their spatial variability on a sub-regional scale.
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2020 |
Bodian, A., Diop, L., Panthou, G., Dacosta, H., Deme, A., Dezetter, A., et al. (2020). Recent Trend in Hydroclimatic Conditions in the Senegal River Basin. Water, 12(2).
Abstract: Analyzing trends of annual rainfall and assessing the impacts of these trends on the hydrological regime are crucial in the context of climate change and increasing water use. This research investigates the recent trend of hydroclimatic variables in the Senegal River basin based on 36 rain gauge stations and three hydrometric stations not influenced by hydraulic structures. The Man Kendall and Pettitt's tests were applied for the annual rainfall time series from 1940 to 2013 to detect the shift and the general trend of the annual rainfall. In addition, trends of average annual flow rate (AAFR), maximum daily flow (MADF), and low flow rate (LFR) were evaluated before and after annual rainfall shift. The results show that the first shift is situated on average at 1969 whereas the second one is at 1994. While the first shift is very consistent between stations (between 1966 and 1972), there is a significant dispersion of the second change-point between 1984 and 2002. After the second shift (1994), an increase of annual rainfall is noticed compared to the previous period (1969-1994) which indicates a not significant, partial rainfall recovery at the basin level. The relative changes of hydrologic variables differ based on the variables and the sub-basin. Relative changes before and after first change-point are significantly negative for all variables. The highest relative changes are observed for the AAFR. Considering the periods before and second shifts, the relative changes are mainly significantly positive except for the LFR.
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Braud, I., Chaffard, V., Coussot, C., Galle, S., Juen, P., Alexandre, H., et al. (2020). Building the information system of the French Critical Zone Observatories network: Theia/OZCAR-IS. Hydrological Sciences Journal-Journal Des Sciences Hydrologiques, .
Abstract: The French Critical Zone research infrastructure, OZCAR-RI, gathers 20 observatories sampling various compartments of the critical zone, each having developed their own data management and distribution systems. A common information system (Theia/OZCAR IS) was built to make theirin situobservation FAIR (findable, accessible, interoperable, reusable). The IS architecture was designed after consultation of the users, data producers and IT teams involved in data management. A common data model based on various metadata standards was defined to create information fluxes between observatories' ISs and the Theia/OZCAR IS. Controlled vocabularies were defined to develop a data discovery web portal offering a faceted search with various criteria, including variables names and categories that were harmonized in a thesaurus published on the web. This paper describes the IS architecture, the pivot data model and open-source solutions used to implement data discovery, and future steps to implement data downloading and interoperability services.
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Brocca, L., Massari, C., Pellarin, T., Filippucci, P., Ciabatta, L., Camici, S., et al. (2020). River flow prediction in data scarce regions: soil moisture integrated satellite rainfall products outperform rain gauge observations in West Africa. Scientific Reports, 10(1).
Abstract: Satellite precipitation products have been largely improved in the recent years particularly with the launch of the global precipitation measurement (GPM) core satellite. Moreover, the development of techniques for exploiting the information provided by satellite soil moisture to complement/enhance precipitation products have improved the accuracy of accumulated rainfall estimates over land. Such satellite enhanced precipitation products, available with a short latency (<1 day), represent an important and new source of information for river flow prediction and water resources management, particularly in developing countries in which ground observations are scarcely available and the access to such data is not always ensured. In this study, three recently developed rainfall products obtained from the integration of GPM rainfall and satellite soil moisture products have been used; namely GPM+SM2RAIN, PRISM-SMOS, and PRISM-SMAP. The prediction of observed daily river discharge at 10 basins located in Europe (4), West Africa (3) and South Africa (3) is carried out. For comparison, we have also considered three rainfall products based on: (1) GPM only, i.e., the Early Run version of the Integrated Multi-Satellite Retrievals for GPM (GPM-ER), (2) rain gauges, i.e., the Global Precipitation Climatology Centre, and (3) the latest European Centre for Medium-Range Weather Forecasts reanalysis, ERA5. Three different conceptual and lumped rainfall-runoff models are employed to obtain robust and reliable results over the 3-year data period 2015-2017. Results indicate that, particularly over scarcely gauged areas (West Africa), the integrated products outperform both ground- and reanalysis-based rainfall estimates. For all basins, the GPM+SM2RAIN product is performing the best among the short latency products with mean Kling-Gupta Efficiency (KGE) equal to 0.87, and significantly better than GPM-ER (mean KGE=0.77). The integrated products are found to reproduce particularly well the high flows. These results highlight the strong need to disseminate such integrated satellite rainfall products for hydrological (and agricultural) applications in poorly gauged areas such as Africa and South America.
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Cappelaere, B., Feurer, D., Vischel, T., Ottle, C., Issoufou, H., Saux-Picart, S., et al. (2020). Modeling Land Surface Fluxes from Uncertain Rainfall: A Case Study in the Sahel with Field-Driven Stochastic Rainfields. Atmosphere, 11(5).
Abstract: In distributed land surface modeling (LSM) studies, uncertainty in the rainfields that are used to force models is a major source of error in predicted land surface response variables. This is particularly true for applications in the African Sahel region, where weak knowledge of highly time/space-variable convective rainfall in a poorly monitored region is a considerable obstacle to such developments. In this study, we used a field-based stochastic rainfield generator to analyze the propagation of the rainfall uncertainty through a distributed land surface model simulating water and energy fluxes in Sahelian ecosystems. Ensemble time/space rainfields were generated from field observations of the local AMMA-CATCH-Niger recording raingauge network. The rainfields were then used to force the SEtHyS-Savannah LSM, yielding an ensemble of time/space simulated fluxes. Through informative graphical representations and innovative diagnosis metrics, these outputs were analyzed to separate the different components of flux variability, among which was the uncertainty represented by ensemble-wise variability. Scale dependence was analyzed for each flux type in the water and energy budgets, producing a comprehensive picture of uncertainty propagation for the various flux types, with its relationship to intrinsic space/time flux variability. The study was performed over a 2530 km(2)domain over six months, covering an entire monsoon season and the subsequent dry-down, using a kilometer/daily base resolution of analysis. The newly introduced dimensionless uncertainty measure, called the uncertainty coefficient, proved to be more effective in describing uncertainty patterns and relationships than a more classical measure based on variance fractions. Results show a clear scaling relationship in uncertainty coefficients between rainfall and the dependent fluxes, specific to each flux type. These results suggest a higher sensitivity to rainfall uncertainty for hydrological than for agro-ecological or meteorological applications, even though eddy fluxes do receive a substantial part of that source uncertainty.
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Chagnaud, G., Gallee, H., Lebel, T., Panthou, G., & Vischel, T. (2020). A Boundary Forcing Sensitivity Analysis of the West African Monsoon Simulated by the Modele Atmospherique Regional. Atmosphere, 11(2).
Abstract: The rainfall regime of West Africa is highly variable over a large range of space and time scales. With rainfall agriculture being predominent in the region, the local population is extremely vulnerable to intraseasonal dry spells and multi-year droughts as well as to intense rainfall over small time steps. Were this variability to increase, it might render the area close from becoming unhabitable. Anticipating any change is thus crucial from both a societal and a scientific perspective. Despite continuous efforts in Global Climate Model (GCM) development, there is still no agreement on the sign of the future rainfall regime change in the region. Regional Climate Models (RCMs) are used for more accurate projections of future changes as well as end-user-oriented impact studies. In this study, the sensitivity of the Modele Atmospherique Regional (MAR) to homogeneous perturbations in boundary forcing air temperature and/or SST is assessed with the aim to better understand (i) the thermodynamical imprint of the recent rainfall regime changes and (ii) the impact of errors in driving data on the West African rainfall regime simulated by an RCM. After an evaluation step where the model is proved to satisfactorily simulate the West African Monsoon (WAM), sensitivity experiments display contrasted, sizable and robust responses of the simulated rainfall regime. The rainfall responses to the boundary forcing perturbations compare in magnitude with the intrinsic model bias, giving support for such an analysis. A physical interpretation of the rainfall anomalies provides confidence in the model response consistency and shows the potential of such an experimental protocol for future climate change downscalling over this region.
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Charrondiere, C., Brun, C., Sicart, J., Cohard, J., Biron, R., & Blein, S. (2020). Buoyancy Effects in the Turbulence Kinetic Energy Budget and Reynolds Stress Budget for a Katabatic Jet over a Steep Alpine Slope. Boundary-Layer Meteorology, 177(1), 97–122.
Abstract: Katabatic winds are very frequent but poorly understood or simulated over steep slopes. This study focuses on a katabatic jet above a steep alpine slope. We assess the buoyancy terms in both the turbulence kinetic energy (TKE) and the Reynolds shear-stress budget equations. We specifically focus on the contribution of the slope-normal and along-slope turbulent sensible heat fluxes to these terms. Four levels of measurements below and above the maximum wind-speed height enable analysis of the buoyancy effect along the vertical profile as follow: (i) buoyancy tends to destroy TKE, as expected in stable conditions, and the turbulent momentum flux in the inner-layer region of the jet below the maximum wind-speed height z(j); (ii) results also suggest buoyancy contributes to the production of TKE in the outer-layer shear region of the jet (well above z(j)) while consumption of the turbulent momentum flux is observed in the same region; (iii) In the region around the maximum wind speed where mechanical shear production is marginal, buoyancy tends to destroy TKE and our results suggest it tends to increase the momentum flux. The present study also provides an analytical condition for the limit between production and consumption of the turbulent momentum flux due to buoyancy as a function of the slope angle, similar to the condition already proposed for TKE. We reintroduce the stress Richardson number, which is the equivalent of the flux Richardson number for the Reynolds shear-stress budget. We point out that the flux Richardson number and the stress Richardson number are complementary stability parameters for characterizing the katabatic flow apart from the region around the maximum wind-speed height.
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Cuthbert, M., Taylor, R., Favreau, G., Todd, M., Shamsudduha, M., Villholth, K., et al. (2020). Observed controls on resilience of groundwater to climate variability in sub-Saharan Africa (vol 572, pg 230, 2019). Nature, 588(7838), E25.
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Descroix, L., Faty, B., Manga, S., Diedhiou, A., Lambert, L., Soumare, S., et al. (2020). Are the Fouta Djallon Highlands Still the Water Tower of West Africa? Water, 12(11).
Abstract: A large share of surface water resources in Sahelian countries originates from Guinea's Fouta Djallon highlands, earning the area the name of “the water tower of West Africa”. This paper aims to investigate the recent dynamics of the Fouta Djallon's hydrological functioning. The evolution of the runoff and depletion coefficients are analyzed as well as their correlations with the rainfall and vegetation cover. The latter is described at three different space scales and with different methods. Twenty-five years after the end of the 1968-1993 major drought, annual discharges continue to slowly increase, nearly reaching a long-term average, as natural reservoirs which emptied to sustain streamflows during the drought have been replenishing since the 1990s, explaining the slow increase in discharges. However, another important trend has been detected since the beginning of the drought, i.e., the increase in the depletion coefficient of most of the Fouta Djallon upper basins, as a consequence of the reduction in the soil water-holding capacity. After confirming the pertinence and significance of this increase and subsequent decrease in the depletion coefficient, this paper identifies the factors possibly linked with the basins' storage capacity trends. The densely populated areas of the summit plateau are also shown to be the ones where vegetation cover is not threatened and where ecological intensification of rural activities is ancient.
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Descroix, L., Sane, Y., Thior, M., Manga, S., Ba, B., Mingou, J., et al. (2020). Inverse Estuaries in West Africa: Evidence of the Rainfall Recovery? Water, 12(3).
Abstract: In West Africa, as in many other estuaries, enormous volumes of marine water are entering the continent. Fresh water discharge is very low, and it is commonly strongly linked to rainfall level. Some of these estuaries are inverse estuaries. During the Great Sahelian Drought (1968-1993), their hyperhaline feature was exacerbated. This paper aims to describe the evolution of the two main West African inverse estuaries, those of the Saloum River and the Casamance River, since the end of the drought. Water salinity measurements were carried out over three to five years according to the sites in order to document this evolution and to compare data with the historical ones collected during the long dry period at the end of 20th century. The results show that in both estuaries, the mean water salinity values have markedly decreased since the end of the drought. However, the Saloum estuary remains a totally inverse estuary, while for the Casamance River, the estuarine turbidity maximum (ETM) is the location of the salinity maximum, and it moves according to the seasons from a location 1-10 km downwards from the upstream estuary entry, during the dry season, to a location 40-70 km downwards from this point, during the rainy season. These observations fit with the functioning of the mangrove, the West African mangrove being among the few in the world that are markedly increasing since the beginning of the 1990s and the end of the dry period, as mangrove growth is favored by the relative salinity reduction. Finally, one of the inverse estuary behavior factors is the low fresh water incoming from the continent. The small area of the Casamance and Saloum basins (20,150 and 26,500 km(2) respectively) is to be compared with the basins of their two main neighbor basins, the Gambia River and the Senegal River, which provide significant fresh water discharge to their estuary.
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Descroix, L., Sane, Y., Thior, M., Manga, S., Ba, B., Mingou, J., et al. (2020). Inverse Estuaries in West Africa: Evidence of the Rainfall Recovery? Water, 12(3).
Abstract: In West Africa, as in many other estuaries, enormous volumes of marine water are entering the continent. Fresh water discharge is very low, and it is commonly strongly linked to rainfall level. Some of these estuaries are inverse estuaries. During the Great Sahelian Drought (1968-1993), their hyperhaline feature was exacerbated. This paper aims to describe the evolution of the two main West African inverse estuaries, those of the Saloum River and the Casamance River, since the end of the drought. Water salinity measurements were carried out over three to five years according to the sites in order to document this evolution and to compare data with the historical ones collected during the long dry period at the end of 20th century. The results show that in both estuaries, the mean water salinity values have markedly decreased since the end of the drought. However, the Saloum estuary remains a totally inverse estuary, while for the Casamance River, the estuarine turbidity maximum (ETM) is the location of the salinity maximum, and it moves according to the seasons from a location 1-10 km downwards from the upstream estuary entry, during the dry season, to a location 40-70 km downwards from this point, during the rainy season. These observations fit with the functioning of the mangrove, the West African mangrove being among the few in the world that are markedly increasing since the beginning of the 1990s and the end of the dry period, as mangrove growth is favored by the relative salinity reduction. Finally, one of the inverse estuary behavior factors is the low fresh water incoming from the continent. The small area of the Casamance and Saloum basins (20,150 and 26,500 km(2) respectively) is to be compared with the basins of their two main neighbor basins, the Gambia River and the Senegal River, which provide significant fresh water discharge to their estuary.
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Dunn, R., Alexander, L., Donat, M., Zhang, X., Bador, M., Herold, N., et al. (2020). Development of an Updated Global Land In Situ-Based Data Set of Temperature and Precipitation Extremes: HadEX3. Journal Of Geophysical Research-Atmospheres, 125(16).
Abstract: We present the second update to a data set of gridded land-based temperature and precipitation extremes indices: HadEX3. This consists of 17 temperature and 12 precipitation indices derived from daily, in situ observations and recommended by the World Meteorological Organization (WMO) Expert Team on Climate Change Detection and Indices (ETCCDI). These indices have been calculated at around 7,000 locations for temperature and 17,000 for precipitation. The annual (and monthly) indices have been interpolated on a1.875 degrees x1.25 degrees longitude-latitude grid, covering 1901-2018. We show changes in these indices by examining “global”-average time series in comparison with previous observational data sets and also estimating the uncertainty resulting from the nonuniform distribution of meteorological stations. Both the short and long time scale behavior of HadEX3 agrees well with existing products. Changes in the temperature indices are widespread and consistent with global-scale warming. The extremes related to daily minimum temperatures are changing faster than the maximum. Spatial changes in the linear trends of precipitation indices over 1950-2018 are less spatially coherent than those for temperature indices. Globally, there are more heavy precipitation events that are also more intense and contribute a greater fraction to the total. Some of the indices use a reference period for calculating exceedance thresholds. We present a comparison between using 1961-1990 and 1981-2010. The differences between the time series of the temperature indices observed over longer time scales are shown to be the result of the interaction of the reference period with a warming climate. The gridded netCDF files and, where possible, underlying station indices are available from and .
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Flechard, C., Ibrom, A., Skiba, U., De Vries, W., Van Oijen, M., Cameron, D., et al. (2020). Carbon-nitrogen interactions in European forests and semi-natural vegetation – Part 1: Fluxes and budgets of carbon, nitrogen and greenhouse gases from ecosystem monitoring and modelling. Biogeosciences, 17(6), 1583–1620.
Abstract: The impact of atmospheric reactive nitrogen (N-r) deposition on carbon (C) sequestration in soils and biomass of unfertilized, natural, semi-natural and forest ecosystems has been much debated. Many previous results of this dC/dN response were based on changes in carbon stocks from periodical soil and ecosystem inventories, associated with estimates of N-r deposition obtained from large-scale chemical transport models. This study and a companion paper (Flechard et al., 2020) strive to reduce uncertainties of N effects on C sequestration by linking multi-annual gross and net ecosystem productivity estimates from 40 eddy covariance flux towers across Europe to local measurement-based estimates of dry and wet N-r deposition from a dedicated collocated monitoring network. To identify possible ecological drivers and processes affecting the interplay between C and N-r inputs and losses, these data were also combined with in situ flux measurements of NO, N2O and CH4 fluxes; soil NO3- leaching sampling; and results of soil incubation experiments for N and greenhouse gas (GHG) emissions, as well as surveys of available data from online databases and from the literature, together with forest ecosystem (BAS-FOR) modelling. Multi-year averages of net ecosystem productivity (NEP) in forests ranged from -70 to 826 gCm(-2) yr(-1) at total wet + dry inorganic N-r deposition rates (N-dep) of 0.3 to 4.3 gNm(-2) yr(-1) and from -4 to 361 g Cm-2 yr(-1) at N-dep rates of 0.1 to 3.1 gNm(-2) yr(-1) in short semi-natural vegetation (moorlands, wetlands and unfertilized extensively managed grasslands). The GHG budgets of the forests were strongly dominated by CO2 exchange, while CH4 and N2O exchange comprised a larger proportion of the GHG balance in short semi-natural vegetation. Uncertainties in elemental budgets were much larger for nitrogen than carbon, especially at sites with elevated N-dep where N-r leaching losses were also very large, and compounded by the lack of reliable data on organic nitrogen and N-2 losses by denitrification. Nitrogen losses in the form of NO, N2O and especially NO3- were on average 27%(range 6 %-54 %) of N-dep at sites with N-dep < 1 gNm(-2) yr(-1) versus 65% (range 35 %-85 %) for N-dep > 3 gNm(-2) yr(-1). Such large levels of N-r loss likely indicate that different stages of N saturation occurred at a number of sites. The joint analysis of the C and N budgets provided further hints that N saturation could be detected in altered patterns of forest growth. Net ecosystem productivity increased with N-r deposition up to 2-2.5 gNm(-2) yr(-1), with large scatter associated with a wide range in carbon sequestration efficiency (CSE, defined as the NEP/GPP ratio). At elevated N-dep levels (> 2.5 gNm(-2) yr(-1)), where inorganic N-r losses were also increasingly large, NEP levelled off and then decreased. The apparent increase in NEP at low to intermediate N-dep levels was partly the result of geographical cross-correlations between N-dep and climate, indicating that the actual mean dC/dN response at individual sites was significantly lower than would be suggested by a simple, straightforward regression of NEP vs. N-dep.
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Flechard, C., Van Oijen, M., Cameron, D., De Vries, W., Ibrom, A., Buchmann, N., et al. (2020). Carbon-nitrogen interactions in European forests and semi-natural vegetation – Part 2: Untangling climatic, edaphic, management and nitrogen deposition effects on carbon sequestration potentials. Biogeosciences, 17(6), 1621–1654.
Abstract: The effects of atmospheric nitrogen deposition (N-dep) on carbon (C) sequestration in forests have often been assessed by relating differences in productivity to spatial variations of N-dep across a large geographic domain. These correlations generally suffer from covariation of other confounding variables related to climate and other growth-limiting factors, as well as large uncertainties in total (dry + wet) reactive nitrogen (N-r) deposition. We propose a methodology for untangling the effects of N-dep from those of meteorological variables, soil water retention capacity and stand age, using a mechanistic forest growth model in combination with eddy covariance CO2 exchange fluxes from a Europe-wide network of 22 forest flux towers. Total N-r deposition rates were estimated from local measurements as far as possible. The forest data were compared with data from natural or semi-natural, non-woody vegetation sites. The response of forest net ecosystem productivity to nitrogen deposition (dNEP/dN(dep)) was estimated after accounting for the effects on gross primary productivity (GPP) of the co-correlates by means of a meta-modelling standardization procedure, which resulted in a reduction by a factor of about 2 of the uncorrected, apparent dGPP/dN(dep) value. This model-enhanced analysis of the C and N-dep flux observations at the scale of the European network suggests a mean overall dNEP/dN(dep) response of forest lifetime C sequestration to N-dep of the order of 40-50 g C per g N, which is slightly larger but not significantly different from the range of estimates published in the most recent reviews. Importantly, patterns of gross primary and net ecosystem productivity versus N-dep were non-linear, with no further growth responses at high N-dep levels (N-dep > 2.5-3 gNm(-2) yr(-1)) but accompanied by increasingly large ecosystem N losses by leaching and gaseous emissions. The reduced increase in productivity per unit N deposited at high N-dep levels implies that the forecast increased N-r emissions and increased N-dep levels in large areas of Asia may not positively impact the continent's forest CO2 sink. The large level of unexplained variability in observed carbon sequestration efficiency (CSE) across sites further adds to the uncertainty in the dC/dN response.
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Foucras, M., Zribi, M., Albergel, C., Baghdadi, N., Calvet, J., & Pellarin, T. (2020). Estimating 500-m Resolution Soil Moisture Using Sentinel-1 and Optical Data Synergy. Water, 12(3).
Abstract: The aim of this study is to estimate surface soil moisture at a spatial resolution of 500 m and a temporal resolution of at least 6 days, by combining remote sensing data from Sentinel-1 and optical data from Sentinel-2 and MODIS (Moderate-Resolution Imaging Spectroradiometer). The proposed methodology is based on the change detection technique, applied to a series of measurements over a three-year period (2015 to 2018). The algorithm described here as “Soil Moisture Estimations from the Synergy of Sentinel-1 and optical sensors (SMES)” proposes different options, allowing information from vegetation densities and seasonal conditions to be taken into account. The output from this algorithm is a moisture index ranging between 0 and 1, with 0 corresponding to the driest soils and 1 to the wettest soils. This methodology has been tested at different test sites (South of France, Central Tunisia, Western Benin and Southwestern Niger), characterized by a wide range of different climatic conditions. The resulting surface soil moisture estimations are compared with in situ measurements and already existing satellite-derived soil moisture ASCAT (Advanced SCATterometer) products. They are found to be well correlated, for the African regions in particular (RMSE below 6 vol.%). This outcome indicates that the proposed algorithm can be used with confidence to estimate the surface soil moisture of a wide range of climatically different sites.
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Hinderer, J., Hector, B., Riccardi, U., Rosat, S., Boy, J., Calvo, M., et al. (2020). A study of the monsoonal hydrology contribution using a 8-yr record (2010-2018) from superconducting gravimeter OSG-060 at Djougou (Benin, West Africa). Geophysical Journal International, 221(1), 431–439.
Abstract: We analyse a nearly 8-yr record (2010-2018) of the superconducting gravimeter OSG-060 located at Djougou (Benin, West Africa). After tidal analysis removing all solid Earth and ocean loading tidal contributions and correcting for the long-term instrumental drift and atmospheric loading, we obtain a gravity residual signal which is essentially a hydrological signal due to the monsoon. This signal is first compared to several global hydrology models (ERA, GLDAS and MERRA). Our superconducting gravimeter residual signal is also superimposed onto episodic absolute gravity measurements and to space gravimetry GRACE data. A further comparison is done using local hydrological data like soil moisture in the very superficial layer (0-1.2 m), water table depth and rainfall. The temporal evolution of the correlation coefficient between the gravity observation and both the soil moisture and the water table is well explained by the direct infiltration process of rain water together with the lateral transfer discharging the water table. Finally, we compute the water storage changes (WSC) using a simulation based on the physically based Parflow-CLM numerical model of the catchment, which solves the water and energy budget from the impermeable bedrock to the top of the canopy layer using the 3-D Richards equation for the water transfers in the ground, the kinematic wave equation for the surface runoff and a land surface model (CLM) for the energy budget and evapotranspiration calculation. This model forced by rain is in agreement with evapotranspiration and stream flow data and leads to simulated water storage changes that nicely fit to the observed gravity signal. This study points out the important role played by surface gravity changes in terms of a reliable proxy for water storage changes occurring in small catchments.
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Hinderer, J., Riccardi, U., Rosat, S., Boy, J., Hector, B., Calvo, M., et al. (2020). A study of the solid earth tides, ocean and atmospheric loadings using an 8-year record (2010-2018) from superconducting gravimeter OSG-060 at Djougou (Benin, West Africa). Journal Of Geodynamics, 134.
Abstract: We investigate a nearly 8-year record (2010-2018) of the superconducting gravimeter OSG-060 located at Djougou (Benin, West Africa). We first perform a tidal analysis with ET34-ANA v7.1 software that leads to the gravimetric amplitude and phase factors for all separable waves according to the available time duration. We test nine different ocean tide models for the main eleven tidal constituents (Ssa, Mm, Mf, Q1, O1, P1, K1, N2, M2, S2, K2). After correction for ocean tidal loading we obtain the real and imaginary parts of the residual vector. We also investigate atmospheric loading which is dominated in this equatorial location by the thermal waves S1 and S2 that are modulated in amplitude by annual and semi-annual components. After correction for ocean loading, we test different air pressure corrections on the tidal gravimetric factors for the waves Sa, Ssa, S1 and S2. We show the rather large discrepancy that exists between the classical single admittance pressure reduction and a hybrid model using global atmospheric models everywhere except in the local zone where the model pressure is replaced by the observed pressure.
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Legchenko, A., Baltassat, J., Duwig, C., Boucher, M., Girard, J., Soruco, A., et al. (2020). Time-lapse magnetic resonance sounding measurements for numerical modeling of water flow in variably saturated media. Journal Of Applied Geophysics, 175.
Abstract: We presented an innovative hydrogeophysical approach that allows numerical modeling of water flow in a variably saturated media. In our model, we approximated the subsurface by horizontally stratified porous media. The model output was a time varying water content profile. Then, we compared the water content provided by the model with the water content measurements carried out using the time-lapse Magnetic Resonance Sounding (MRS) method. Each MRS sounding provided a water content profile in the unsaturated zone down to twenty meters. The time shift between the profiles corresponded to the time lapse between individual MRS soundings. We minimized the discrepancy between the observed and the modeled MRS signals by varying hydraulic parameters of soil layers in the water flow model. For measuring and processing MRS data, we used NUMIS MRS instrument and SAMOVAR software. We carried out water flow modeling with HYDRUS-1D software. This paper reports our results and summarizes the limitations of the MRS method applied to water content measurements in the unsaturated zone. (C) 2020 Elsevier B.V. All rights reserved.
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Legchenko, A., Texier, B., Girard, J., Vouillamoz, J., Lawson, F., Alle, I., et al. (2020). Feasibility study of a surface-borehole NMR method. Journal Of Applied Geophysics, 177.
Abstract: We present results of a feasibility study of a borehole induction-coil sensor for surface-borehole NMR (SBNMR) investigations. This sensor of 7 cm diameter and 180 cm length is connected to a standard MRS (Magnetic Resonance Sounding) instrument. Thus, SBNMR is a cost-effective extension of the MRS method. Using a downhole sensor increases the depth of investigation and the resolution of MRS. In the near-horizontal Earth's magnetic field, the sensitive area of the sensor is represented by a cylinder of a few meters in diameter. A blind zone of 0.5 to 1 m around the borehole is due to the disturbance of the Earth's magnetic field by the magnetic core of the sensor. The relatively large volume investigated with SBNMR and the blind zone around borehole may represent an advantage of SBNMR over the NMR(nuclear magnetic resonance) borehole tool investigating a narrow zone around the borehole. However, using the Earth's magnetic field renders the SBNMR performance site dependent with an inherently low signal-to-noise ratio. Our first results show a good correspondence between SBNMR, MRS and borehole data. (C) 2020 Elsevier B.V. All rights reserved.
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Lesparre, N., Girard, J., Jeannot, B., Weill, S., Dumont, M., Boucher, M., et al. (2020). Magnetic resonance sounding dataset of a hard-rock headwater catchment for assessing the vertical distribution of water contents in the subsurface. Data In Brief, 31.
Abstract: Magnetic Resonance Sounding (MRS) measurements are acquired at 16 stations in the Strengbach headwater catchment (Vosges Mountains – France). These data, rendering the vertical distribution of water contents in the subsurface, are used to show their potential in conditioning a hydrological model of the catchment, as described in the article “Magnetic resonance sounding measurements as posterior information to condition hydrological model parameters: Application to a hard-rock headwater catchment” – Journal of Hydrology (2020). Acquisition protocols follow a free induction decay scheme. Data are filtered by applying a band-pass filter at the Larmor frequency. A filter removing the 50 Hz noise is also applied with the exception of data at a Larmor frequency close to the 50 Hz harmonic. The signal envelopes are then fitted by a decaying exponential function over time to estimate the median characteristic relaxation time of each MRS sounding. (c) 2020 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY license. (http://creativecommons.org/licenses/by/4.0/)
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Lesparre, N., Girard, J., Jeannot, B., Weill, S., Dumont, M., Boucher, M., et al. (2020). Magnetic resonance sounding measurements as posterior information to condition hydrological model parameters: Application to a hard-rock headwater catchment. Journal Of Hydrology, 587.
Abstract: In headwater catchment, the calibration of hydrological models is complex due to the scarcity of data in mountainous areas. Here, an innovative methodology is developed to condition hydrological model parameters by using magnetic resonance sounding (MRS) measurements in combination with stream flow rate data. MRS has the specificity in the various geophysical imaging techniques of being mainly sensitive to the vertical distribution of water content among the subsurface. In a way very similar to hydraulic head observations, these local distributions of water content may serve as information in a hydrological model to pattern subsurface flow by seeking model parameters. Simulations are run with different sets of parameters of a hydrological model. Each simulation provides as an output a 4-D map (3-D spatial plus time) of the vertical water content distributions over the whole catchment and their fluctuations over time. This output is then used to simulate the MRS signal that would be produced by the estimated water content. The simulated MRS signal is compared to measured MRS data to determine which hydrological simulations (which model parameters) are close to observations. The approach is applied on a hard-rock headwater catchment housing a very shallow and thin aquifer where an MRS survey covers the whole studied site. Hydraulic parameters of an integrated hydrological model of the catchment are spatially distributed by zones with uniform values, the prior delineation of the zones being guided by pedological studies. As MRS measurements supply local but spatially distributed information, the method conditions the various zones on their parameter values in a much better way than the classical (in headwater catchments) measure of the stream flow rate at the outlet of the system. Finally, hydrological simulation and time-dependent MRS forward calculations can help identifying possible locations for MRS stations to monitor the transient behavior of the hydrological state of the catchment.
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Massari, C., Brocca, L., Pellarin, T., Abramowitz, G., Filippucci, P., Ciabatta, L., et al. (2020). A daily 25km short-latency rainfall product for data-scarce regions based on the integration of the Global Precipitation Measurement mission rainfall and multiple-satellite soil moisture products. Hydrology And Earth System Sciences, 24(5), 2687–2710.
Abstract: Rain gauges are unevenly spaced around the world with extremely low gauge density over developing countries. For instance, in some regions in Africa the gauge density is often less than one station per 10 000 km(2). The availability of rainfall data provided by gauges is also not always guaranteed in near real time or with a timeliness suited for agricultural and water resource management applications, as gauges are also subject to malfunctions and regulations imposed by national authorities. A potential alternative is satellite-based rainfall estimates, yet comparisons with in situ data suggest they are often not optimal. In this study, we developed a short-latency (i.e. 2-3 d) rainfall product derived from the combination of the Integrated Multi-Satellite Retrievals for GPM (Global Precipitation Measurement) Early Run (IMERG-ER) with multiplesatellite soil-moisture-based rainfall products derived from ASCAT (Advanced Scatterometer), SMOS (Soil Moisture and Ocean Salinity) and SMAP (Soil Moisture Active and Passive) L3 (Level 3) satellite soil moisture (SM) retrievals. We tested the performance of this product over four regions characterized by high-quality ground-based rainfall datasets (India, the conterminous United States, Australia and Europe) and over data-scarce regions in Africa and South America by using triple-collocation (TC) analysis. We found that the integration of satellite SM observations with in situ rainfall observations is very beneficial with improvements of IMERG-ER up to 20% and 40% in terms of correlation and error, respectively, and a generalized enhancement in terms of categorical scores with the integrated product often outperforming reanalysis and ground-based long-latency datasets. We also found a relevant overestimation of the rainfall variability of GPM-based products (up to twice the reference value), which was significantly reduced after the integration with satellite soil-moisture-based rainfall estimates. Given the importance of a reliable and readily available rainfall product for water resource management and agricultural applications over data-scarce regions, the developed product can provide a valuable and unique source of rainfall information for these regions.
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Moreaux, V., Longdoz, B., Berveiller, D., Delpierre, N., Dufrene, E., Bonnefond, J., et al. (2020). Environmental control of land-atmosphere CO2 fluxes from temperate ecosystems: a statistical approach based on homogenized time series from five land-use types. Tellus Series B-Chemical And Physical Meteorology, 72(1), 1–25.
Abstract: We assembled homogenized long-term time series, up to 19 years, of measurements of net ecosystem exchange of CO2 (NEE) and its partitioning between gross primary production (GPP) and respiration (R-eco) for five different ecosystems representing the main plant functional types (PFTs) in France. Part of these data was analyzed to determine the influence of the main environmental variables on carbon fluxes between temperate ecosystems and the atmosphere, and to investigate the temporal patterns of their variations. A multi-temporal statistical analysis of the time series was conducted using random forest (RF) and wavelet coherence approaches. The RF analysis showed that, in all ecosystems, the incident solar radiation was highly correlated with GPP and that GPP was better correlated with the temporal variations of NEE than R-eco. The air temperature was the second most important driver in ecosystems with seasonal foliage, i.e., deciduous forest, cropland and grassland; whereas variables related to air or soil drought were prominent in evergreen forest sites. The environmental control on CO2 fluxes was tighter at high frequency suggesting an increased resilience to environmental variations at longer time spans. The spectral analysis performed on three of the five sites selected revealed contrasting temporal patterns of the cross-coherence between CO2 fluxes and climate variables among ecosystems; these were related to the respective PFT, management and soil conditions. In all PFTs, the power spectrum of GPP was well correlated with NEE and clearly different from R-eco. The spectral correlation analysis showed that the canopy phenology and disturbance regime condition the spectral correlation patterns of GPP and R-eco with the soil moisture and atmospheric vapour deficit.
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Moreaux, V., Martel, S., Bosc, A., Picart, D., Achat, D., Moisy, C., et al. (2020). Energy, water and carbon exchanges in managed forest ecosystems: description, sensitivity analysis and evaluation of the INRAE GO plus model, version 3.0. Geoscientific Model Development, 13(12), 5973–6009.
Abstract: The mechanistic model GO+ describes the functioning and growth of managed forests based upon biophysical and biogeochemical processes. The biophysical and biogeochemical processes included are modelled using standard formulations of radiative transfer, convective heat exchange, evapotranspiration, photosynthesis, respiration, plant phenology, growth and mortality, biomass nutrient content, and soil carbon dynamics. The forest ecosystem is modelled as three layers, namely the tree overstorey, understorey and soil. The vegetation layers include stems, branches and foliage and are partitioned dynamically between sunlit and shaded fractions. The soil carbon submodel is an adaption of the Roth-C model to simulate the impact of forest operations. The model runs at an hourly time step. It represents a forest stand covering typically 1 ha and can be straightforwardly upscaled across gridded data at regional, country or continental levels. GO+ accounts for both the immediate and long-term impacts of forest operations on energy, water and carbon exchanges within the soil-vegetation-atmosphere continuum. It includes exhaustive and versatile descriptions of management operations (soil preparation, regeneration, vegetation control, selective thinning, clear-cutting, coppicing, etc.), thus permitting the effects of a wide variety of forest management strategies to be estimated: from close to nature to intensive. This paper examines the sensitivity of the model to its main parameters and estimates how errors in parameter values are propagated into the predicted values of its main output variables.The sensitivity analysis demonstrates an interaction between the sensitivity of variables, with the climate and soil hydraulic properties being dominant under dry conditions but the leaf biochemical properties being most influential with wet soil. The sensitivity profile of the model changes from short to long timescales due to the cumulative effects of the fluxes of carbon, energy and water on the stand growth and canopy structure. Apart from a few specific cases, the model simulations are close to the values of the observations of atmospheric exchanges, tree growth, and soil carbon and water stock changes monitored over Douglas fir, European beech and pine forests of different ages. We also illustrate the capacity of the GO+ model to simulate the provision of key ecosystem services, such as the long-term storage of carbon in biomass and soil under various management and climate scenarios.
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Nguyen, T., N?Mery, J., Gratiot, N., Garnier, J., Strady, E., Nguyen, D., et al. (2020). Nutrient budgets in the Saigon-Dongnai River basin: Past to future inputs from the developing Ho Chi Minh megacity (Vietnam). River Research And Applications, 36(6), 974–990.
Abstract: Ho Chi Minh City (HCMC, Vietnam) is one of the fastest growing megacities in the world. In this paper, we attempt to analyse the dynamics of nutrients, suspended sediments, and water discharges in its aquatic systems today and in the future. The work is based on nine sampling sites along the Saigon River and one on the Dongnai River to identify the reference water status upstream from the urban area and the increase in fluxes that occur within the city and its surroundings. For the first time, the calculated fluxes allow drawing up sediment and nutrient budgets at the basin scale and the quantification of total nutrient loading to the estuarine and coastal zones (2012-2016 period). Based on both national Vietnamese and supplementary monitoring programs, we estimated the water, total suspended sediment, and nutrients (Total N, Total P, and dissolved silica: DSi) fluxes at 137 m(3)year(-1), 3,292 x 10(3)tonSS year(-1), 5,323 tonN year(-1), 450 tonP year(-1), and 2,734 tonSi year(-1)for the Saigon River and 1,693 m(3)year(-1), 1,175 x 10(3)tonSS year(-1), 31,030 tonN year(-1), 1,653 tonP year(-1), and 31,138 tonSi year(-1)for the Dongnai River, respectively. Nutrient fluxes provide an indicator of coastal eutrophication potential (indicator of coastal eutrophication potential), using nutrient stoichiometry ratios. Despite an excess of nitrogen and phosphorus over silica, estuarine waters downstream of the megacity are not heavily impacted by HCMC. Finally, we analysed scenarios of future trends (2025-2050) for the nutrient inputs on the basis of expected population growth in HCMC and improvement of wastewater treatment capacity. We observed that without the construction of a large number of additional wastewater treatment plants, the eutrophication problem is likely to worsen. The results are discussed in the context of the wastewater management policy.
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Noncent, D., Strady, E., Nemery, J., Thanh-Nho, N., Denis, H., Mourier, B., et al. (2020). Sedimentological and geochemical data in bed sediments from a tropical river-estuary system impacted by a developing megacity, Ho Chi Minh City-Vietnam. Data In Brief, 31.
Abstract: Sedimentological and geochemical data were obtained for bed sediments from a tropical estuary environment in Vietnam in October 2014, January 2016, and November 2016. The data include grain-size distribution, percentage of clay, silt and sand, percentage of organic matter, concentration of total particulate phosphorus (TPP), concentration of particulate inorganic phosphorus (PIP), concentration of particulate organic phosphorus (POP), percentage of total nitrogen (TN), percentage of total carbon (TC), trace metals concentrations (V, Cr, Co, Ni, Cu, Zn, As, Mo, Cd, Pb) and major elements (Al, Fe, Mn). Geochemical indexes (Enrichment factor EF and Geo-accumulation Index I-geo) and sediment quality guideline (mean Effect Range Median quotients) were calculated. (C) 2020 The Author(s). Published by Elsevier Inc.
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Pastorello, G., Trotta, C., Canfora, E., Chu, H., Christianson, D., Cheah, Y., et al. (2020). The FLUXNET2015 dataset and the ONEFlux processing pipeline for eddy covariance data. Scientific Data, 7(1).
Abstract: The FLUXNET2015 dataset provides ecosystem-scale data on CO2, water, and energy exchange between the biosphere and the atmosphere, and other meteorological and biological measurements, from 212 sites around the globe (over 1500 site-years, up to and including year 2014). These sites, independently managed and operated, voluntarily contributed their data to create global datasets. Data were quality controlled and processed using uniform methods, to improve consistency and intercomparability across sites. The dataset is already being used in a number of applications, including ecophysiology studies, remote sensing studies, and development of ecosystem and Earth system models. FLUXNET2015 includes derived-data products, such as gap-filled time series, ecosystem respiration and photosynthetic uptake estimates, estimation of uncertainties, and metadata about the measurements, presented for the first time in this paper. In addition, 206 of these sites are for the first time distributed under a Creative Commons (CC-BY 4.0) license. This paper details this enhanced dataset and the processing methods, now made available as open-source codes, making the dataset more accessible, transparent, and reproducible.
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Pellarin, T., Roman-Cascon, C., Baron, C., Bindlish, R., Brocca, L., Camberlin, P., et al. (2020). The Precipitation Inferred from Soil Moisture (PrISM) Near Real-Time Rainfall Product: Evaluation and Comparison. Remote Sensing, 12(3).
Abstract: Near real-time precipitation is essential to many applications. In Africa, the lack of dense rain-gauge networks and ground weather radars makes the use of satellite precipitation products unavoidable. Despite major progresses in estimating precipitation rate from remote sensing measurements over the past decades, satellite precipitation products still suffer from quantitative uncertainties and biases compared to ground data. Consequently, almost all precipitation products are provided in two modes: a real-time mode (also called early-run or raw product) and a corrected mode (also called final-run, adjusted or post-processed product) in which ground precipitation measurements are integrated in algorithms to correct for bias, generally at a monthly timescale. This paper describes a new methodology to provide a near-real-time precipitation product based on satellite precipitation and soil moisture measurements. Recent studies have shown that soil moisture intrinsically contains information on past precipitation and can be used to correct precipitation uncertainties. The PrISM (Precipitation inferred from Soil Moisture) methodology is presented and its performance is assessed for five in situ rainfall measurement networks located in Africa in semi-arid to wet areas: Niger, Benin, Burkina Faso, Central Africa, and East Africa. Results show that the use of SMOS (Soil Moisture and Ocean Salinity) satellite soil moisture measurements in the PrISM algorithm most often improves the real-time satellite precipitation products, and provides results comparable to existing adjusted products, such as TRMM (Tropical Rainfall Measuring Mission), GPCC (Global Precipitation Climatology Centre) and IMERG (Integrated Multi-satellitE Retrievals for GPM), which are available a few weeks or months after their detection.
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Roman-Cascon, C., Lothon, M., Lohou, F., Ojha, N., Merlin, O., Aragones, D., et al. (2020). Can We Use Satellite-Based Soil-Moisture Products at High Resolution to Investigate Land-Use Differences and Land-Atmosphere Interactions? A Case Study in the Savanna. Remote Sensing, 12(11).
Abstract: The use of soil moisture (SM) measurements from satellites has grown in recent years, fostering the development of new products at high resolution. This opens the possibility of using them for certain applications that were normally carried out using in situ data. We investigated this hypothesis through two main analyses using two high-resolution satellite-based soil moisture (SBSM) products that combined microwave with thermal and optical data: (1) The Disaggregation based on Physical And Theoretical scale Change (DISPATCH) and, (2) The Soil Moisture Ocean Salinity-Barcelona Expert Center (SMOS-BEC Level 4). We used these products to analyse the SM differences among pixels with contrasting vegetation. This was done through the comparison of the SM measurements from satellites and the measurements simulated with a simple antecedent precipitation index (API) model, which did not account for the surface characteristics. Subsequently, the deviation of the SM from satellite with respect to the API model (bias) was analysed and compared for contrasting land use categories. We hypothesised that the differences in the biases of the varied categories could provide information regarding the water retention capacity associated with each type of vegetation. From the satellite measurements, we determined how the SM depended on the tree cover, i.e., the denser the tree cover, the higher the SM. However, in winter periods with light rain events, the tree canopy could dampen the moistening of the soil through interception and conducted higher SM in the open areas. This evolution of the SM differences that depended on the characteristics of each season was observed both from satellite and from in situ measurements taken beneath a tree and in grass on the savanna landscape. The agreement between both types of measurements highlighted the potential of the SBSM products to investigate the SM of each type of vegetation. We found that the results were clearer for DISPATCH, whose data was not smoothed spatially as it was in SMOS-BEC. We also tested whether the relationships between SM and evapotranspiration could be investigated using satellite data. The answer to this question was also positive but required removing the unrealistic high-frequency SM oscillations from the satellite data using a low pass filter. This improved the performance scores of the products and the agreement with the results from the in situ data. These results demonstrated the possibility of using SM data from satellites to substitute ground measurements for the study of land-atmosphere interactions, which encourages efforts to improve the quality and resolution of these measurements.
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Segura, H., Espinoza, J., Junquas, C., Lebel, T., Vuille, M., & Garreaud, R. (2020). Recent changes in the precipitation-driving processes over the southern tropical Andes/western Amazon. Climate Dynamics, .
Abstract: Analyzing December-February (DJF) precipitation in the southern tropical Andes-STA (12 circle S}; > 3000 m.a.s.l) allows revisiting regional atmospheric circulation features accounting for its interannual variability over the past 35 years (1982-2018). In a region where in-situ rainfall stations are sparse, the CHIRPS precipitation product is used to identify the first mode of interannual DJF precipitation variability (PC1-Andes). A network of 98 rain-gauge stations further allows verifying that PC1-Andes properly represents the spatio-temporal rainfall distribution over the region; in particular a significant increase in DJF precipitation over the period of study is evident in both in-situ data and PC1-Andes. Using the ERA-Interim data set, we found that aside from the well-known relationship between precipitation and upper-level easterlies over the STA, PC1-Andes is also associated with upward motion over the western Amazon (WA), a link that has not been reported before. The ascent over the WA is a component of the meridional circulation between the tropical North Atlantic and western tropical South America-WTSA (80 circle W). Indeed, the precipitation increase over the last 2 decades is concomitant with the strengthening of this meridional circulation. An intensified upward motion over the WA has moistened the mid-troposphere over WTSA, and as a consequence, a decreased atmospheric stability between the mid- and the upper troposphere is observed over this region, including the STA. We further show that, over the last 15 years or so, the year-to-year variability of STA precipitation (periodicity < 8 years) has been significantly associated with upward motion over the WA, while upper-level easterlies are no longer significantly correlated with precipitation. These observations suggests that the STA have experienced a transition from a dry to a wet state in association with a change in the dominant mode of atmospheric circulation. In the former dominant state, zonal advection of momentum and moisture from the central Amazon, associated with upper-level easterlies, is necessary to develop convection over the STA. Since the beginning of the 21st century, DJF precipitation over the STA seems to respond directly and primarily to upward motion over the WA. Beyond improving our understanding of the factors influencing STA precipitation nowadays, these results point to the need of exploring their possible implications for the long-term evolution of precipitation in a context of global warming.
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Strady, E., Kieu-Le, T., Gasperi, J., & Tassin, B. (2020). Temporal dynamic of anthropogenic fibers in a tropical river-estuarine system. Environmental Pollution, 259.
Abstract: Anthropogenic fibers, gathering synthetic fibers, artificial fibers and natural fibers are ubiquitous in the natural environment. Tremendous concentrations of anthropogenic fibers were previously measured in the tropical Saigon River (Vietnam), i.e. a river impacted by textile and apparel industries. In the present study, we want to examine the role of contrasted seasonal variation (e.g., dry and rainy seasons), via the rainfall and monthly water discharges, and of water's physico-chemical conditions on the concentrations of anthropogenic fibers in the surface water. The one year and half monthly survey evidenced that concentrations of anthropogenic fibers varied from 22 to 251 items L-1 and their variations were not related to rainfall, water discharge or abiotic factors. However, their color and length distribution varied monthly suggesting variations in sources and sinks. Based on the 2017 survey, we estimated an annual emission of anthropogenic fibers from the river to the downstream coastal zone of 115-164 x 10(12) items yr(-1). (C) 2020 Elsevier Ltd. All rights reserved.
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Tran, H., Zhang, J., Cohard, J., Condon, L., & Maxwell, R. (2020). Simulating Groundwater-Streamflow Connections in the Upper Colorado River Basin. Groundwater, 58(3), 392–405.
Abstract: In mountain, snow driven catchments, snowmelt is supposed to be the primary contribution to river streamflows during spring. In these catchments the contribution of groundwater is not well documented because of the difficulty to monitor groundwater in such complex environment with deep aquifers. In this study we use an integrated hydrologic model to conduct numerical experiments that help quantify the effect of lateral groundwater flow on total annual and peak streamflow in predevelopment conditions. Our simulations focus on the Upper Colorado River Basin (UCRB; 2.8 x 10(5) km(2)) a well-documented mountain catchment for which both streamflow and water table measurements are available for several important sub-basins. For the simulated water year, our results suggest an increase in peak flow of up to 57% when lateral groundwater flow processes are included-an unexpected result for flood conditions generally assumed independent of groundwater. Additionally, inclusion of lateral groundwater flow moderately improved the model match to observations. The correlation coefficient for mean annual flows improved from 0.84 for the no lateral groundwater flow simulation to 0.98 for the lateral groundwater flow one. Spatially we see more pronounced differences between lateral and no lateral groundwater flow cases in areas of the domain with steeper topography. We also found distinct differences in the magnitude and spatial distribution of streamflow changes with and without lateral groundwater flow between Upper Colorado River Sub-basins. A sensitivity test that scaled hydraulic conductivity over two orders of magnitude was conducted for the lateral groundwater flow simulations. These results show that the impact of lateral groundwater flow is as large or larger than an order of magnitude change in hydraulic conductivity. While our results focus on the UCRB, we feel that these simulations have relevance to other headwaters systems worldwide.
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Tran, V., Nguyen, P., & Strady, E. (2020). Bioaccumulation of trace elements in the hard clam, Meretrix lyrata, reared downstream of a developing megacity, the Saigon-Dongnai River Estuary, Vietnam. Environmental Monitoring And Assessment, 192(9).
Abstract: A large number of white hard clam farms are in the estuary shoreline of Saigon-Dongnai Rivers, which flow through Ho Chi Minh City, a megacity, and numerous industrial zones in the basin catchment area. In this study, eleven trace elements (Mn, Fe, Co, Ni, Cu, Zn, As, Se, Cd, Hg, and Pb) in the hard clam Meretrix lyrata and its habitats including surface water, suspended particulate matter, and sediment were evaluated to understand the bioaccumulation of trace metals from the environment into the whole tissues of the hard clam as well as its different organs. The samples were collected monthly in dry, transition, and wet seasons of the southern part of Vietnam from March to September 2016. The results showed that seasonal and spatial variations of the studied metal concentrations in the hard clam M. lyrata might be influenced by the sea current as well as the surface runoff in the rainy season. The relationship between condition index and the element concentrations in M. lyrata might be affected by the living environment conditions and farming methods. In addition, the hazard index values of all trace elements in the hard clam M. lyrata harvested in the sampling time show that the hard clams farmed in the study area were safe for local consumers.
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2019 |
Al-Yaari, A., Wigneron, J., Dorigo, W., Colliander, A., Pellarin, T., Hahn, S., et al. (2019). Assessment and inter-comparison of recently developed/reprocessed microwave satellite soil moisture products using ISMN ground-based measurements. Remote Sensing Of Environment, 224, 289–303.
Abstract: Soil moisture (SM) is a key state variable in understanding the climate system through its control on the land surface energy, water budget partitioning, and the carbon cycle. Monitoring SM at regional scale has become possible thanks to microwave remote sensing. In the past two decades, several satellites were launched carrying on board either radiometer (passive) or radar (active) or both sensors in different frequency bands with various spatial and temporal resolutions. Soil moisture algorithms are in rapid development and their improvements/revisions are ongoing. The latest SM retrieval products and versions of products that have been recently released are not yet, to our knowledge, comprehensively evaluated and inter-compared over different ecoregions and climate conditions. The aim of this paper is to comprehensively evaluate the most recent microwave-based SM retrieval products available from NASA's (National Aeronautics and Space Administration) SMAP (Soil Moisture Active Passive) satellite, ESA's led mission (European Space Agency) SMOS (Soil Moisture and Ocean Salinity) satellite, ASCAT (Advanced Scatterometer) sensor on board the meteorological operational (Metop) platforms Metop-A and Metop-B, and the ESA Climate Change Initiative (CCI) blended long-term SM time series. More specifically, in this study we compared SMAPL3 V4, SMOSL3 V300, SMOSL2 V650, ASCAT H111, and CCI V04.2 and the new SMOS-IC (V105) SM product. This evaluation was achieved using four statistical scores: Pearson correlation (considering both original observations and anomalies), RMSE, unbiased RMSE, and Bias between remotely-sensed SM retrievals and ground-based measurements from > 1000 stations from 17 monitoring networks, spread over the globe, disseminated through the International Soil Moisture Network (ISMN). The analysis reveals that the performance of the remotely-sensed SM retrievals generally varies depending on ecoregions, land cover types, climate conditions, and between the monitoring networks. It also reveals that temporal sampling of the data, the frequency of data in time and the spatial coverage, affect the performance metrics. Overall, the performance of SMAP and SMOS-IC products compared slightly better with respect to the ISMN in situ observations than the other remotely-sensed products.
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Babaye, M., Orban, P., Ousmane, B., Favreau, G., Brouyere, S., & Dassargues, A. (2019). Characterization of recharge mechanisms in a Precambrian basement aquifer in semi-arid south-west Niger. Hydrogeology Journal, 27(2), 475–491.
Abstract: In the central part of the semi-arid Dargol Basin of southwestern Niger, most of the groundwater resource is contained in the fractured aquifers of the Precambrian basement. The groundwater resource is poorly characterized and this study is the first attempt to better describe the recharge mechanisms and hydrogeochemical behaviour of the aquifers. Hydrogeochemical and piezometric methods were combined to determine changes in recharge rate and origin of groundwaters for the shallow weathered aquifer and the deep fissured/fractured aquifer. At the basin scale, the groundwater fluxes towards the Niger River are influenced mainly by topography, with no visual long-term trend in groundwater levels (1980-2009). The hydro-geochemical signature is dominated by the calcic-bicarbonate to magnesian (70%) type. It shows evolution from an open environment with CO2 and low mineralized water (granitoids, alterites) towards a more confined environment with more mineralized waters (schists). Stable water isotopes (O-18, H-2) analysis suggests two main groundwater recharge mechanisms: (1) direct recharge with nearly no post-rainfall fractionation signature and (2) indirect recharge from evaporated surface waters and/or stream-channel beds. Groundwater tritium content indicates that recharge is mostly recent, with an age less than 50years (H-3>3 TU), with only 10% indicating low or even no recharge for the past decades. A median value of the groundwater renewal rate estimated from individual values of tritium is equivalent to 1.3% year(-1), close to the one determined for groundwater samples dating to the early 1980s, thus indicating no measurable long-term change.
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Bamba, A., Diallo, I., Toure, N., Kouadio, K., Konare, A., Drame, M., et al. (2019). Effect of the African greenbelt position on West African summer climate: a regional climate modeling study. Theoretical And Applied Climatology, 137(1-2), 309–322.
Abstract: This modeling study is conducted to examine the potential impact of the reforestation (greenbelt) location (either in Sahel or in Guinean region) on West Africansummer climate system. To this end, three simulations using the regional climate model RegCM4 driven by ERA-Interim reanalysis were performed at 50km horizontal resolution over a West African domain for the period 2000-2011. The first experiment, namely the control (CTRL), uses the standard vegetation cover, while the two others incorporate throughout the model integration, a zonal reforestation band of evergreen broadleaf over different locations: (i) over a 13 degrees N-17 degrees N band latitudes in a Sahel-Sahara region (experiment hereafter referred to as GB15N) and (ii) between 8.5 degrees N-11.5 degrees N in the Guinea Coast region (experiment hereafter referred to as GB10N). A comparison of the CTRL experiment with observation reveals a faithful reproduction of the mean boreal and summer seasonal precipitation pattern, though substantial dry/wet biases remain, especially in the Atlantic Ocean. In addition, the seasonal cycle over sub-regions matches satisfactory the observed pattern. The GB15N reforestation leads to a precipitation increase in the range of 2-4mm/day over the forested areas, whereas in the GB10N reforestation, precipitation increase is weaker and not necessarily located in the forested areas. Temperature cooling is observed over the reforested area and may be explained by a decrease of ground heat flux related to a reduction of the surface albedo.
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Berthou, S., Rowell, D., Kendon, E., Roberts, M., Stratton, R., Crook, J., et al. (2019). Improved climatological precipitation characteristics over West Africa at convection-permitting scales. Climate Dynamics, 53(3-4), 1991–2011.
Abstract: The West African climate is unique and challenging to reproduce using standard resolution climate models as a large proportion of precipitation comes from organised deep convection. For the first time, a regional 4.5km convection permitting simulation was performed on a pan-African domain for a period of 10years (1997-2006). The 4.5km simulation (CP4A) is compared with a 25x40km convection-parameterised model (R25) over West Africa. CP4A shows increased mean precipitation, which results in improvements in the mature phase of the West African monsoon but deterioration in the early and late phases. The distribution of precipitation rates is improved due to more short lasting intense rainfall events linked with mesoscale convective systems. Consequently, the CP4A model shows a better representation of wet and dry spells both at the daily and sub-daily time-scales. The diurnal cycle of rainfall is improved, which impacts the diurnal cycle of monsoon winds and increases moisture convergence in the Sahel. Although shortcomings were identified, with implications for model development, this convection-permitting model provides a much more reliable precipitation distribution than its convection-parameterised counterpart at both daily and sub-daily time-scales. Convection-permitting scales will therefore be useful to address the crucial question of how the precipitation distribution will change in the future.
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Camberlin, P., Barraud, G., Bigot, S., Dewitte, O., Imwangana, F., Mateso, J., et al. (2019). Evaluation of remotely sensed rainfall products over Central Africa. Quarterly Journal Of The Royal Meteorological Society, 145(722), 2115–2138.
Abstract: An intercomparison of seven gridded rainfall products incorporating satellite data (ARC, CHIRPS, CMORPH, PERSIANN, TAPEER, TARCAT, TMPA) is carried out over Central Africa, by evaluating them against three observed datasets: (a) the WaTFor database, consisting of 293 (monthly records) and 154 (daily records) rain-gauge stations collected from global datasets, national meteorological services and monitoring projects, (b) the WorldClim v2 gridded database, and (c) a set of stations expanded from the FAOCLIM network, these two latter sets describing climate normals. All products fairly well reproduce the mean rainfall regimes and the spatial patterns of mean annual rainfall, although with some discrepancies in the east-west gradient. A systematic positive bias is found in the CMORPH product. Despite its lower spatial resolution, TAPEER shows reasonable skills. When considering daily rainfall amounts, TMPA shows best skills, followed by CMORPH, but over the central part of the Democratic Republic of the Congo, TARCAT is amongst the best products. Skills ranking is however different at the interannual time-scale, with CHIRPS and TMPA performing best, though PERSIANN has comparable skills when only fully independent stations are used as reference. A preliminary study of Southern Hemisphere dry season variability, from the example of Kinshasa, shows that it is a difficult variable to capture with satellite-based rainfall products. Users should still be careful when using any product in the most data-sparse regions, especially for trend assessment.
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Cuthbert, M., Taylor, R., Favreau, G., Todd, M., Shamsudduha, M., Villholth, K., et al. (2019). Observed controls on resilience of groundwater to climate variability in sub-Saharan Africa (vol 572, pg 230, 2019). Nature, .
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Dupire, S., Curt, T., Bigot, S., & Frejaville, T. (2019). Vulnerability of forest ecosystems to fire in the French Alps. European Journal Of Forest Research, 138(5), 813–830.
Abstract: Forest fires are expected to be more frequent and more intense with climate change, including in temperate and mountain forest ecosystems. In the Alps, forest vulnerability to fire resulting from interactions between climate, fuel types, vegetation structure and tree resistance to fire is little understood. This paper aims at identifying trends in the vulnerability of Alpine forest ecosystems to fire at different scales (tree species, stand level and biogeographic level) and according to three different climatic conditions (cold season, average summer and extremely dry summer). To explore Alpine forest vulnerability to fire, we used surface fuel measurements, forest inventory and fire weather data to simulate fire behaviour and ultimately post-fire tree mortality across 4438 forest plots in the French Alps. The results showed that cold season fires (about 50% of the fires in the French Alps) have a limited impact except on low-elevation forests of the Southern Alps (mainly Oak, Scots pine). In average summer conditions, mixed and broadleaved forests of low elevations suffer the highest mortality rates (up to 75% in coppices). Finally, summer fires occurring in extremely dry conditions promote high mortality across all forest communities. Lowest mortality rates were observed in high forest stands composed of tree species presenting adaptation to surface fires (e.g. thick bark, high canopy) such as Larch forests of the internal Alps. This study provides insights on the vulnerability of the main tree species and forest ecosystems of the French Alps useful for the adaptation of forest management practices to climate changes.
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Duwig, C., Prado, B., Tinet, A., Delmas, P., Dal Ferro, N., Vandervaere, J., et al. (2019). Impacts of land use on hydrodynamic properties and pore architecture of volcanic soils from the Mexican Highlands. Soil Research, 57(6), 629–641.
Abstract: Volcanic soils are important resources because of their unique mineralogical and physical characteristics, and allophanic Andosols represent some of the world's most fertile soils. However, their unique properties can be lost when cultivated. Most soils in the Central Valley, Mexico, are derived from volcanic materials. This valley encompasses one of the largest water supply systems in the world by volume, but is affected by soil degradation and deforestation. Sustainably managing volcanic soils requires understanding how land use affects their hydrodynamic properties. Gas adsorption and mercury intrusion porosimetry, water retention curves, tension infiltrometry and X-ray tomography were used to describe pore structure characteristics. Two volcanic soils (one Andosol and one derived from indurated tuff – Tepetates), three land uses (maize monoculture, maize-wheat rotation and fallow) and two horizons (Ap and A2 for maize monoculture and maize-wheat rotation) were studied. Tillage affected topsoil by increasing the sand fraction by 38% and decreasing total porosity and macroporosity by 23% and 40% respectively. Macropore size was reduced and the number of isolated macropores was higher in the tilled layer under maize, compared with untilled subsoil. The plot under maize-wheat rotation had lower allophane content, and saturated hydraulic conductivity was reduced by nearly an order of magnitude and water retention by half, compared with maize and fallow plots. Compared with Andosols, Tepetates showed differences in mineralogical composition with lower contents of amorphous compounds and in its porous network characteristics with twice the total and percolating macroporosity compared with the maize plot. Its high content of organic carbon (3.5%) seemed beneficial for its hydrodynamic properties. Sustainable agricultural management of these volcanic soils requires reducing mechanised tillage, avoiding periods when soil is bare, not applying maize-wheat rotation and applying maize-fallow rotation allowing natural vegetation growth.
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Kotchoni, D., Vouillamoz, J., Lawson, F., Adjomayi, P., Boukari, M., & Taylor, R. (2019). Relationships between rainfall and groundwater recharge in seasonally humid Benin: a comparative analysis of long-term hydrographs in sedimentary and crystalline aquifers. Hydrogeology Journal, 27(2), 447–457.
Abstract: Groundwater is a vital source of freshwater throughout the tropics enabling access to safe water for domestic, agricultural and industrial purposes close to the point of demand. The sustainability of groundwater withdrawals is controlled, in part, by groundwater recharge, yet the conversion of rainfall into recharge remains inadequately understood, particularly in the tropics. This study examines a rare set of 19-25-year records of observed groundwater levels and rainfall under humid conditions (mean rainfall is similar to 1,200mmyear(-1)) in three common geological environments of Benin and other parts of West Africa: Quaternary sands, Mio-Pliocene sandstone, and crystalline rocks. Recharge is estimated from groundwater-level fluctuations and employs values of specific yield derived from magnetic resonance soundings. Recharge is observed to occur seasonally and linearly in response to rainfall exceeding an apparent threshold of between 140 and 250mmyear(-1). Inter-annual changes in groundwater storage correlate well to inter-annual rainfall variability. However, recharge varies substantially depending upon the geological environment: annual recharge to shallow aquifers of Quaternary sands amounts to as much as 40% of annual rainfall, whereas in deeper aquifers of Mio-Pliocene sandstone and weathered crystalline rocks, annual fractions of rainfall generating recharge are 13 and 4%, respectively. Differences are primarily attributed to the thickness of the unsaturated zone and to the lithological controls on the transmission and storage of rain-fed recharge.
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Massazza, G., Tamagnone, P., Wilcox, C., Belcore, E., Pezzoli, A., Vischel, T., et al. (2019). Flood Hazard Scenarios of the Sirba River (Niger): Evaluation of the Hazard Thresholds and Flooding Areas. Water, 11(5).
Abstract: In Sahelian countries, a vast number of people are still affected every year by flood despite the efforts to prevent or mitigate these catastrophic events. This phenomenon is exacerbated by the incessant population growth and the increase of extreme natural events. Hence, the development of flood management strategies such as flood hazard mapping and Early Warning Systems has become a crucial objective for the affected nations. This study presents a comprehensive hazard assessment of the Nigerien reach of the Sirba River, the main tributary Middle Niger River. Hazard thresholds were defined both on hydrological analysis and field effects, according to national guidelines. Non-stationary analyses were carried out to consider changes in the hydrological behavior of the Sirba basin over time. Data from topographical land surveys and discharge gauges collected during the 2018 dry and wet seasons were used to implement the hydraulic numerical model of the analyzed reach. The use of the proposed hydraulic model allowed the delineation of flood hazard maps as well the calculation of the flood propagation time from the upstream hydrometric station and the validation of the rating curves of the two gauging sites. These significative outcomes will allow the implementation of the Early Warning System for the river flood hazard and risk reduction plans preparation for each settlement.
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Mimeau, L., Esteves, M., Zin, I., Jacobi, H., Brun, F., Wagnon, P., et al. (2019). Quantification of different flow components in a high-altitude glacierized catchment (Dudh Koshi, Himalaya): some cryospheric-related issues. Hydrology And Earth System Sciences, 23(9), 3969–3996.
Abstract: In a context of climate change and water demand growth, understanding the origin of water flows in the Himalayas is a key issue for assessing the current and future water resource availability and planning the future uses of water in downstream regions. Two of the main issues in the hydrology of high-altitude glacierized catchments are (i) the limited representation of cryospheric processes controlling the evolution of ice and snow in distributed hydrological models and (ii) the difficulty in defining and quantifying the hydrological contributions to the river outflow. This study estimates the relative contribution of rainfall, glaciers, and snowmelt to the Khumbu River streamflow (Upper Dudh Koshi, Nepal, 146 km(2), 43% glacierized, elevation range from 4260 to 8848 ma.s.l.) as well as the seasonal, daily, and sub-daily variability during the period 2012-2015 by using the DHSVM-GDM (Distributed Hydrological Soil Vegetation Model – Glaciers Dynamics Model) physically based glacio-hydrological model. The impact of different snow and glacier parameterizations was tested by modifying the snow albedo parameterization, adding an avalanche module, adding a reduction factor for the melt of debris-covered glaciers, and adding a conceptual englacial storage. The representation of snow, glacier, and hydrological processes was evaluated using three types of data (MODIS satellite images, glacier mass balances, and in situ discharge measurements). The relative flow components were estimated using two different definitions based on the water inputs and contributing areas. The simulated hydrological contributions differ not only depending on the used models and implemented processes, but also on different definitions of the estimated flow components. In the presented case study, ice melt and snowmelt contribute each more than 40% to the annual water inputs and 69% of the annual stream flow originates from glacierized areas. The analysis of the seasonal contributions highlights that ice melt and snowmelt as well as rain contribute to monsoon flows in similar proportions and that winter outflow is mainly controlled by the release from the englacial water storage. The choice of a given parametrization for snow and glacier processes, as well as their relative parameter values, has a significant impact on the simulated water balance: for instance, the different tested parameterizations led to ice melt contributions ranging from 42% to 54 %. The sensitivity of the model to the glacier inventory was also tested, demonstrating that the uncertainty related to the glacierized surface leads to an uncertainty of 20% for the simulated ice melt component.
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Mougin, E., Diawara, M., Soumaguel, N., Maiga, A., Demarez, V., Hiernaux, P., et al. (2019). A leaf area index data set acquired in Sahelian rangelands of Gourma in Mali over the 2005-2017 period. Earth System Science Data, 11(2), 675–686.
Abstract: The leaf area index of Sahelian rangelands and related variables such as the vegetation cover fraction, the fraction of absorbed photosynthetically active radiation and the clumping index were measured between 2005 and 2017 in the Gourma region of northern Mali. These variables, known as climate essential variables, were derived from the acquisition and the processing of hemispherical photographs taken along 1 km linear sampling transects for five contrasted canopies and one millet field. The same sampling protocol was applied in a seasonally inundated Acacia open forest, along a 0.5 km transect, by taking photographs of the understorey and the tree canopy. These observations collected over more than a decade, in a remote and not very accessible region, provide a relevant and unique data set that can be used for a better understanding of the Sahelian vegetation response to the current rainfall changes. The collected data can also be used for satellite product evaluation and land surface model development and validation. This paper aims to present the field work that was carried out during 13 successive rainy seasons, the measured vegetation variables, and the associated open database. Finally, a few examples of data use are shown. DOI of the referenced data set: https://doi.org/10.17178/AMMA-CATCH.CE.Veg_Gh.
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Ndarwe, D., Bongue, D., Monkam, D., Moudi, P., Philippon, N., & Kenfack, C. (2019). Analysis of the diurnal to seasonal variability of solar radiation in Douala, Cameroon. Theoretical And Applied Climatology, 138(1-2), 249–261.
Abstract: One of the solutions for resolving the problem of energy production deficit in Central Africa is to promote renewable energy sources. The knowledge of the solar variability represents a determining factor for design, dimensioning, performance assessment, and energetic management of renewable energy conversion systems. In this work, we analyze the behaviour of solar radiation from diurnal to seasonal time scales for the region of Douala, the largest industrial city of Cameroon. Observed data of temperature, sunshine duration and precipitation, and satellite estimates of solar radiation (from Soda Solar Project) and cloudiness (acquired from Meteosat Second Generation) were used. The results show that the solar radiation annual cycle at Douala can be decomposed into four seasons: the main dry season in December-January-February (DJF) which corresponds to the most illuminated season, the main rainy season in June-July-August-September (JJAS) which is the least illuminated, and two intermediate periods, March-April-May (MAM) and October-November (ON) which correspond to semi-illuminated periods. Using a hierarchical clustering analysis (HCA), we found that Douala usually experiences five main types of solar radiation diurnal cycles depicting very bright to obscure days. A characterization of sky conditions during these five diurnal cycles shows a predominance of low and high opaque clouds during obscure days.
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Nkrumah, F., Vischel, T., Panthou, G., Klutse, N., Adukpo, D., & Diedhiou, A. (2019). Recent Trends in the Daily Rainfall Regime in Southern West Africa. Atmosphere, 10(12).
Abstract: Extreme climate events, either being linked to dry spells or extreme precipitation, are of major concern in Africa, a region in which the economy and population are highly vulnerable to climate hazards. However, recent trends in climate events are not often documented in this poorly surveyed continent. This study makes use of a large set of daily rain gauge data covering Southern West Africa (extending from 10 degrees W to 10 degrees E and from 4 degrees N to 12 degrees N) from 1950 to 2014. The evolution of the number and the intensity of daily rainfall events, especially the most extremes, were analyzed at the annual and seasonal scales. During the first rainy season (April-July), mean annual rainfall is observed to have a minor trend due to less frequent but more intense rainfall mainly along the coast of Southern West Africa (SWA) over the last two decades. The north-south seasonal changes exhibit an increase in mean annual rainfall over the last decade during the second rainy season (September-November) linked by both an increase in the frequency of occurrence of rainy days as well as an increase in the mean intensity and extreme events over the last decade. The study also provides evidence of a disparity that exists between the west and east of SWA, with the east recording a stronger increase in the mean intensity of wet days and extreme rainfall during the second rainy season (September-November).
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Philippon, N., Cornu, G., Monteil, L., Gond, V., Moron, V., Pergaud, J., et al. (2019). The light-deficient climates of western Central African evergreen forests. Environmental Research Letters, 14(3).
Abstract: Rainfall thresholds under which forests grow in Central Africa are lower than those of Amazonia and southeast Asia. Attention is thus regularly paid to rainfall whose seasonality and interannual variability has been shown to control Central African forests' water balance and photosynthetic activity. Nonetheless, light availability is also recognized as a key factor to tropical forests. Therefore this study aims to explore the light conditions prevailing across Central Africa, and their potential impact on forests' traits. Using satellite estimates of hourly irradiance, we find first that the four main types of diurnal cycles of irradiance extracted translate into different levels of rainfall, evapotranspiration, direct and diffuse light. Then accounting for scale interactions between the diurnal and annual cycles, we show that the daily quantity and quality of light considerably vary across Central African forests during the annual cycle: the uniqueness of western Central Africa and Gabon in particular, with strongly light-deficient climates especially during the main dry season, points out. Lastly, using an original map of terra firme forests, we also show that most of the evergreen forests are located in western Central Africa and Gabon. We postulate that despite mean annual precipitation below 2000 mm yr(-1), the light-deficient climates of western Central Africa can harbour evergreen forests because of an extensive low-level cloudiness developing during the June-September main dry season, which strongly reduces the water demand and enhances the quality of light available for tree photosynthesis. These findings pave the way for further analyses of the past and future changes in the light-deficient climates of western Central Africa and the vulnerability of evergreen forests to these changes.
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Sanchez-Roman, A., Gomez-Navarro, L., Fablet, R., Oro, D., Mason, E., Arcos, J., et al. (2019). Rafting behaviour of seabirds as a proxy to describe surface ocean currents in the Balearic Sea. Scientific Reports, 9.
Abstract: Spatio-temporal variability of surface geostrophic mesoscale currents in the Balearic Sea (western Mediterranean) is characterized from satellite altimetry in combination with in-situ velocity measurements collected, among others, by drifting buoys, gliders and high-frequency radar. Here, we explore the use of tracking data from living organisms in the Balearic Sea as an alternative way to acquire in-situ velocity measurements. Specifically, we use GPS-tracks of resting Scopoli's shearwaters Calonectris diomedea, that act as passive drifters, and compare them with satellite-derived velocity patterns. Results suggest that animal-borne GPS data can be used to identify rafting behaviour outside of the breeding colonies and, furthermore, as a proxy to describe local sea surface currents. Four rafting patterns were identified according to the prevailing driving forces responsible for the observed trajectories. We find that 76% of the bird trajectories are associated with the combined effects of slippage and Ekman drift and/or surface drag; 59% are directly driven by the sea surface currents. Shearwaters are therefore likely to be passively transported by these driving forces while resting. The tracks are generally consistent with the mesoscale features observed in satellite data and identified with eddy-tracking software.
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Segura, H., Junquas, C., Espinoza, J., Vuille, M., Jauregui, Y., Rabatel, A., et al. (2019). New insights into the rainfall variability in the tropical Andes on seasonal and interannual time scales. Climate Dynamics, 53(1-2), 405–426.
Abstract: In this study, we analyze the atmospheric mechanisms associated with the main rainfall patterns in the tropical Andes (20 degrees S-1*DEG;N) on seasonal and interannual time scales. Using a homogeneous and high spatial resolution precipitation data set (0.05 degrees x0.05 degrees) at monthly time step (CHIRPS; 1981-2016), in-situ precipitation from 206 rain-gauge stations, power spectrum and EOF analysis, we identify three Andean regions characterized by specific seasonal and interannual rainfall modes: the equatorial Andes (EA, 5 degrees S-1*DEG;N), the transition zone (TZ, 8 degrees S-5*DEG;S) and the southern tropical Andes (STA, 20 degrees S-8*DEG;S). On seasonal time scales, the main mode of precipitation in the EA and STA are characterized by a unimodal regime, while the TZ is represented by a bimodal regime. The EA and the TZ share the same wet season in the February-April period, which is associated with a weakened Walker Cell, the southerly position of the Intertropical Convergence Zone (ITCZ) and a strong westward humidity transport from the equatorial Amazon. This latter mechanism and a reduced elevation of the Andes are associated with the October-November wet season in the TZ. The presence of the Bolivian High and the northward extension of the Low Level Jet are associated with the precipitation over Andean regions between 20 degrees S and 8 degrees S in the December-March period. On interannual time scales, extreme monthly wet events (EMWE) in the STA (TZ) are related to convection over the western (equatorial) Amazon during the December-March (February-April) period, showing an atmospheric relationship between the Amazon and the Andes. Extreme monthly dry events (EMDE) in the TZ and in the EA during the February-April period are related to a strengthened Walker Cell, especially in the eastern Pacific. In addition, EMWE (EMDE) in the EA are associated with an anomalous southward (northward) displaced eastern PacificITCZ. Moreover, we find a relationship between precipitation at higher elevations in the Andes north of 10 degrees S and easterly winds at 200 hPa during February-April EMWE. Finally, extreme monthly events in the EA (STA) are related to sea surface temperature anomalies in the western (central) equatorial Pacific.
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Segura, H., Junquas, C., Espinoza, J., Vuille, M., Jauregui, Y., Rabatel, A., et al. (2019). New insights into the rainfall variability in the tropical Andes on seasonal and interannual time scales (vol 53, pg 405, 2019). Climate Dynamics, .
Abstract: The original version of the article contained errors in Fig.
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Tchouta, K., Marie, B., Emmanuel, M., Guillaume, F., Benjamin, N., Nicaise, Y., et al. (2019). Contribution of time domain electromagnetic and magnetic resonance soundings to groundwater assessment at the margin of lake chad basin, cameroon. Journal Of Applied Geophysics, 170.
Abstract: At the edge of a semi-arid sedimentary basin, the Piedmont plain of the Mandara mountains is potentially a key area for the recharge of the Lake Chad Quaternary aquifer. We conducted two geophysical surveys based on magnetic resonance sounding (MRS) and Time-Domain Electromagnetism (TDEM) techniques for a better understanding of the aquifer structure at.the piedmont scale in aim to identify preferential groundwater recharge pathways. MRS water content and electrical resistivity of the medium confirmed the very heterogeneous and clayey nature of the quaternary aquifer, limiting the recharge to the sandiest areas. Thanks to the geophysical methods, these areas have been highlighted near temporary rivers and upstream of the study area near Maroua city. Because of the thickening of the aquifer downstream, the most transmissive zones are in the mid-slope of the piedmont. The geographical distribution of aquifer properties defined by geophysics and the range of MRS water content (from <2% to 26%) may be used in the future to constrain groundwater modelling and specify the recharge of the quaternary aquifer at mountain front. (C) 2019 Elsevier B.V. All rights reserved.
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Viguier, B., Jourde, H., Leonardi, V., Daniele, L., Batiot-Guilhe, C., Favreau, G., et al. (2019). Water table variations in the hyperarid Atacama Desert: Role of the increasing groundwater extraction in the pampa del tamarugal (Northern Chile). Journal Of Arid Environments, 168, 9–16.
Abstract: In the hyperarid Atacama Desert (Northern Chile), the economic and social development is supported using fossil groundwater. The groundwater extraction (GWE) has significantly increased over the last 30 years, reaching similar to 4.2 m(3).s(-1) in 2018 (+1890%) at the Pampa del Tamarugal Aquifer (PTA). But opposite assumptions lead to uncertainties concerning the role of the increasing anthropogenic pressures and the ephemeral recharge events in the water table (WT) variations. This paper analyzes: (i) the long-term groundwater levels changes between the late 1950s (post Saltpeter Work) and the early 2010s, and (ii) the short-term response of groundwater levels, based on the analysis of the 1998-2018 WT time series at 10 observation boreholes. Results indicate that the WT variations in space and time are strongly related to the anthropogenic pressure changes. Since the late 1950s, the WT is declining in the major part of the PTA. Nevertheless, local reduction of GWE together with ephemeral recharge events in alluvial fans allowed local WT rises. But after a large GWE increase (+114%) between 2004 and 2006, all observation boreholes highlight a general WT decline (-9.8 +/- 5.8 cm.yr(-1)). Over the years, anthropogenic pressures became the dominant factor of the WT variations and led to overuse the aquifer.
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Alle, I. C., Descloitres, M., Vouillamoz, J. M., Yalo, N., Lawson, F. M. A., & Adihou, A. C. (2018). Why 1D electrical resistivity techniques can result in inaccurate siting of boreholes in hard rock aquifers and why electrical resistivity tomography must be preferred: the example of Benin, West Africa. Journal Of African Earth Sciences, 139, 341–353.
Abstract: Hard rock aquifers are of particular importance for supplying people with drinking water in Africa and in the world. Although the common use of one-dimensional (1D) electrical resistivity techniques to locate drilling site, the failure rate of boreholes is usually high. For instance, about 40% of boreholes drilled in hard rock aquifers in Benin are unsuccessful. This study investigates why the current use of 1D techniques (e.g. electrical profiling and electrical sounding) can result in inaccurate siting of boreholes, and checks the interest and the limitations of the use of two-dimensional (2D) Electrical Resistivity Tomography (ERT). Geophysical numerical modeling and comprehensive 1D and 2D resistivity surveys were carried out in hard rock aquifers in Benin. The experiments carried out at 7 sites located in different hard rock groups confirmed the results of the numerical modeling: the current use of 1D techniques can frequently leads to inaccurate siting, and ERT better reveals hydrogeological targets such as thick weathered zone (e.g. stratiform fractured layer and preferential weathering associated with subvertical fractured zone). Moreover, a cost analysis demonstrates that the use of ERT can save money at the scale of a drilling programme if ERT improves the success rate by only 5% as compared to the success rate obtained with 1D techniques. Finally, this study demonstrates, using the example of Benin, that the use of electrical resistivity profiling and sounding for siting boreholes in weathered hard rocks of western Africa should be discarded and replaced by the use of ERT technique, more efficient. (C) 2017 Elsevier Ltd. All rights reserved.
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Bellier, J., Zin, I., & Bontron, G. (2018). Generating Coherent Ensemble Forecasts After Hydrological Postprocessing: Adaptations of ECC-Based Methods. Water Resources Research, 54(8), 5741–5762.
Abstract: Hydrological ensemble forecasts are frequently miscalibrated, and need to be statistically postprocessed in order to account for the total predictive uncertainty. Very often, this step relies on parametric, univariate techniques that ignore the between-basins and between-lead times dependencies. This calls for a procedure referred to as sampling-reordering, which generates a coherent multivariate ensemble from the marginal postprocessed distributions. The ensemble copula coupling (ECC) approach, which is already popular in the field of meteorological postprocessing, is attractive for hydrological forecasts as it preserves the dependence structure of the raw ensemble assumed as spatially and temporally coherent. However, the existing implementations of ECC have strong limitations when applied to hourly streamflow, due to raw ensembles being frequently nondispersive and to streamflow data being strongly autocorrelated. Based on this diagnosis, this paper investigates several variants of ECC, in particular the addition of a perturbation to the raw ensemble to handle the nondispersive cases, and the smoothing of the temporal trajectories to make them more realistic. The evaluation is conducted on a case study of hydrological forecasting over a set of French basins. The results show that the new variants improve upon the existing ECC implementations, while they remain simple and computationally inexpensive.
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Blanchet, J., Aly, C., Vischel, T., Panthou, G., Sane, Y., & Kane, M. D. (2018). Trend in the Co-Occurrence of Extreme Daily Rainfall in West Africa Since 1950. Journal Of Geophysical Research-Atmospheres, 123(3), 1536–1551.
Abstract: We propose in this paper a statistical framework to study the evolution of the co-occurrence of extreme daily rainfall in West Africa since 1950. We consider two regions subject to contrasted rainfall regimes: Senegal and the central Sahel. We study the likelihood of the 3% largest daily rainfall (considering all days) in each region to occur simultaneously and, in a 20year moving window approach, how this likelihood has evolved with time. Our method uses an anisotropic max-stable process allowing us to properly represent the co-occurrence of daily extremes and including the possibility of a preferred direction of co-occurrence. In Senegal, a change is found in the 1980s, with preferred co-occurrence along the E-50-N direction (i.e., along azimuth 50 degrees) before the 1980s and weaker isotropic co-occurrence afterward. In central Sahel, a change is also found in the 1980s but surprisingly with contrasting results. Anisotropy along the E-W direction is found over the whole period, with greater extension after the 1980s. The paper discusses how the co-occurrence of extremes can provide a qualitative indicator on change in size and propagation of the strongest storms. This calls for further research to identify the atmospheric processes responsible for such contrasted changes in storm properties. Plain Language Summary We propose in this paper a statistical framework to study the evolution of the co-occurrence of extreme daily rainfall in West Africa since 1950. We consider two regions subject to contrasted rainfall regimes: Senegal and the central Sahel. In Senegal, a change is found in the 1980s, with preferred co-occurrence along the E-50-N direction (i.e., along azimuth 50 degrees) before the 1980s and weaker isotropic co-occurrence afterward. In the central Sahel, a change is also found in the 1980s but surprisingly with contrasting results. Anisotropy along the E-W direction is found over the whole period, with greater extension after the 1980s. The paper discusses how the co-occurrence of extremes can provide a qualitative indicator on change in size and propagation of the strongest storms. This calls for further research to identify the atmospheric processes responsible for such contrasted changes in storm properties.
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Camberlin, P., Gitau, W., Planchon, O., Dubreuil, V., Funatsu, B. M., & Philippon, N. (2018). Major role of water bodies on diurnal precipitation regimes in Eastern Africa. International Journal Of Climatology, 38(2), 613–629.
Abstract: Mean diurnal rainfall regimes over Eastern Africa (also referred to as the Greater Horn of Africa) are studied based on 3-hourly data from the TRMM 3B42 data set, averaged over a 17-year period (1998-2014). The consistency with long-term mean raingauge data, available for partly independent periods, varies from good (Sudan, Ethiopia, Eritrea and Somalia) to very good (Kenya, Tanzania and Uganda). Over sea (Indian Ocean and Red Sea), the diurnal rainfall distribution is quite uniform; however, a morning peak dominates and there is evidence of offshore phase propagation south of the equator. Over land, both rainfall frequency and rainfall amounts show a dominant afternoon maximum (1500-1800 East African Time, i.e. GMT + 3). However, many inland regions show a delayed rainfall maximum (evening, night-time or morning). The evening to night-time maximum found over some land areas is associated with a phase propagation from areas showing an afternoon peak. This occurs west of high ground areas (Sudan and parts of the Great Lakes region) and in belts parallel to the seashores (Eritrea, northeastern Ethiopia, Somalia and eastern Kenya). The latter provide indirect evidence that sea breeze effects can be detected at unexpectedly great distances from the coast (up to 300-400 km) in parts of Eastern Africa. A remarkable ring of early afternoon (1500) maxima is found around most lakes, although some east-west asymmetries occur. Over the lakes, a morning or late night maximum is mostly found. It is generally inversely related to the distance to the shorelines for the larger lakes, but over the mid-size lakes it is replaced by or competes with a late afternoon to evening maximum.
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Carriere, S., Chalikakis, K., Ollivier, C., Heath, T., Mangin, M., Kempf, J., et al. (2018). Sustainable groundwater resources exploration and management in a complex geological setting as part of a humanitarian project (Mahafaly Plateau, Madagascar). Environmental Earth Sciences, 77(21).
Abstract: Southwestern Madagascar is a semi-arid region and a hot-spot of global change. On the Mahafaly plateau, people live with quasi-permanent water stress and groundwater, the only available resource, is difficult to exploit due to a complex hydrogeological environment. A methodology (suitable for humanitarian projects; <40kEuro) was developed in four phases to assess the sustainable exploitation of the water resource: (A) regional scale exploration, (B) village scale exploration, (C) drilling campaign, and (D) hydro-climatic monitoring. This integrated hydrogeophysical approach involves geophysical measurements (262 TEM-fast soundings, 2588 Slingram measurements, 35 electrical soundings), hydrochemical analyses (112 samples), and a piezometric survey (127 measurements). Two groundwater resources were identified, one deep (below 150m) and one shallow (<20m). Hydrochemical results highlighted the vulnerability of both resources: anthropic contamination for the shallower and seawater intrusion for the deeper. Therefore, subsequent geophysical surveys supported the siting of six boreholes and three wells in the shallow aquifer. This methodological approach was successful in this complex geological setting and requires testing at other sites in and outside Madagascar. The study demonstrates that geophysical results should be used in addition to drilling campaigns and to help monitor the water resource. In fact, to prevent over-exploitation, piezometric and meteorological sensors were installed to monitor the water resource. This unique hydro-climatic observatory may help (1) non-governmental organization and local institutions prevent future water shortages and (2) scientists to understand better how global change will affect this region of the world.
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Cohard, J. - M., Rosant, J. - M., Rodriguez, F., Andrieu, H., Mestayer, P. G., & Guillevic, P. (2018). Energy and water budgets of asphalt concrete pavement under simulated rain events. Urban Climate, 24, 675–691.
Abstract: Urban areas are subject to high human pressure and forthcoming enhanced hydrologic and climatic risks due to both city development and climate change. An asphalt concrete parking lot was instrumented in Nantes, France, to quantify the energy and hydrological responses of the surface to simulated rainfalls. The surface fluxes (precipitation, evaporation, radiation exchanges, sensible heat convection and conduction, runoff) were measured in situ and used to close the water budget with residual closure errors lower than 10%, depending on the surface evaporation retrieval method. The latent heat flux estimated from scintillometry measurements provided a better water budget closure than the direct eddy-correlation measurements. Runoff was the primary component of the water budget and represented around 80% of the total precipitation, compared to 17% for surface evaporation. The scintillometry method provided water evaporation time series at a 1-min time scale during the experiment. These series were used to characterize the rapid changes in the hydrological and energetic budgets of the asphalt surface after a precipitation event. During the drying phase the surface evaporation was significantly active, yielding 80% of the turbulent fluxes with a Bowen ratio of 0.25.
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Cretaux, J., Berge-Nguyen, M., Calmant, S., Jamangulova, N., Satylkanov, R., Lyard, F., et al. (2018). Absolute Calibration or Validation of the Altimeters on the Sentinel-3A and the Jason-3 over Lake Issykkul (Kyrgyzstan). Remote Sensing, 10(11).
Abstract: Calibration/Validation (C/V) studies using sites in the oceans have a long history and protocols are well established. Over lakes, C/V allows addressing problems such as the performance of the various retracking algorithms and evaluating the accuracy of the geophysical corrections for continental waters. This is achievable when measurements of specific and numerous field campaigns and a ground permanent network of level gauges and weather stations are processed. C/V consists of installation of permanent sites (weather stations, limnigraphs, and GPS reference points) and the organization of regular field campaigns. The lake Issykkul serves as permanent site of C/V, for a multi-mission purpose. The objective of this paper is to calculate the altimeter biases of Jason-3 and Sentinel-3A, both belonging to an operational satellite system which is used for the long-term monitoring of lake level variations. We have also determined the accuracy of the altimeters of these two satellites, through a comparison analysis with in situ data. In 2016 and 2017, three campaigns have been organized over this lake in order to estimate the absolute bias of the nadir altimeter onboard the Jason-3 and Sentinel-3A. The fieldwork consisted of measuring water height using a GPS system, carried on a boat, along the track of the altimeter satellite across the lake. It was performed at the time of the pass of the altimeter. Absolute altimeter biases were calculated by averaging the water height differences along the pass of the satellite (GPS from the boat system versus altimetry). Jason-3 operates in a Low Resolution Mode (LRM), while the Sentinel-3A operates in Synthetic Aperture Radar (SAR) mode. In this study we found that the absolute biases measured for Jason-3 were -28 +/- 40 mm with the Ocean retracker and 206 +/- 30 mm with the Ice-1 retracker. The biases for Sentinel-3A were -14 +/- 20 mm with the Samosa (Ocean like) retracker and 285 +/- 20 mm with the OCOG (Ice-1-like) retracker. We have also evaluated the accuracy of these two altimeters over Lake Issykkul which reached to 3 cm, for both the instruments, using the Ocean retracker.
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Descroix, L., Guichard, F., Grippa, M., Lambert, L. A., Panthou, G., Mahe, G., et al. (2018). Evolution of Surface Hydrology in the Sahelo-Sudanian Strip: An Updated Review. Water, 10(6), 748.
Abstract: In the West African Sahel, two paradoxical hydrological behaviors have occurred during the last five decades. The first paradox was observed during the 1968-1990s Great Drought' period, during which runoff significantly increased. The second paradox appeared during the subsequent period of rainfall recovery (i.e., since the 1990s), during which the runoff coefficient continued to increase despite the general re-greening of the Sahel. This paper reviews and synthesizes the literature on the drivers of these paradoxical behaviors, focusing on recent works in the West African Sahelo/Sudanian strip, and upscaling the hydrological processes through an analysis of recent data from two representative areas of this region. This paper helps better determine the respective roles played by Land Use/Land Cover Changes (LULCC), the evolution of rainfall intensity and the occurrence of extreme rainfall events in these hydrological paradoxes. Both the literature review and recent data converge in indicating that the first Sahelian hydrological paradox was mostly driven by LULCC, while the second paradox has been caused by both LULCC and climate evolution, mainly the recent increase in rainfall intensity.
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Dickson, N. E. M., Comte, J. - C., Koussoube, Y., Ofterdinger, U. S., & Vouillamoz, J. - M. (2018). Analysis and numerical modelling of large-scale controls on aquifer structure and hydrogeological properties in the African basement (Benin, West Africa). Geological Society, London, Special Publications, 479.
Abstract: The metamorphic basement units of the Upper Ouémé watershed in Benin have been investigated to identify the structural controls on aquifer properties, groundwater flow and water balance at large scale. Spatial analysis of borehole and hydrogeophysical data suggests that large-scale weathering profiles, aquifer transmissivity and storage properties are better correlated to a palaeo-weathering surface. Multi-model analysis, combined with assessment of nine transient numerical groundwater models against observations, suggests the best conceptualizations are those where hydraulic conductivity and specific yield are distributed within a weathered zone determined through interpolation of weathered zone thickness. When compared to previous studies, the general groundwater balance of simulated models suggests the groundwater system contributes, on average, 49.8 m3 s-1 to the river flow (mostly during the rainy season). The same volumetric flow would be lost to groundwater evapo-transpiration and deep/lateral drainage of the catchment. Borehole abstraction (about 7.5 m3 s-1) represents only 6% of the average groundwater recharge and 1% of the average rainfall. This suggests that despite relatively low borehole productivity, the basement aquifer system still has an important unused potential for rural to mid-scale water supply and that, at present, the main external drivers for groundwater resource sustainability are changes in climate and land use.
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Dommo, A., Philippon, N., Vondou, D., Seze, G., & Eastman, R. (2018). The June-September Low Cloud Cover in Western Central Africa: Mean Spatial Distribution and Diurnal Evolution, and Associated Atmospheric Dynamics. Journal Of Climate, 31(23), 9585–9603.
Abstract: Western central Africa (WCA) was recently shown to be one of the cloudiest areas of the tropics. Analyzing an ensemble of satellite products and surface cloud observations, we show that in June-September, WCA cloud cover is dominated by single-layered low stratiform clouds. Despite an underestimation of low cloud frequency in satellite estimates at night, comparisons with surface observations bring insights into the spatial distribution and diurnal cycle of low clouds. Both appear strongly influenced by orography: to the west, the coastal plains and the ocean-facing valleys have the largest cloud cover and a lower-amplitude diurnal cycle with a maximum cloud phase at 0400 local time (LT). To the east, across the windward slopes, plateaus, and downwind slopes, the cloud cover becomes progressively reduced and the diurnal cycle has a larger amplitude with a maximum cloud phase at 1000 LT. In terms of atmospheric dynamics, the east/west gradient observed in low cloud frequency and amount is related to a foehn effect without substantial rainfall on windward slopes. The diurnal cycle of low clouds on the windward slopes and plateaus is related to the reversal, from mean subsidence at 0700 LT over the Atlantic and inland to rising motion inland at 1300 LT. In addition, the airmass stability in low levels prevents the vertical development of cloud cover. Last, we could not detect in the European reanalyses any nocturnal jet as observed in southern West Africa (SWA), suggesting different mechanisms triggering low cloud formation in WCA compare to SWA.
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Gaillardet, J., Braud, I., Hankard, F., Anquetin, S., Bour, O., Dorfliger, N., et al. (2018). OZCAR: The French Network of Critical Zone Observatories. Vadose Zone Journal, 17(1).
Abstract: The French critical zone initiative, called OZCAR (Observatoires de la Zone Critique-Application et Recherche or Critical Zone Observatories-Application and Research) is a National Research Infrastructure (RI). OZCAR-RI is a network of instrumented sites, bringing together 21 pre-existing research observatories monitoring different compartments of the zone situated between “the rock and the sky,” the Earth's skin or critical zone (CZ), over the long term. These observatories are regionally based and have specific initial scientific questions, monitoring strategies, databases, and modeling activities. The diversity of OZCAR-RI observatories and sites is well representative of the heterogeneity of the CZ and of the scientific communities studying it. Despite this diversity, all OZCAR-RI sites share a main overarching mandate, which is to monitor, understand, and predict (“earthcast”) the fluxes of water and matter of the Earth's near surface and how they will change in response to the “new climatic regime.” The vision for OZCAR strategic development aims at designing an open infrastructure, building a national CZ community able to share a systemic representation of the CZ, and educating a new generation of scientists more apt to tackle the wicked problem of the Anthropocene. OZCAR articulates around: (i) a set of common scientific questions and cross-cutting scientific activities using the wealth of OZCAR-RI observatories, (ii) an ambitious instrumental development program, and (iii) a better interaction between data and models to integrate the different time and spatial scales. Internationally, OZCAR-RI aims at strengthening the CZ community by providing a model of organization for pre-existing observatories and by offering CZ instrumented sites. OZCAR is one of two French mirrors of the European Strategy Forum on Research Infrastructure (eLTER-ESFRI) project.
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Galle, S., Grippa, M., Peugeot, C., Moussa, I., Cappelaere, B., Demarty, J., et al. (2018). AMMA-CATCH, a Critical Zone Observatory in West Africa Monitoring a Region in Transition. Vadose Zone Journal, 17(1).
Abstract: West Africa is a region in fast transition from climate, demography, and land use perspectives. In this context, the African Monsoon Multidisciplinary Analysis (AMMA)-Couplage de l'Atmosphere Tropicale et du Cycle eco-Hydrologique (CATCH) long-term regional observatory was developed to monitor the impacts of global change on the critical zone of West Africa and to better understand its current and future dynamics. The observatory is organized into three thematic axes, which drive the observation and instrumentation strategy: (i) analyze the long-term evolution of eco-hydrosystems from a regional perspective; (ii) better understand critical zone processes and their variability; and (iii) meet socioeconomic and development needs. To achieve these goals, the observatory has gathered data since 1990 from four densely instrumented mesoscale sites (similar to 10(4) km(2) each), located at different latitudes (Benin, Niger, Mali, and Senegal) so as to sample the sharp eco-climatic gradient that is characteristic of the region. Simultaneous monitoring of the vegetation cover and of various components of the water balance at these four sites has provided new insights into the seemingly paradoxical eco-hydrological changes observed in the Sahel during the last decades: groundwater recharge and/ or runoff intensification despite rainfall deficit and subsequent re-greening with still increasing runoff. Hydrological processes and the role of certain key landscape features are highlighted, as well as the importance of an appropriate description of soil and subsoil characteristics. Applications of these scientific results for sustainable development issues are proposed. Finally, detecting and attributing eco-hydrological changes and identifying possible regime shifts in the hydrologic cycle are the next challenges that need to be faced.
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Gibon, F., Pellarin, T., Roman-Cascon, C., Alhassane, A., Traore, S., Kerr, Y., et al. (2018). Millet yield estimates in the Sahel using satellite derived soil moisture time series. Agricultural And Forest Meteorology, 262, 100–109.
Abstract: In the Sahel, crop growth and yield are strongly linked to climate fluctuations. The low and erratic rainfall the Sahel region has experienced for several years led to poor harvests, associated with dramatic food crises and famines. Consequently, numerous studies were conducted to develop innovative techniques to estimate crop yield based on satellite measurements. Unlike most approaches which use rainfall, temperature or vegetation indices to derive crop yield estimates, the present study investigates the potential of satellite-derived soil moisture products. This study focuses on millet, a major food crop in Africa. A first step was devoted to analyzing the relation between soil moisture and millet yield at the local scale using ground-based soil moisture and millet yield measurements obtained at ten site locations in Niger. Then, the statistical relationship obtained at the local scale was assessed at the regional scale (Niger, Mali, Senegal and Burkina Faso) using satellite-based soil moisture mapping (based on a simple land-surface model and a satellite precipitation product) and compared to millet yield estimates from the Food and Agriculture Organization (FAO) database. It was shown that millet yield variations are closely linked to soil moisture variations during two key periods of the plant growth: the “grain filling” and the “reproductive” periods. Soil moisture variations during these two periods led to explain 81% (R-2 = 0.81) of the FAO millet yield variations from 1998 to 2014 in the Sahel.
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Gomez-Navarro, L., Fablet, R., Mason, E., Pascual, A., Mourre, B., Cosme, E., et al. (2018). SWOT Spatial Scales in the Western Mediterranean Sea Derived from Pseudo-Observations and an Ad Hoc Filtering. Remote Sensing, 10(4).
Abstract: The aim of this study is to assess the capacity of the Surface Water Ocean Topography (SWOT) satellite to resolve fine scale oceanic surface features in the western Mediterranean. Using as input the Sea Surface Height (SSH) fields from a high-resolution Ocean General Circulation Model (OGCM), the SWOT Simulator for Ocean Science generates SWOT-like outputs along a swath and the nadir following the orbit ground tracks. Given the characteristic temporal and spatial scales of fine scale features in the region, we examine temporal and spatial resolution of the SWOT outputs by comparing them with the original model data which are interpolated onto the SWOT grid. To further assess the satellite's performance, we derive the absolute geostrophic velocity and relative vorticity. We find that instrument noise and geophysical error mask the whole signal of the pseudo-SWOT derived dynamical variables. We therefore address the impact of removal of satellite noise from the pseudo-SWOT data using a Laplacian diffusion filter, and then focus on the spatial scales that are resolved within a swath after this filtering. To investigate sensitivity to different filtering parameters, we calculate spatial spectra and root mean square errors. Our numerical experiments show that noise patterns dominate the spectral content of the pseudo-SWOT fields at wavelengths below 60 km. Application of the Laplacian diffusion filter allows recovery of the spectral signature within a swath down to the 40-60 km wavelength range. Consequently, with the help of this filter, we are able to improve the observation of fine scale oceanic features in pseudo-SWOT data, and in the estimation of associated derived variables such as velocity and vorticity.
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Hector, B., Cohard, J., Seguis, L., Galle, S., & Peugeot, C. (2018). Hydrological functioning of western African inland valleys explored with a critical zone model. Hydrology And Earth System Sciences, 22(11), 5867–5900.
Abstract: Inland valleys are seasonally waterlogged headwater wetlands, widespread across western Africa. Their role in the hydrological cycle in the humid, hard-rock-dominated Sudanian savanna is not yet well understood. Thus, while in the region recurrent floods are a major issue, and hydropower has been recognized as an important development pathway, the scientific community lacks precise knowledge of streamflow (Q) generation processes and how they could be affected by the presence of inland valleys. Furthermore, inland valleys carry an important agronomic potential, and with the strong demographic rates of the region, they are highly subject to undergoing land cover changes. We address both the questions of the hydrological functioning of inland valleys in the Sudanian savanna of western Africa and the impact of land cover changes on these systems through deterministic sensitivity experiments using a physically based critical zone model (ParFlow-CLM) applied to a virtual generic catchment which comprises an inland valley. Model forcings are based on 20 years of data from the AMMA-CATCH observation service and parameters are evaluated against multiple field data (Q, evapotranspiration – ET -, soil moisture, water table levels, and water storage) acquired on a pilot elementary catchment. The hydrological model applied to the conceptual lithological/pedological model proposed in this study reproduces the main behaviours observed, which allowed those virtual experiments to be conducted. We found that yearly water budgets were highly sensitive to the vegetation distribution: average yearly ET for a tree-covered catchment (944 mm) exceeds that of herbaceous cover (791 mm) ET differences between the two covers vary between 12 % and 24 % of the precipitation of the year for the wettest and driest years, respectively. Consequently, the tree-covered catchment produces a yearly Q amount of 28 % lower on average as compared to a herbaceous-covered catchment, ranging from 20 % for the wettest year to 47 % for a dry year. Trees also buffer interannual variability in ET by 26 % (with respect to herbaceous). On the other hand, pedological features (presence – or absence – of the low-permeability layer commonly found below inland valleys, upstream and lateral contributive areas) had limited impact on yearly water budgets but marked consequences for intraseasonal hydrological processes (sustained/non-sustained baseflow in the dry season, catchment water storage redistribution). Therefore, subsurface features and vegetation cover of inland valleys have potentially significant impacts on downstream water-dependent ecosystems and water uses as hydropower generation, and should focus our attention.
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Junquas, C., Takahashi, K., Condom, T., Espinoza, J. C., Chavez, S., Sicart, J. E., et al. (2018). Understanding the influence of orography on the precipitation diurnal cycle and the associated atmospheric processes in the central Andes. Climate Dynamics, 50(11-12), 3995–4017.
Abstract: In the tropical Andes, the identification of the present synoptic mechanisms associated with the diurnal cycle of precipitation and its interaction with orography is a key step to understand how the atmospheric circulation influences the patterns of precipitation variability on longer time-scales. In particular we aim to better understand the combination of the local and regional mechanisms controlling the diurnal cycle of summertime (DJF) precipitation in the Northern Central Andes (NCA) region of Southern Peru. A climatology of the diurnal cycle is obtained from 15 wet seasons (2000-2014) of 3-hourly TRMM-3B42 data (0.25A degrees x 0.25A degrees) and swath data from the TRMM-2A25 precipitation radar product (5 km x 5 km). The main findings are: (1) in the NCA region, the diurnal cycle shows a maximum precipitation occurring during the day (night) in the western (eastern) side of the Andes highlands, (2) in the valleys of the Cuzco region and in the Amazon slope of the Andes the maximum (minimum) precipitation occurs during the night (day). The WRF (Weather Research and Forecasting) regional atmospheric model is used to simulate the mean diurnal cycle in the NCA region for the same period at 27 km and 9 km horizontal grid spacing and 3-hourly output, and at 3 km only for the month of January 2010 in the Cuzco valleys. Sensitivity experiments were also performed to investigate the effect of the topography on the observed rainfall patterns. The model reproduces the main diurnal precipitation features. The main atmospheric processes identified are: (1) the presence of a regional-scale cyclonic circulation strengthening during the afternoon, (2) diurnal thermally driven circulations at local scale, including upslope (downslope) wind and moisture transport during the day (night), (3) channelization of the upslope moisture transport from the Amazon along the Apurimac valleys toward the western part of the cordillera.
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Koïta, M., Yonli, H., Soro, D., Dara, A., & Vouillamoz, J. - M. (2018). Groundwater Storage Change Estimation Using Combination of Hydrogeophysical and Groundwater Table Fluctuation Methods in Hard Rock Aquifers. Resources, 7(1), 5.
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Kone, B., Diedhiou, A., Toure, N., Sylla, M., Giorgi, F., Anquetin, S., et al. (2018). Sensitivity study of the regional climate model RegCM4 to different convective schemes over West Africa. Earth System Dynamics, 9(4), 1261–1278.
Abstract: The latest version of RegCM4 with CLM4.5 as a land surface scheme was used to assess the performance and sensitivity of the simulated West African climate system to different convection schemes. The sensitivity studies were performed over the West African domain from November 2002 to December 2004 at a spatial resolution of 50 km x 50 km and involved five convective schemes: (i) Emanuel; (ii) Grell; (iii) Emanuel over land and Grell over ocean (Mix1); (iv) Grell over land and Emanuel over ocean (Mix2); and (v) Tiedtke. All simulations were forced with ERA-Interim data. Validation of surface temperature at 2 m and precipitation were conducted using data from the Climate Research Unit (CRU), Global Precipitation Climatology Project (GPCP) and the Tropical Rainfall Measurement Mission (TRMM) during June to September (rainy season), while the simulated atmospheric dynamic was compared to ERA-Interim data. It is worth noting that the few previous similar sensitivity studies conducted in the region were performed using BATS as a land surface scheme and involved less convective schemes. Compared with the previous version of RegCM, RegCM4-CLM also shows a general cold bias over West Africa whatever the convective scheme used. This cold bias is more reduced when using the Emanuel convective scheme. In terms of precipitation, the dominant feature in model simulations is a dry bias that is better reduced when using the Emanuel convective scheme. Considering the good performance with respect to a quantitative evaluation of the temperature and precipitation simulations over the entire West African domain and its subregions, the Emanuel convective scheme is recommended for the study of the West African climate system.
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Kotchoni, D. O. V., Vouillamoz, J. - M., Lawson, F. M. A., Adjomayi, P., Boukari, M., & Taylor, R. G. (2018). Relationships between rainfall and groundwater recharge in seasonally humid Benin: a comparative analysis of long-term hydrographs in sedimentary and crystalline aquifers. Hydrogeology Journal, .
Abstract: Groundwater is a vital source of freshwater throughout the tropics enabling access to safe water for domestic, agricultural and industrial purposes close to the point of demand. The sustainability of groundwater withdrawals is controlled, in part, by groundwater recharge, yet the conversion of rainfall into recharge remains inadequately understood, particularly in the tropics. This study examines a rare set of 19-25-year records of observed groundwater levels and rainfall under humid conditions (mean rainfall is ~1,200 mm year-1) in three common geological environments of Benin and other parts of West Africa: Quaternary sands, Mio-Pliocene sandstone, and crystalline rocks. Recharge is estimated from groundwater-level fluctuations and employs values of specific yield derived from magnetic resonance soundings. Recharge is observed to occur seasonally and linearly in response to rainfall exceeding an apparent threshold of between 140 and 250 mm year-1. Inter-annual changes in groundwater storage correlate well to inter-annual rainfall variability. However, recharge varies substantially depending upon the geological environment: annual recharge to shallow aquifers of Quaternary sands amounts to as much as 40% of annual rainfall, whereas in deeper aquifers of Mio-Pliocene sandstone and weathered crystalline rocks, annual fractions of rainfall generating recharge are 13 and 4%, respectively. Differences are primarily attributed to the thickness of the unsaturated zone and to the lithological controls on the transmission and storage of rain-fed recharge.
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Lac, C., Chaboureau, J. P., Masson, V., Pinty, J. P., Tulet, P., Escobar, J., et al. (2018). Overview of the Meso-NH model version 5.4 and its applications. Geoscientific Model Development, 11(5), 1929–1969.
Abstract: This paper presents the Meso-NH model version 5.4. Meso-NH is an atmospheric non hydrostatic research model that is applied to a broad range of resolutions, from synoptic to turbulent scales, and is designed for studies of physics and chemistry. It is a limited-area model employing advanced numerical techniques, including monotonic advection schemes for scalar transport and fourth-order centered or odd-order WENO advection schemes for momentum. The model includes state-of-the-art physics parameter-ization schemes that are important to represent convectivescale phenomena and turbulent eddies, as well as flows at larger scales. In addition, Meso-NH has been expanded to provide capabilities for a range of Earth system prediction applications such as chemistry and aerosols, electricity and lightning, hydrology, wildland fires, volcanic eruptions, and cyclones with ocean coupling. Here, we present the main innovations to the dynamics and physics of the code since the pioneer paper of Lafore et al. (1998) and provide an overview of recent applications and couplings.
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Legchenko, A., Miege, C., Koenig, L. S., Forster, R. R., Miller, O., Solomon, D. K., et al. (2018). Estimating water volume stored in the south-eastern Greenland firn aquifer using magnetic-resonance soundings. Journal Of Applied Geophysics, 150, 11–20.
Abstract: Recent observations of the Greenland ice sheet show an increase of the area affected by progressive melt of snow and ice, thus resulting in production of the additional meltwater. In 2011, an important storage of meltwater in the firn has been observed in the S-E Greenland. This water does not freeze during the wintertime and forms a perennial firn aquifer. The aquifer spatial extent has been initially monitored with combined ground and airborne radar observations, but these geophysical techniques are not able to inform us on the amount of meltwater stored at depth. In this study, we use the magnetic resonance soundings (MRS) method for estimating the volume of water stored in the Greenland ice sheet firn and mapping its spatial variability. Our study area covers a firn aquifer along a 16-km E-W transect, ranging between elevations of 1520 and 1760 m. In July 2015 and July 2016, we performed MRS measurements that allow estimating the water volume in the studied area as well as the one-year water volume evolution. Water storage is not homogeneous, fluctuating between 0.2 and 2 m(3)/m(2), and contains discontinuities in the hydrodynamic properties. We estimate an average volume of water stored in the firn in 2016 to be 0.76 m(3)/m(2), which corresponds to a 0.76-m-thick layer of bulk water. MRS monitoring reveals that from April 2015 to July 2016 the volume of water stored at the location of our transect increases by about 36%. We found MRS-estimated depth to water in a good agreement with that obtained with the ground penetrating radar (GPR). (C) 2018 Elsevier B.V. All rights reserved.
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Maley, J., Doumenge, C., Giresse, P., Mahe, G., Philippon, N., Hubau, W., et al. (2018). Late Holocene forest contraction and fragmentation in central Africa. Quaternary Research, 89(1), 43–59.
Abstract: During the warmer Holocene Period, two major climatic crises affected the Central African rainforests. The first crisis, around 4000 cal yr BP, caused the contraction of the forest in favor of savanna expansion at its northern and southern periphery. The second crisis, around 2500 cal yr BP, resulted in major perturbation at the forest core, leading to forest disturbance and fragmentation with a rapid expansion of pioneer-type vegetation, and a marked erosional phase. The major driver of these two climatic crises appears to be rapid sea-surface temperature variations in the equatorial eastern Atlantic, which modified the regional atmospheric circulation. The change between ca. 2500 to 2000 cal yr BP led to a large increase in thunderstorm activity, which explains the phase of forest fragmentation. Ultimately, climatic data obtained recently show that the present-day major rise in thunderstorms and lightning activity in Central Africa could result from some kind of solar influence, and hence the phase of forest fragmentation between ca. 2500 to 2000 cal yr BP may provide a model for the present-day global warming-related environmental changes in this region.
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Panthou, G., Lebel, T., Vischel, T., Quantin, G., Sane, Y., Ba, A., et al. (2018). Rainfall intensification in tropical semi-arid regions: the Sahelian case. Environmental Research Letters, 13(6).
Abstract: An anticipated consequence of ongoing global warming is the intensification of the rainfall regimes meaning longer dry spells and heavier precipitation when it rains, with potentially high hydrological and socio-economic impacts. The semi-arid regions of the intertropical band, such as the Sahel, are facing particularly serious challenges in this respect since their population is strongly vulnerable to extreme climatic events. Detecting long term trends in the Sahelian rainfall regime is thus of great societal importance, while being scientifically challenging because datasets allowing for such detection studies are rare in this region. This study addresses this challenge by making use of a large set of daily rain gauge data covering the Sahel (defined in this study as extending from 20 degrees W-10 degrees E and from 11 degrees N-18 degrees N) since 1950, combined with an unparalleled 5 minute rainfall observations available since 1990 over the AMMA-CATCH Niger observatory. The analysis of the daily data leads to the assertion that a hydro-climatic intensification is actually taking place in the Sahel, with an increasing mean intensity of rainy days associated with a higher frequency of heavy rainfall. This leads in turn to highlight that the return to wetter annual rainfall conditions since the beginning of the 2000s-succeeding the 1970-2000 drought-is by no mean a recovery towards the much smoother regime that prevailed during the 1950s and 1960s. It also provides a vision of the contrasts existing between theWest Sahel and the East Sahel, the East Sahel experiencing a stronger increase of extreme rainfall. This regional vision is complemented by a local study at sub-daily timescales carried out thanks to the 5 minute rainfall series of the AMMA-CATCH Niger observatory (12000 km(2)). The increasing intensity of extreme rainfall is also visible at sub-daily timescales, the annual maximum intensities have increased at an average rate of 2%-6% per decade since 1990 for timescales ranging from 5 min to 1 hour. Both visions-regional/long term/daily on the one hand, and local/27/years/sub-daily, on the other-converge to the conclusion that, rather than a rainfall recovery, the Sahel is experiencing a new era of climate extremes that roughly started at the beginning of this century.
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Pierret, M., Cotel, S., Ackerer, P., Beaulieu, E., Benarioumlil, S., Boucher, M., et al. (2018). The Strengbach Catchment: A Multidisciplinary Environmental Sentry for 30 Years. Vadose Zone Journal, 17(1).
Abstract: Research activity associated with various observations at the Strengbach catchment in the Vosges Massif (880-1150 m) addresses many questions in the domains of hydrology and geochemistry. The catchment is the observation and experimental site of the Observatoire Hydro-Geochimique de l'Environnement appointed by the Centre National de la Recherche Scientifique. It also belongs to the research facilities that form the French Network of Critical Zone Observatories (OZCAR), which supports a network of critical zone observatories. The catchment is small (0.8 km(2)) with steep slopes (20-30%) on granitic bedrock that mainly allow for forestry (spruce and beech stands) as the main land cover. Meteorological, hydrological, and geochemical data have been monitored since 1986. The first studies conducted were dedicated to the elucidation of acid rain effects on forest ecosystems and particularly on forest decline. Multidisciplinary research studies conducted on the Strengbach catchment enable exploration of the following issues: (i) hydrological functioning at the scale of a small catchment and questions regarding the evolution and preservation of the water resources in mountainous environments (stock, recharge, infiltration, and water pathways), (ii) exchange processes observed at the soil-plant-atmosphere continuum and in particular weathering processes and the evolution of soil mineral fertility (Ca, Mg, K, P), (iii) processes responsible for the export of water and for associated fluxes (dissolved chemicals, suspended materials, bed loads) and their dynamic at the outlet, and (iv) responses of the ecosystems to environmental disturbances (acid rain, forest management, and climate change) and their current and future modeling.
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Revuelto, J., Lecourt, G., Lafaysse, M., Zin, I., Charrois, L., Vionnet, V., et al. (2018). Multi-Criteria Evaluation of Snowpack Simulations in Complex Alpine Terrain Using Satellite and In Situ Observations. Remote Sensing, 10(8).
Abstract: This work presents an extensive evaluation of the Crocus snowpack model over a rugged and highly glacierized mountain catchment (Arve valley, Western Alps, France) from 1989 to 2015. The simulations were compared and evaluated using in-situ point snow depth measurements, in-situ seasonal and annual glacier surface mass balance, snow covered area evolution based on optical satellite imagery at 250 m resolution (MODIS sensor), and the annual equilibrium-line altitude of glaciers, derived from satellite images (Landsat, SPOT, and ASTER). The snowpack simulations were obtained using the Crocus snowpack model driven by the same, originally semi-distributed, meteorological forcing (SAFRAN) reanalysis using the native semi-distributed configuration, but also a fully distributed configuration. The semi-distributed approach addresses land surface simulations for discrete topographic classes characterized by elevation range, aspect, and slope. The distributed approach operates on a 250-m grid, enabling inclusion of terrain shadowing effects, based on the same original meteorological dataset. Despite the fact that the two simulations use the same snowpack model, being potentially subjected to same potential deviation from the parametrization of certain physical processes, the results showed that both approaches accurately reproduced the snowpack distribution over the study period. Slightly (although statistically significantly) better results were obtained by using the distributed approach. The evaluation of the snow cover area with MODIS sensor has shown, on average, a reduction of the Root Mean Squared Error (RMSE) from 15.2% with the semi-distributed approach to 12.6% with the distributed one. Similarly, surface glacier mass balance RMSE decreased from 1.475 m of water equivalent (W.E.) for the semi-distributed simulation to 1.375 m W.E. for the distribution. The improvement, observed with a much higher computational time, does not justify the recommendation of this approach for all applications; however, for simulations that require a precise representation of snowpack distribution, the distributed approach is suggested.
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Sane, Y., Panthou, G., Bodian, A., Vischel, T., Lebel, T., Dacosta, H., et al. (2018). Intensity-duration-frequency (IDF) rainfall curves in Senegal. Natural Hazards And Earth System Sciences, 18(7), 1849–1866.
Abstract: Urbanization resulting from sharply increasing demographic pressure and infrastructure development has made the populations of many tropical areas more vulnerable to extreme rainfall hazards. Characterizing extreme rainfall distribution in a coherent way in space and time is thus becoming an overarching need that requires using appropriate models of intensity-duration-frequency (IDF) curves. Using a 14 series of 5 min rainfall records collected in Senegal, a comparison of two generalized extreme value (GEV) and scaling models is carried out, resulting in the selection of the more parsimonious one (four parameters), as the recommended model for use. A bootstrap approach is proposed to compute the uncertainty associated with the estimation of these four parameters and of the related rainfall return levels for durations ranging from 1 to 24 h. This study confirms previous works showing that simple scaling holds for characterizing the temporal scaling of extreme rainfall in tropical regions such as sub-Saharan Africa. It further provides confidence intervals for the parameter estimates and shows that the uncertainty linked to the estimation of the GEV parameters is 3 to 4 times larger than the uncertainty linked to the inference of the scaling parameter. From this model, maps of IDF parameters over Senegal are produced, providing a spatial vision of their organization over the country, with a north to south gradient for the location and scale parameters of the GEV. An influence of the distance from the ocean was found for the scaling parameter. It is acknowledged in conclusion that climate change renders the inference of IDF curves sensitive to increasing non-stationarity effects, which requires warning end-users that such tools should be used with care and discernment.
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Uber, M., Vandervaere, J., Zin, I., Braud, I., Heistermann, M., Legout, C., et al. (2018). How does initial soil moisture influence the hydrological response? A case study from southern France. Hydrology And Earth System Sciences, 22(12), 6127–6146.
Abstract: The Cevennes-Vivarais region in southern France is prone to heavy rainfall that can lead to flash floods which are one of the most hazardous natural risks in Europe. The results of numerous studies show that besides rainfall and physical catchment characteristics the catchment's initial soil moisture also impacts the hydrological response to rain events. The aim of this paper is to analyze the relationship between catchment mean initial soil moisture (theta) over bar (ini) and the hydrological response that is quantified using the event-based runoff coefficient phi(ev) in the two nested catchments of the Gazel (3.4 km(2)) and the Claduegne (43 km(2)). Thus, the objectives are twofold: (1) obtaining meaningful estimates of soil moisture at catchment scale from a dense network of in situ measurements and (2) using this estimate of (theta) over bar (ini) to analyze its relation with phi(ev) calculated for many runoff events. A sampling setup including 45 permanently installed frequency domain reflectancy probes that continuously measure soil moisture at three depths is applied. Additionally, on-alert surface measurements at approximate to 10 locations in each one of 11 plots are conducted. Thus, catchment mean soil moisture can be confidently assessed with a standard error of the mean of <= 1.7 vol% over a wide range of soil moisture conditions. The phi(ev) is calculated from high-resolution discharge and precipitation data for several rain events with a cumulative precipitation P-cum ranging from less than 5mm to more than 80 mm. Because of the high uncertainty of phi(ev) associated with the hydrograph separation method, phi(ev) is calculated with several methods, including graphical methods, digital filters and a tracer-based method. The results indicate that the hydrological response depends on (theta) over bar (ini): during dry conditions phi(ev) is consistently below 0.1, even for events with high and intense precipitation. Above a threshold of (theta) over bar (ini) = 34 vol % phi(ev) can reach values up to 0.99 but there is a high scatter. Some variability can be explained with a weak correlation of phi(ev) with P-cum and rain intensity, but a considerable part of the variability remains unexplained. It is concluded that threshold-based methods can be helpful to prevent overestimation of the hydrological response during dry catchment conditions. The impact of soil moisture on the hydrological response during wet catchment conditions, however, is still insufficiently understood and cannot be generalized based on the present results.
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Valois, R., Vouillamoz, J. M., Lun, S., & Arnout, L. (2018). Mapping groundwater reserves in northwestern Cambodia with the combined use of data from lithologs and time-domain-electromagnetic and magnetic-resonance soundings. Hydrogeology Journal, 26(4), 1187–1200.
Abstract: Lack of access to water is the primary constraint to development in rural areas of northwestern Cambodia. Communities lack water for both domestic and irrigation purposes. To provide access to drinking water, governmental and aid agencies have focused on drilling shallow boreholes but they have not had a clear understanding of groundwater potential. The goal of this study has been to improve hydrogeological knowledge of two districts in Oddar Meanchey Province by analyzing borehole lithologs and geophysical data sets. The comparison of 55 time-domain electromagnetic (TEM) soundings and lithologs, as well as 66 magnetic-resonance soundings (MRS) with TEM soundings, allows a better understanding of the links between geology, electrical resistivity and hydrogeological parameters such as the specific yield (S (y)) derived from MRS. The main findings are that water inflow and S (y) are more related to electrical resistivity and elevation than to the litholog description. Indeed, conductive media are associated with a null value of S (y), whereas resistive rocks at low elevation are always linked to strictly positive S (y). A new methodology was developed to create maps of groundwater reserves based on 612 TEM soundings and the observed relationship between resistivity and S (y). TEM soundings were inverted using a quasi-3D modeling approach called 'spatially constrained inversion'. Such maps will, no doubt, be very useful for borehole siting and in the economic development of the province because they clearly distinguish areas of high groundwater-reserves potential from areas that lack reserves.
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Viani, A., Condom, T., Vincent, C., Rabatel, A., Bacchi, B., Sicart, J. E., et al. (2018). Glacier-wide summer surface mass-balance calculation: hydrological balance applied to the Argentiere and Mer de Glace drainage basins (Mont Blanc). Journal Of Glaciology, 64(243), 119–131.
Abstract: We present the glacier-wide summer surface mass balances determined by a detailed hydrological balance (sSMBhydro) and the quantification of the uncertainties of the calculations on the Argentiere and Mer de Glace-Leschaux drainage basins, located in the upper Arve watershed (French Alps), over the period 1996-2004. The spatial distribution of precipitation within the study area was adjusted using in situ winter mass-balance measurements. The sSMBhydro performance was assessed via a comparison with the summer surface mass balances based on in situ glaciological observations (sSMBglacio). Our results show that the sSMBhydro has an uncertainty of +/- 0.67 m w.e.a(-1) at Argentiere and +/- 0.66 m w.e.a(-1) at Mer de Glace-Leschaux. Estimates of the Argentiere sSMBhydro values are in good agreement with the sSMBglacio values. These time series show almost the same interannual variability. From the marked difference between the sSMBhydro and sSMBglacio values for the Mer de Glace-Leschaux glacier, we suspect a significant role of groundwater fluxes in the hydrological balance. This study underlines the importance of taking into account the groundwater transfers to represent and predict the hydro-glaciological behaviour of a catchment.
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Wilcox, C., Vischel, T., Panthou, G., Bodian, A., Blanchet, J., Descroix, L., et al. (2018). Trends in hydrological extremes in the Senegal and Niger Rivers. Journal Of Hydrology, 566, 531–545.
Abstract: In recent years, West Africa has witnessed an increasing number of damaging floods that raise the question of a possible intensification of the hydrological hazards in the region. In this study, the evolution of extreme floods is analyzed over the period 1950-2015 for seven tributaries in the Sudano-Guinean part of the Senegal River basin and four data sets in the Sahelian part of the Niger River basin. Non-stationary Generalized Extreme Value (NS-GEV) distributions including twelve models with time-dependent parameters plus a stationary GEV are applied to annual maxima of daily discharge (AMAX) series. An original methodology is proposed for comparing GEV models and selecting the best for use. The stationary GEV is rejected for all stations, demonstrating the significant non-stationarity of extreme discharge values in West Africa over the past six decades. The model of best fit most commonly selected is a double-linear model for the central tendency parameter (mu), with the dispersion parameter (sigma) modeled as either stationary, linear, or a double-linear. Change points in double-linear models are relatively consistent for the Senegal basin, with stations switching from a decreasing streamflow trend to an increasing streamflow trend in the early 1980s. In the Niger basin the trend in μis generally positive since the 1970s with an increase in slope after the change point, but the change point location is less consistent. The recent increasing trends in extreme discharges are reflected in an especially marked increase in return level magnitudes since the 1980s in the studied Sahelian rivers. The rate of the increase indicated by the study results raises urgent considerations for stakeholders and engineers who are in charge of river basin management and hydraulic works sizing.
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Zaharia, L., Ioana-Toroimac, G., Morosanu, G. A., Galie, A. C., Moldoveanu, M., Canjevac, I., et al. (2018). Review of national methodologies for rivers' hydromorphological assessment: A comparative approach in France, Romania, and Croatia. Journal Of Environmental Management, 217, 735–746.
Abstract: Conducting hydromorphological assessments for evaluating the ecological status of rivers is a key requirement of the Directive 2000/60/EC (Water Framework Directive – WFD) within European Union (EU) Member States. This paper aims at understanding how this requirement was implemented, through an original comparative review of methodologies for rivers' hydromorphological assessment in three EU Member States, which joined the EU at different times, and with many differences in terms of hydro graphic features, socio-economic and water management systems: France, Romania, and Croatia. More precisely, the paper aims at identifying and understanding the main principles guiding the hydro morphological assessment methodologies, elements and data used, giving an overview of the results of hydromorphological river status assessment, and concluding on the stage of hydromorphological assessment implementation. France developed numerous methodologies for physical habitat survey since the 1990s and it is currently conducting a rigorous hydromorphological field survey, but it does not yet have any national methodology for rivers' hydromorphological status assessment, nevertheless foreseen for the next cycle of the WFD. Besides, Romania and Croatia have already started the assessment of the hydromorphological status of rivers within the two cycles of the River Basin Management Plans and are making efforts to improve the hydromorphological monitoring activity. The methods generally differ in indicators, data used, and spatial scale of analysis, which makes it difficult to compare the results of the assessments. Despite a common water policy, the methodological dissimilarities seem to be rather usual between EU Member States. Therefore, the standardization of methodologies appears to be necessary, but the current results could be useful for setting priorities for river restoration and for achieving a better status at a national scale. (C) 2018 Elsevier Ltd. All rights reserved.
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2017 |
Al Bitar, A., Mialon, A., Kerr, Y. H., Cabot, F., Richaume, P., Jacquette, E., et al. (2017). The global SMOS Level 3 daily soil moisture and brightness temperature maps. Earth System Science Data, 9(1), 293–315.
Abstract: The objective of this paper is to present the multi-orbit (MO) surface soil moisture (SM) and angle-binned brightness temperature (TB) products for the SMOS (Soil Moisture and Ocean Salinity) mission based on a new multi-orbit algorithm. The Level 3 algorithm at CATDS (Centre Aval de Traitement des Donnees SMOS) makes use of MO retrieval to enhance the robustness and quality of SM retrievals. The motivation of the approach is to make use of the longer temporal autocorrelation length of the vegetation optical depth (VOD) compared to the corresponding SM autocorrelation in order to enhance the retrievals when an acquisition occurs at the border of the swath. The retrieval algorithm is implemented in a unique operational processor delivering multiple parameters (e.g. SM and VOD) using multi-angular dual-polarisation TB from MO. A subsidiary angle-binned TB product is provided. In this study the Level 3 TB V310 product is showcased and compared to SMAP (Soil Moisture Active Passive) TB. The Level 3 SM V300 product is compared to the single-orbit (SO) retrievals from the Level 2 SM processor from ESA with aligned configuration. The advantages and drawbacks of the Level 3 SM product (L3SM) are discussed. The comparison is done on a global scale between the two datasets and on the local scale with respect to in situ data from AMMA-CATCH and USDA ARS Watershed networks. The results obtained from the global analysis show that the MO implementation enhances the number of retrievals: up to 9% over certain areas. The comparison with the in situ data shows that the increase in the number of retrievals does not come with a decrease in quality, but rather at the expense of an increased time lag in product availability from 6 h to 3.5 days, which can be a limiting factor for applications like flood forecast but reasonable for drought monitoring and climate change studies. The SMOS L3 soil moisture and L3 brightness temperature products are delivered using an open licence and free of charge using a web application(https://www.catds.fr/sipad/).TheRE04 products, versions 300 and 310, used in this paper are also available at ftp://ext-catds-cpdc:catds2010@ftp.ifremer.fr/Land_products/GRIDDED/L3SM/RE04/.
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Awessou, K. G. B., Peugeot, C., Rocheteau, A., Seguis, L., Do, F. C., Galle, S., et al. (2017). Differences in transpiration between a forest and an agroforestry tree species in the Sudanian belt. Agroforestry Systems, 91(3), 403–413.
Abstract: Average population growth in the African Sudanian belt is 3 % per year. This leads to a significant increase in cultivated areas at the expense of fallows and forests. For centuries, rural populations have been practicing agroforestry dominated by Vitellaria paradoxa parklands. We wanted to know whether agroforestry can improve local rainfall recycling as well as forest. We compared transpiration and its seasonal variations between Vitellaria paradoxa, the dominant species in fallows, and Isoberlinia doka, the dominant species in dry forests in the Sudanian belt. The fallow and dry forest we studied are located in northwestern Benin, where average annual rainfall is 1200 mm. Sap flow density (SFD) was measured by transient thermal dissipation, from which tree transpiration was deduced. Transpiration of five trees per species was estimated by taking into account the radial profile of SFD. The effect of the species and of the season on transpiration was tested with a generalized linear mixed model. Over the three-year study period, daily transpiration of the agroforestry trees, V. paradoxa (diameters 8-38 cm) ranged between 4.4 and 26.8 L day(-1) while that of the forest trees, I. doka, (diameters 20-38 cm) ranged from 9.8 to 92.6 L day(-1). Daily transpiration of V. paradoxa was significantly lower (15 %) in the dry season than in the rainy season, whereas daily transpiration by I. doka was significantly higher (13 %) in the dry season than in the rainy season. Our results indicate that the woody cover of agroforestry systems is less efficient in recycling local rainfall than forest cover, not only due to lower tree density but also to species composition.
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Bellier, J., Bontron, G., & Zin, I. (2017). Using Meteorological Analogues for Reordering Postprocessed Precipitation Ensembles in Hydrological Forecasting. Water Resources Research, 53(12), 10085–10107.
Abstract: Meteorological ensemble forecasts are nowadays widely used as input of hydrological models for probabilistic streamflow forecasting. These forcings are frequently biased and have to be statistically postprocessed, using most of the time univariate techniques that apply independently to individual locations, lead times and weather variables. Postprocessed ensemble forecasts therefore need to be reordered so as to reconstruct suitable multivariate dependence structures. The Schaake shuffle and ensemble copula coupling are the two most popular methods for this purpose. This paper proposes two adaptations of them that make use of meteorological analogues for reconstructing spatiotemporal dependence structures of precipitation forecasts. Performances of the original and adapted techniques are compared through a multistep verification experiment using real forecasts from the European Centre for Medium-Range Weather Forecasts. This experiment evaluates not only multivariate precipitation forecasts but also the corresponding streamflow forecasts that derive from hydrological modeling. Results show that the relative performances of the different reordering methods vary depending on the verification step. In particular, the standard Schaake shuffle is found to perform poorly when evaluated on streamflow. This emphasizes the crucial role of the precipitation spatiotemporal dependence structure in hydrological ensemble forecasting.
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Bellier, J., Zin, I., & Bontron, G. (2017). Sample Stratification in Verification of Ensemble Forecasts of Continuous Scalar Variables: Potential Benefits and Pitfalls. Monthly Weather Review, 145(9), 3529–3544.
Abstract: In the verification field, stratification is the process of dividing the sample of forecast-observation pairs into quasi-homogeneous subsets, in order to learn more on how forecasts behave under specific conditions. A general framework for stratification is presented for the case of ensemble forecasts of continuous scalar variables. Distinction is made between forecast-based, observation-based, and external-based stratification, depending on the criterion on which the sample is stratified. The formalism is applied to two widely used verification measures: the continuous ranked probability score (CRPS) and the rank histogram. For both, new graphical representations that synthesize the added information are proposed. Based on the definition of calibration, it is shown that the rank histogram should be used within a forecast-based stratification, while an observation-based stratification leads to significantly nonflat histograms for calibrated forecasts. Nevertheless, as previous studies have warned, statistical artifacts created by a forecast-based stratification may still occur, thus a graphical test to detect them is suggested. To illustrate potential insights about forecast behavior that can be gained from stratification, a numerical example with two different datasets of mean areal precipitation forecasts is presented.
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Braud, I., Desprats, J. F., Ayral, P. A., Bouvier, C., & Vandervaere, J. P. (2017). Mapping topsoil field-saturated hydraulic conductivity from point measurements using different methods. Journal Of Hydrology And Hydromechanics, 65(3), 264–275.
Abstract: Topsoil field-saturated hydraulic conductivity, Kf(s), is a parameter that controls the partition of rainfall between infiltration and runoff and is a key parameter in most distributed hydrological models. There is a mismatch between the scale of local in situ Kf(s) measurements and the scale at which the parameter is required in models for regional mapping. Therefore methods for extrapolating local Kf(s) values to larger mapping units are required. The paper explores the feasibility of mapping Kf(s) in the Cevennes-Vivarais region, in south-east France, using more easily available GIS data concerning geology and land cover. Our analysis makes uses of a data set from infiltration measurements performed in the area and its vicinity for more than ten years. The data set is composed of Kf(s) derived from infiltration measurements performed using various methods: Guelph permeameters, double ring and single ring infiltrotrometers and tension infiltrometers. The different methods resulted in a large variation in Kf(s) up to several orders of magnitude. A method is proposed to pool the data from the different infiltration methods to create an equivalent set of Kf(s). Statistical tests showed significant differences in Kf(s) distributions in function of different geological formations and land cover. Thus the mapping of Kf(s) at regional scale was based on geological formations and land cover. This map was compared to a map based on the Rawls and Brakensiek (RB) pedotransfer function (mainly based on texture) and the two maps showed very different patterns. The RB values did not fit observed equivalent Kf(s) at the local scale, highlighting that soil texture alone is not a good predictor of Kf(s).
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Colliander, A., Jackson, T. J., Bindlish, R., Chan, S., Das, N., Kim, S. B., et al. (2017). Validation of SMAP surface soil moisture products with core validation sites. Remote Sensing Of Environment, 191, 215–231.
Abstract: The NASA Soil Moisture Active Passive (SMAP) mission has utilized a set of core validation sites as the primary methodology in assessing the soil moisture retrieval algorithm performance. Those sites provide well calibrated in situ soil moisture measurements within SMAP product grid pixels for diverse conditions and locations. The estimation of the average soil moisture within the SMAP product grid pixels based on in situ measurements is more reliable when location specific calibration of the sensors has been performed and there is adequate replication over the spatial domain, with an up-scaling function based on analysis using independent estimates of the soil moisture distribution. SMAP fulfilled these requirements through a collaborative Cal/Val Partner program. This paper presents the results from 34 candidate core validation sites for the first eleven months of the SMAP mission. As a result of the screening of the sites prior to the availability of SMAP data, out of the 34 candidate sites 18 sites fulfilled all the requirements at one of the resolution scales (at least). The rest of the sites are used as secondary information in algorithm evaluation. The results indicate that the SMAP radiometer-based soil moisture data product meets its expected performance of 0.04 m(3)/m(3) volumetric soil moisture (unbiased root mean square error); the combined radar-radiometer product is close to its expected performance of 0.04 m(3)/m(3), and the radar-based product meets its target accuracy of 0.06 m(3)/m(3) (the lengths of the combined and radar-based products are truncated to about 10 weeks because of the SMAP radar failure). Upon completing the intensive Cal/Val phase of the mission the SMAP project will continue to enhance the products in the primary and extended geographic domains, in co-operation with the Cal/Val Partners, by continuing the comparisons over the existing core validation sites and inclusion of candidate sites that can address shortcomings. (C) 2017 Elsevier Inc. All rights reserved.
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Dickson, N., Comte, J. - C., Koussoube, Y., Ofterdinger, U., & Vouillamoz, J. - M. (2017). Analysis and numerical modelling of large-scale controls on aquifer structure and hydrogeological properties in the African basement (Benin, West Africa). Geological Society Special Publications, .
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Dupire, S., Curt, T., & Bigot, S. (2017). Spatio-temporal trends in fire weather in the French Alps. Science Of The Total Environment, 595, 801–817.
Abstract: The Alpine area is particularly sensitive to climatic and environmental changes that might impact socioecosystems and modify the regime of natural hazards. Among them, wildfire is of major importance as it threatens both ecosystems and human lives and infrastructures. Wildfires result from complex interactions between available vegetation fuels, climate and weather, and humans who decide of the land use and are the main source of fire ignitions. The changes in fire weather during the past decades are rather unknown in the French Alps especially due to their complex topography. Moreover, local institutions and managers wonder if the ongoing climate changes might increase fire risk and affect the environmental quality and the different ecosystem services provided by the mountain forests. In this context, we used the national forest fires database together with daily meteorological observations from 1959 to 2015 to investigate the changes in wildfire danger in the French Alps. We analysed the spatial and temporal variations in terms of intensity, frequency, seasonality and window of opportunity of two fire weather indices: the fine fuel moisture code (FFMC) and the fire weather index (FWI) that measure the daily water content of vegetation and the potential intensity of fires, respectively. Our results showed a major contrast between Southern Alps with a high fire weather danger on average and a significant increase in the past decades, and Northern Alps with low to moderate danger on average that increased only at low elevations. This study contributes to the understanding of the consequences of ongoings climate changes on wildfires in the French Alps. It produced high resolution results that account for the topographic and climatic variability of the area. Finally, the maps of the different fire weather components have practical implications for fire management and modelling and for preventing indirect effects of fires on ecosystems and human assets. (C) 2017 Elsevier B.V. All rights reserved.
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Ezersky, M. G., Legchenko, A., Eppelbaum, L., & Al-Zoubi, A. (2017). Overview of the geophysical studies in the Dead Sea coastal area related to evaporite karst and recent sinkhole development. International Journal Of Speleology, 46(2), 277–302.
Abstract: Since the early 80s, a progressively increasing number of sinkholes appeared along the Dead Sea coastal line. It has been found that their appearance is strongly correlating with the lowering of the Dead Sea level taking place with the rate of approximately 1 m/yr. Location of areas affected by sinkhole development corresponds to location of the salt formation deposited during the latest Pleistocene, when the Lake Lisan receded to later become the Dead Sea. Water flowing to the Dead Sea from adjacent and underlying aquifers dissolves salt and creates caverns that cause ground subsidence and consequent formation of sinkholes. Before subsidence, these caverns are not visible on the surface but can be investigated with surface geophysical methods. For that, we applied Surface Nuclear Magnetic Resonance (SNMR), Transient Electromagnetic (TEM) Seismic refraction and reflection, Multichannel Analysis of Surface waves (MASW), microgravity and magnetic surveys and their combinations. Our geophysical results allowed us to locate the salt formation and to detect caverns in salt thus contributing to better understanding sinkhole development mechanisms. Comparison of sinkhole appearance along the western DS shore derived from the recent database (2017) shows that predictions made on the base of geophysical data (2005-2008) are now confirmed thus demonstrating efficiency of our study. In this paper, we briefly present a summary of up to date knowledge of the geology and hydrogeology of Dead Sea basin, of the physical properties of the salt rock and the most popular models explaining mechanisms of sinkhole development. We also share our experience gained during geophysical studies carried out in the framework of national and international research projects in this area for the last 20 years.
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Fichez, R., Archundia, D., Grenz, C., Douillet, P., Mendieta, F. G., Moreno, M. O., et al. (2017). Global climate change and local watershed management as potential drivers of salinity variation in a tropical coastal lagoon (Laguna de Terminos, Mexico). Aquatic Sciences, 79(2), 219–230.
Abstract: The wide range of ecological goods and services provided by tropical coastal lagoons and wetlands are under considerable pressure due to the synergistic effects of local anthropogenic impact and global climate change. In transitional waters, salinity is a key driver of ecological processes mostly depending on the balance between marine and river inputs, a balance that can be significantly modified by climate change and by anthropogenic alteration of the watershed. Mesoamerica being considered as a climate change hot-spot and as an ecoregion strongly vulnerable to global change, our study aimed at analyzing the relationship between salinity, river runoff, and rainfall variability in a tropical coastal lagoon and to assess the respective influence of climate change and watershed management. The study focusing on the large and shallow coastal lagoon of Laguna de Terminos in south eastern Mexico established: (1) the variability in salinity distribution along the yearly cycle and the occurrence of a high salinity anomaly period during the wet season of 2009; (2) the relationship between lagoon waters salinity and river inputs further underlying the anomalous situation encountered in 2009; (3) a long term increase in river discharge during the past 60 years, indicating potential salinity decrease in the lagoon during that same period; (4) an absence of any change in rainfall linking the increase in runoff to watershed management rather than long term trend in climate change. Additionally, the specific context of the 2009-2010 Central-Equatorial Pacific El Nio is underlined and the potential relationship between river discharge and ENSO is discussed. Those results should be of significant practical value to decision-makers who are often keen to point the finger at global climate change when local environmental management is also and sometime most significantly responsible.
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Getirana, A., Boone, A., Peugeot, C., & ALMIP2, W. G. (2017). Streamflows over a West African Basin from the ALMIP2 Model Ensemble. Journal of Hydrometeorology, 18(7), 1831–1845.
Abstract: Comparing streamflow simulations against observations has become a straightforward way to evaluate a land surface model�s (LSM) ability in simulating water budget within a catchment. Using a mesoscale river routing scheme (RRS), this study evaluates simulated streamflows over the upper Ouémé River basin resulting from 14 LSMs within the framework of phase 2 of the African Monsoon Multidisciplinary Analysis (AMMA) Land Surface Model Intercomparison Project (ALMIP2). The ALMIP2 RRS (ARTS) has been used to route LSM outputs. ARTS is based on the nonlinear Muskingum�Cunge method and a simple deep water infiltration formulation representing water-table recharge as previously observed in that region. Simulations are performed for the 2005�08 period during which ground observations are largely available. Experiments are designed using different ground-based rainfall datasets derived from two interpolation methods: the Thiessen technique and a combined kriging�Lagrangian methodology. LSM-based total runoff (TR) averages vary from 0.07 to 1.97 mm day−1, while optimal TR was estimated as ~0.65 mm day−1. This highly affected the RRS parameterization and streamflow simulations. Optimal Nash�Sutcliffe coefficients for LSM-averaged streamflows varied from 0.66 to 0.92, depending on the gauge station. However, individual LSM performances show a wider range. A more detailed rainfall distribution provided by the kriging�Lagrangian methodology resulted in overall better streamflow simulations. The early runoff generation related to reduced infiltration rates during early rainfall events features as one of the main reasons for poor LSM performances.
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Grippa, M., Kergoat, L., Boone, A., Peugeot, C., Demarty, J., Cappelaere, B., et al. (2017). Modeling Surface Runoff and Water Fluxes over Contrasted Soils in the Pastoral Sahel: Evaluation of the ALMIP2 Land Surface Models over the Gourma Region in Mali. Journal of Hydrometeorology, 18(7), 1847–1866.
Abstract: Land surface processes play an important role in the West African monsoon variability. In addition, the evolution of hydrological systems in this region, and particularly the increase of surface water and runoff coefficients observed since the 1950s, has had a strong impact on water resources and on the occurrence of floods events. This study addresses results from phase 2 of the African Monsoon Multidisciplinary Analysis (AMMA) Land Surface Model Intercomparison Project (ALMIP2), carried out to evaluate the capability of different state-of-the-art land surface models to reproduce surface processes at the mesoscale. Evaluation of runoff and water fluxes over the Mali site is carried out through comparison with runoff estimations over endorheic watersheds as well as evapotranspiration (ET) measurements. Three remote-sensing-based ET products [ALEXI, MODIS, and Global Land Evaporation Amsterdam Model (GLEAM)] are also analyzed. It is found that, over deep sandy soils, surface runoff is generally overestimated, but the ALMIP2 multimodel mean reproduces in situ measurements of ET and water stress events rather well. However, ALMIP2 models are generally unable to distinguish among the two contrasted hydrological systems typical of the study area. Employing as input a soil map that explicitly represents shallow soils improves the representation of water fluxes for the models that can account for their representation. Shallow soils are shown to be also quite challenging for remote-sensing-based ET products, even if their effect on evaporative loss was captured by the diagnostic thermal-based ALEXI. A better representation of these soils, in soil databases, model parameterizations, and remote sensing algorithms, is fundamental to improve the estimation of water fluxes in this part of the Sahel.
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Guedron, S., Point, D., Acha, D., Bouchet, S., Baya, P. A., Tessier, E., et al. (2017). Mercury contamination level and speciation inventory in Lakes Titicaca & Uru-Uru (Bolivia): Current status and future trends. Environmental Pollution, 231, 262–270.
Abstract: Aquatic ecosystems of the Bolivian Altiplano (similar to 3800 m a.s.l.) are characterized by extreme hydro-climatic constrains (e.g., high UV-radiations and low oxygen) and are under the pressure of increasing anthropogenic activities, unregulated mining, agricultural and urban development. We report here a complete inventory of mercury (Hg) levels and speciation in the water column, atmosphere, sediment and key sentinel organisms (i.e., plankton, fish and birds) of two endorheic Lakes of the same watershed differing with respect to their size, eutrophication and contamination levels. Total Hg (THg) and mono-methylmercury (MMHg) concentrations in filtered water and sediment of Lake Titicaca are in the lowest range of reported levels in other large lakes worldwide. Downstream, Hg levels are 3-10 times higher in the shallow eutrophic Lake Uru-Uru than in Lake Titicaca due to high Hg inputs from the surrounding mining region. High percentages of MMHg were found in the filtered and unfiltered water rising up from <1 to similar to 50% THg from the oligoihetero-trophic Lake Titicaca to the eutrophic Lake Uru-Uru. Such high % MMHg is explained by a high in situ MMHg production in relation to the sulfate rich substrate, the low oxygen levels of the water column, and the stabilization of MMHg due to abundant ligands present in these alkaline waters. Differences in MMHg concentrations in water and sediments compartments between Lake Titicaca and Uru-Uru were found to mirror the offset in MMHg levels that also exist in their respective food webs. This suggests that in situ MMHg baseline production is likely the main factor controlling MMHg levels in fish species consumed by the local population. Finally, the increase of anthropogenic pressure in Lake Titicaca may probably enhance eutrophication processes which favor MMHg production and thus accumulation in water and biota. (C) 2017 Elsevier Ltd. All rights reserved.
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Koïta, M., Yonli, H., Soro, D., Dara, A., & Vouillamoz, J. - M. (2017). Taking into Account the Role of the Weathering Profile in Determining Hydrodynamic Properties of Hard Rock Aquifers. Geosciences, 7(3), 89.
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Lafore, J. - P., Beucher, F., Peyrillé, P., Diongue-Niang, A., Chapelon, N., Bouniol, D., et al. (2017). A multi-scale analysis of the extreme rain event of Ouagadougou in 2009: High-impact weather system, West Africa, African Easterly Waves. Quarterly Journal Of The Royal Meteorological Society, .
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Lafore, J. P., Beucher, F., Peyrille, P., Diongue-Niang, A., Chapelon, N., Bouniol, D., et al. (2017). A multi-scale analysis of the extreme rain event of Ouagadougou in 2009. Quarterly Journal Of The Royal Meteorological Society, 143(709), 3094–3109.
Abstract: This study presents a multi-scale analysis of an extreme rain event that occurred in Burkina Faso on 1 September 2009 with an absolute record of 263 mm rainfall observed at Ouagadougou. This high-impact weather system results from the combination of several favourable ingredients at different scales. The sea-surface temperature anomaly patterns in July-August 2009 of both the Atlantic cold tongue, the Tropical Atlantic Dipole and the Mediterranean Sea are favourable factors for the northward penetration of the West African monsoon. The intense convective activity of the last 10-day period in August is associated with the crossing of a convectively coupled Kelvin wave increasing the African easterly wave (AEW) activity, and of an equatorial Rossby wave. At the synoptic scale this event corresponds to the passage of a train of three AEWs with increasing magnitude. Behind the first AEW trough axis, an intense and deep southerly monsoon burst develops. It contributes to the amplification of the second AEW and its breaking is associated with the formation of an intense meso-vortex on the southern flank of the African easterly jet. Compared to the fast-moving squall line, the dominant type of precipitating weather system over the Sahel, the Ouagadougou precipitating system appears to be a moist vortex propagating slowly, allowing rainfall accumulation, without wind gusts or convective cold pools observed at the surface. The main precipitation area is located about 2 degrees longitude downshear (westward due to the African easterly jet) of the centre of this strong meso-vortex.
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Leauthaud, C., Cappelaere, B., Demarty, J., Guichard, F., Velluet, C., Kergoat, L., et al. (2017). A 60-year reconstructed high-resolution local meteorological data set in Central Sahel (1950-2009): evaluation, analysis and application to land surface modelling. International Journal Of Climatology, 37(5), 2699–2718.
Abstract: The Sahel has experienced strong climate variability in the past decades. Understanding its implications for natural and cultivated ecosystems is pivotal in a context of high population growth and mainly agriculture-based livelihoods. However, efforts to model processes at the land-atmosphere interface are hindered, particularly when the multi-decadal timescale is targeted, as climatic data are scarce, largely incomplete and often unreliable. This study presents the generation of a long-term, high-temporal resolution, multivariate local climatic data set for Niamey, Central Sahel. The continuous series spans the period 1950-2009 at a 30-min timescale and includes ground station-based meteorological variables (precipitation, air temperature, relative and specific humidity, air pressure, wind speed, downwelling long- and short-wave radiation) as well as process-modelled surface fluxes (upwelling long- and short-wave radiation, latent, sensible and soil heat fluxes and surface temperature). A combination of complementary techniques (linear/spline regressions, a multivariate analogue method, artificial neural networks and recursive gap filling) was used to reconstruct missing meteorological data. The complete surface energy budget was then obtained for two dominant land cover types, fallow bush and millet, by applying the meteorological forcing data set to a finely field-calibrated land surface model. Uncertainty in reconstructed data was expressed by means of a stochastic ensemble of plausible historical time series. Climatological statistics were computed at sub-daily to decadal timescales and compared with local, regional and global data sets such as CRU and ERA-Interim. The reconstructed precipitation statistics, approximate to 1 degrees C increase in mean annual temperature from 1950 to 2009, and mean diurnal and annual cycles for all variables were in good agreement with previous studies. The new data set, denoted NAD (Niamey Airport-derived set) and publicly available, can be used to investigate the water and energy cycles in Central Sahel, while the methodology can be applied to reconstruct series at other stations.
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Legchenko, A., Comte, J. C., Ofterdinger, U., Vouillamoz, J. M., Lawson, F. M. A., & Walsh, J. (2017). Joint use of singular value decomposition and Monte-Carlo simulation for estimating uncertainty in surface NMR inversion. Journal Of Applied Geophysics, 144, 28–36.
Abstract: We propose a simple and robust approach for investigating uncertainty in the results of inversion in geophysics. We apply this approach to inversion of Surface Nuclear Magnetic Resonance (SNMR) data, which is also known as Magnetic Resonance Sounding (MRS). Solution of this inverse problem is known to be non-unique. We inverse MRS data using the well-known Tikhonov regularization method, which provides an optimal solution as a trade-off between the stability and accuracy. Then, we perturb this model by random values and compute the fitting error for the perturbed models. The magnitude of these perturbations is limited by the uncertainty estimated with the singular value decomposition (SVD) and taking into account experimental errors. We use 106 perturbed models and show that the large majority of these models, which have all the water content within the variations given by the SVD estimate, do not fit data with an acceptable accuracy. Thus, we may limit the solution space by only the equivalent inverse models that fit data with the accuracy close to that of the initial inverse model. For representing inversion results, we use three equivalent solutions instead of the only one: the “best” solution given by the regularization or other inversion technic and the extreme variations of this solution corresponding to the equivalent models with the minimum and the maximum volume of water. For demonstrating our approach, we use synthetic data sets and experimental data acquired in the framework of investigation of a hard rock aquifer in the Ireland (County Donegal). (C) 2017 Elsevier B.V. All rights reserved.
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Miller, O. L., Solomon, D. K., Miège, C., Koenig, L. S., Forster, R. R., Montgomery, L., et al. (2017). Hydraulic Conductivity of a Firn Aquifer in Southeast Greenland. Frontiers in Earth Science, , 5–38.
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Montgomery, L. N., Schmerr, N., Burdick, S., RR., F., Koenig, L. S., Legchenko, A., et al. (2017). Investigation of Firn Aquifer Structure in Southeastern Greenland Using Active Source Seismology. Frontiers in Earth Science, , 5–10.
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Nord, G., Boudevillain, B., Berne, A., Branger, F., Braud, I., Dramais, G., et al. (2017). A high space-time resolution dataset linking meteorological forcing and hydro-sedimentary response in a mesoscale Mediterranean catchment (Auzon) of the Ardeche region, France. Earth System Science Data, 9(1).
Abstract: A comprehensive hydrometeorological dataset is presented spanning the period 1 January 201131 December 2014 to improve the understanding of the hydrological processes leading to flash floods and the relation between rainfall, runoff, erosion and sediment transport in a mesoscale catchment (Auzon, 116 km(2)) of the Mediterranean region. Badlands are present in the Auzon catchment and well connected to high-gradient channels of bedrock rivers which promotes the transfer of suspended solids downstream. The number of observed variables, the various sensors involved (both in situ and remote) and the space-time resolution (similar to km(2), similar to min) of this comprehensive dataset make it a unique contribution to research communities focused on hydrometeorology, surface hydrology and erosion. Given that rainfall is highly variable in space and time in this region, the observation system enables assessment of the hydrological response to rainfall fields. Indeed, (i) rainfall data are provided by rain gauges (both a research network of 21 rain gauges with a 5 min time step and an operational network of 10 rain gauges with a 5 min or 1 h time step), S-band Doppler dual-polarization radars (1 km(2), 5 min resolution), disdrometers (16 sensors working at 30 s or 1 min time step) and Micro Rain Radars (5 sensors, 100m height resolution). Additionally, during the special observation period (SOP-1) of the HyMeX (Hydrological Cycle in the Mediterranean Experiment) project, two X-band radars provided precipitation measurements at very fine spatial and temporal scales (1 ha, 5 min). (ii) Other meteorological data are taken from the operational surface weather observation stations of Meteo-France (including 2m air temperature, atmospheric pressure, 2 m relative humidity, 10m wind speed and direction, global radiation) at the hourly time resolution (six stations in the region of interest). (iii) The monitoring of surface hydrology and suspended sediment is multi-scale and based on nested catchments. Three hydrometric stations estimate water discharge at a 2-10 min time resolution. Two of these stations also measure additional physico-chemical variables (turbidity, temperature, conductivity) and water samples are collected automatically during floods, allowing further geochemical characterization of water and suspended solids. Two experimental plots monitor overland flow and erosion at 1 min time resolution on a hillslope with vineyard. A network of 11 sensors installed in the intermittent hydrographic network continuously me
asures water level and water temperature in headwater subcatchments (from 0.17 to 116 km(2)) at a time resolution of 2-5 min. A network of soil moisture sensors enables the continuous measurement of soil volumetric water content at 20 min time resolution at 9 sites. Additionally, concomitant observations (soil moisture measurements and stream gauging) were performed during floods between 2012 and 2014. Finally, this dataset is considered appropriate for understanding the rainfall variability in time and space at fine scales, improving areal rainfall estimations and progressing in distributed hydrological and erosion modelling.
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Raynaud, D., Hingray, B., Zin, I., Anquetin, S., Debionne, S., & Vautard, R. (2017). Atmospheric analogues for physically consistent scenarios of surface weather in Europe and Maghreb. International Journal Of Climatology, 37(4), 2160–2176.
Abstract: This study compares the multivariate predictions of daily temperature, temperature range, precipitation, surface wind and solar radiation of a single-model analogue approach with an original multi-model analogy over 12 regions in Europe and Maghreb. Both approaches are based on two-level analogue models where atmospheric predictors are either dynamic or thermodynamic. In the multi-model approach, independent analogue models with predictand-specific predictors are used. In the single-model one, a unique analogue model and its associated set of predictors is applied to all predictands. Testing numerous large-scale predictors, we first identify the best predictor sets for each modelling strategy. Those obtained for the single-model approach are significantly different from those of the predictand-specific models. This is especially the case for local temperature and wind speed. Both methods perform similarly for precipitation, temperature range and radiation. We next assess the ability of both approaches to simulate physically coherent multivariate weather scenarios. With the single-model method, weather scenarios are obtained for each prediction day from observations sampled simultaneously on one analogue day. The physical consistency between variables is thus automatically fulfilled each day. This allows the single-model method to reproduce well the observed inter-predictand correlations, especially the significant correlations between radiation and precipitation and between radiation and temperature range. These results suggest a hybrid analogue model using a single-model for radiation, temperature range and precipitation, combined with a univariate approach for wind. Two options are proposed for temperature for which either the predictand-specific method or a single-model approach with an additional correction are conceivable. This hybrid approach leads to a possible compromise between reasonable univariate prediction skills and realistic inter-predictands correlations, both classically required for many impact studies.
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Reichle, R. H., De Lannoy, G. J. M., Liu, Q., Ardizzone, J. V., Colliander, A., Conaty, A., et al. (2017). Assessment of the SMAP Level-4 Surface and Root-Zone Soil Moisture Product Using In Situ Measurements. Journal Of Hydrometeorology, 18(10), 2621–2645.
Abstract: The Soil Moisture Active Passive (SMAP) mission Level-4 Surface and Root-Zone Soil Moisture (L4SM) data product is generated by assimilating SMAP L-band brightness temperature observations into the NASA Catchment land surface model. The L4SM product is available from 31 March 2015 to present (within 3 days from real time) and provides 3-hourly, global, 9-km resolution estimates of surface (0-5 cm) and root-zone (0-100 cm) soil moisture and land surface conditions. This study presents an overview of the L4SM algorithm, validation approach, and product assessment versus in situ measurements. Core validation sites provide spatially averaged surface (root zone) soil moisture measurements for 43 (17) "reference pixels'' at 9- and 36-km gridcell scales located in 17 (7) distinct watersheds. Sparse networks provide point-scale measurements of surface (root zone) soil moisture at 406 (311) locations. Core validation site results indicate that the L4SM product meets its soil moisture accuracy requirement, specified as an unbiased RMSE (ubRMSE, or standard deviation of the error) of 0.04 m(3) m(-3) or better. The ubRMSE for L4SM surface (root zone) soil moisture is 0.038 m(3) m(-3) (0.030 m(3) m(-3)) at the 9-km scale and 0.035 m(3) m(-3) (0.026 m(3) m(-3)) at the 36-km scale. The L4SM estimates improve (significantly at the 5% level for surface soil moisture) over model-only estimates, which do not benefit from the assimilation of SMAP brightness temperature observations and have a 9-km surface (root zone) ubRMSE of 0.042 m(3) m(-3) (0.032 m(3) m(-3)). Time series correlations exhibit similar relative performance. The sparse network results corroborate these findings over a greater variety of climate and land cover conditions.
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Roman-Cascon, C., Pellarin, T., Gibon, F., Brocca, L., Cosme, E., Crow, W., et al. (2017). Correcting satellite-based precipitation products through SMOS soil moisture data assimilation in two land-surface models of different complexity: API and SURFEX. Remote Sensing Of Environment, 200, 295–310.
Abstract: Global rainfall information is useful for many applications. However, real-time versions of satellite-based rainfall products are known to contain errors. Recent studies have demonstrated how the information about rainfall intrinsically contained in soil moisture data can be utilised for improving rainfall estimates. That is, soil moisture dynamics are impacted for several days by the accumulated amount of rainfall following within a particular event. In this context, soil moisture data from the Soil Moisture Ocean Salinity (SMOS) satellite is used in this study to correct rainfall accumulation estimates provided by satellite-based real-time precipitation products such as CMORPH, TRMM-3B42RT or PERSIANN. An algorithm based on the SMOS measurements data assimilation is tested in two land-surface models of different complexity: a simple hydrological model (Antecedent Precipitation Index (API)) and a more sophisticated state-of-the-art land-surface model (SURFEX (Surface Extemalisee)). We show how the assimilation technique, based on a particle filter method, generally leads to a significant improvement in rainfall estimates, with slightly better results for the simpler (and less computationally demanding) API model. This methodology has been evaluated for six years at ten sites around the world with different land use and climatological features. The results also show the limitations of the methodology in regions highly affected by mountainous terrain, forest or intense radio-frequency interference (RFI), which can notably affect the quality of the retrievals. The satisfactory results shown here invite the future operational application of the methodology in near-real time on a global scale.
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Soro, D. D., Koita, M., Biaou, C. A., Outoumbe, E., Vouillamoz, J. M., Yacouba, H., et al. (2017). Geophysical demonstration of the absence of correlation between lineaments and hydrogeologically usefull fractures: Case study of the Sanon hard rock aquifer (central northern Burkina Faso). Journal Of African Earth Sciences, 129, 842–852.
Abstract: The conceptualization of hard rock aquifers in terms of their geometry and structure has undergone considerable progress over the last two decades. Despite these advances, hydrogeologists are still divided by the models used to describe two central concepts: (i) the influence of weathering processes on hydraulic conductivity; (ii) the influence of tectonics on the hydraulic conductivity of hard rock aquifers. In order to provide further insight into this debate, the present study proposes a conceptual model for hard rock aquifers, based on an integrated hydrogeological and geophysical approach, using information acquired at different scales. The data and observations used for this case study were derived from the Sanon experimental site, located in Burkina Faso, which is presently exposed to a Sudano-Sahelian climate. The methodological approach consisted firstly in developing a description of the site's weathering profile at the scale of a borehole, based on lithologs and electrical resistivity logs. In a second step, the site's ridge to ridge (longitudinal) weathering profile was established from several 2D resistivity sections crossing a maximum number of lineament structures, which in some prior studies were considered to be the superficial manifestation of tectonic fractures. The results show that at that scale the weathering profile is comprised of three main layers, which from top to bottom are referred to as: the saprolite, the fissured layer and the fresh rock. This weathering profile model is consistent with other models proposed in recent years, suggesting that the hydraulic conductivity of hard rock aquifers is a consequence of weathering processes, rather than tectonic fracturing. Tectonic fractures are not visible on the 2D sections of the ridge to ridge profiles, and the lineaments originally thought to be overground representations of tectonic fractures are likely to have different origins. The lack of a substantial correlation between tectonic lineaments and fractures appears to account for the high incidence of negative boreholes in hard rock aquifers, where the siting of drillings has systematically been based on lineament studies and on geophysical studies looking for vertical fractures such as profiling and vertical electrical sounding. There is thus a need to revise current hydrogeological concepts and methodologies to site wells based on tectonic fractures represented by lineaments. (C) 2017 Elsevier Ltd. All rights reserved.
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Taylor, C. M., Belusic, D., Guichard, F., Arker, D. J. P., Vischel, T., Bock, O., et al. (2017). Frequency of extreme Sahelian storms tripled since 1982 in satellite observations. Nature, 544(7651), 475–+.
Abstract: The hydrological cycle is expected to intensify under global warming(1), with studies reporting more frequent extreme rain events in many regions of the world(2-4), and predicting increases in future flood frequency(5). Such early, predominantly mid-latitude observations are essential because of shortcomings within climate models in their depiction of convective rainfall(6,7). A globally important group of intense storms-mesoscale convective systems (MCSs) 8-poses a particular challenge, because they organize dynamically on spatial scales that cannot be resolved by conventional climate models. Here, we use 35 years of satellite observations from the West African Sahel to reveal a persistent increase in the frequency of the most intense MCSs. Sahelian storms are some of the most powerful on the planet(9), and rain gauges in this region have recorded a rise in 'extreme'(17) daily rainfall totals. We find that intense MCS frequency is only weakly related to the multidecadal recovery of Sahel annual rainfall, but is highly correlated with global land temperatures. Analysis of trends across Africa reveals that MCS intensification is limited to a narrow band south of the Sahara desert. During this period, wet-season Sahelian temperatures have not risen, ruling out the possibility that rainfall has intensified in response to locally warmer conditions. On the other hand, the meridional temperature gradient spanning the Sahel has increased in recent decades, consistent with anthropogenic forcing driving enhanced Saharan warming(10). We argue that Saharan warming intensifies convection within Sahelian MCSs through increased wind shear and changes to the Saharan air layer. The meridional gradient is projected to strengthen throughout the twenty-first century, suggesting that the Sahel will experience particularly marked increases in extreme rain. The remarkably rapid intensification of Sahelian MCSs since the 1980s sheds new light on the response of organized tropical convection to global warming, and challenges conventional projections made by general circulation models.
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Valois, R., Vouillamoz, J. M., Lun, S., & Arnout, L. (2017). Assessment of water resources to support the development of irrigation in northwest Cambodia: a water budget approach. Hydrological Sciences Journal-Journal Des Sciences Hydrologiques, 62(11), 1840–1855.
Abstract: Water availability is the primary constraint on the improvement of food security in rural areas in northwestern Cambodia. A 4-year study was carried out in the upper Stung Sreng watershed to assess water resources. Four sub-watersheds with different land cover types, ranging in size from 1.5 to 185km(2), were monitored using dedicated weather stations and rain- and streamgauges. Geophysics and observation boreholes were used to characterize aquifers. Rainwater is mostly split into evapotranspiration (annual mean of 54% rainfall) and streamflow components (49%), because groundwater recharge is low (1%). Thus, rainwater and streamflow are the main sources for irrigation development. Groundwater can be used only in specific locations for low water-demand crops. A total of 186 household ponds and three village-scale dams were built and 31 wells were installed. The household pond was determined to be the best solution for irrigation development because of its simple management.
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Vignon, E., Genthon, C., Barral, H., Amory, C., Picard, G., Gallee, H., et al. (2017). Momentum- and Heat-Flux Parametrization at Dome C, Antarctica: A Sensitivity Study. Boundary-Layer Meteorology, 162(2), 341–367.
Abstract: An extensive meteorological observational dataset at Dome C, East Antarctic Plateau, enabled estimation of the sensitivity of surface momentum and sensible heat fluxes to aerodynamic roughness length and atmospheric stability in this region. Our study reveals that (1) because of the preferential orientation of snow micro-reliefs (sastrugi), the aerodynamic roughness length varies by more than two orders of magnitude depending on the wind direction; consequently, estimating the turbulent fluxes with a realistic but constant of 1 mm leads to a mean friction velocity bias of in near-neutral conditions; (2) the dependence of the ratio of the roughness length for heat to on the roughness Reynolds number is shown to be in reasonable agreement with previous models; (3) the wide range of atmospheric stability at Dome C makes the flux very sensitive to the choice of the stability functions; stability function models presumed to be suitable for stable conditions were evaluated and shown to generally underestimate the dimensionless vertical temperature gradient; as these models differ increasingly with increases in the stability parameter z / L, heat flux and friction velocity relative differences reached when ; (4) the shallowness of the stable boundary layer is responsible for significant sensitivity to the height of the observed temperature and wind data used to estimate the fluxes. Consistent flux results were obtained with atmospheric measurements at heights up to 2 m. Our sensitivity study revealed the need to include a dynamical parametrization of roughness length over Antarctica in climate models and to develop new parametrizations of the surface fluxes in very stable conditions, accounting, for instance, for the divergence in both radiative and turbulent fluxes in the first few metres of the boundary layer.
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Wubda, M., Descloitres, M., Yalo, N., Ribolzi, O., Vouillamoz, J. M., Boukari, M., et al. (2017). Time-lapse electrical surveys to locate infiltration zones in weathered hard rock tropical areas. Journal Of Applied Geophysics, 142, 23–37.
Abstract: In West Africa, infiltration and groundwater recharge processes in hard rock areas are depending on climatic, surface and subsurface conditions, and are poorly documented. Part of the reason is that identification, location and monitoring of these processes is still a challenge. Here, we explore the potential for time-lapse electrical surveys to bring additional information on these processes for two different climate situations: a semi-arid Sahelian site (north of Burkina and a humid Sudanian site (north of Benin), respectively focusing on indirect (localized) and direct (diffuse) recharge processes. The methodology is based on surveys in dry season and rainy season on typical pond or gully using Electrical Resistivity Tomography (ERT) and frequency electromagnetic (FEM) apparent conductivity mapping. The results show that in the Sahelian zone an indirect recharge occurs as expected, but infiltration doesn't takes place at the center of the pond to the aquifer, but occurs laterally in the banks. In Sudanian zone, the ERT survey shows a direct recharge process as expected, but also a complicated behavior of groundwater dilution, as well as the role of hardpans for fast infiltration. These processes are ascertained by groundwater monitoring in adjacent observing wells. At last, FEM time lapse mapping is found to be difficult to quantitatively interpreted due to the non-uniqueness of the model, clearly evidenced comparing FEM result to auger holes monitoring. Finally, we found that time-lapse ERT can be an efficient way to track infiltration processes across ponds and gullies in both climatic conditions, the Sahelian setting providing results easier to interpret, due to significant resistivity contrasts between dry and rain seasons. Both methods can be used for efficient implementation of punctual sensors for complementary studies. However, FEM time-lapse mapping remains difficult to practice without external information that renders this method less attractive for quantitative interpretation purposes. (C) 2017 Elsevier B.V. All rights reserved.
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Ago, E. E., Agbossou, E. K., Cohard, J. M., Galle, S., & Aubinet, M. (2016). Response of CO2 fluxes and productivity to water availability in two contrasting ecosystems in northern Benin (West Africa). Annals Of Forest Science, 73(2), 483–500.
Abstract: CO (2) fluxes were measured during 18 months in a forest and a savannah in northern Benin. Higher values of carbon fluxes were found during the wet season at each site. A strong dependency of carbon fluxes on water relations was found in two contrasting sites. The forest sequestered 640 +/- 50 and the savannah 190 +/- 40 g C m (-2) year (-1) . In West Africa, the main mechanisms or factors governing the dynamics of ecosystems, especially the dynamics of the carbon fluxes and productivity, still remain less known. This study reports the carbon fluxes over two contrasting ecosystems, notably a protected forest (lat 9.79A degrees N, long 1.72A degrees E, alt 414 m) and a cultivated savannah (lat 9.74A degrees N, long 1.60A degrees E, alt 449 m) in northern Benin. The two sites were among those equipped by the AMMA-CATCH observatory and Ou,m, 2025 project. Flux data were analyzed at the daily and seasonal scales in order to understand their controlling variables. We discussed the patterns of CO2 fluxes and the characteristics of the two ecosystems. The study also focused on the different water usage strategies developed by the two ecosystems since the alternation between dry and wet seasons highly influenced the seasonal dynamics. Finally, the annual carbon sequestration was estimated together with its uncertainty. The carbon fluxes were measured during 18 months (July 2008-December 2009) by an eddy-covariance system over two contrasting sites in northern Benin. Fluxes data were computed following the standard procedure. The responses of CO2 fluxes to the principal climatic and edaphic factors, and the canopy conductance were studied. A clear CO2 fluxes response to main environmental factors was observed, however with difference according to the seasons and vegetation types. The ecosystem respiration showed the highest values during the wet season and a progressive decrease from wet to dry periods. Also, the carbon uptake values were high during the wet period, but low during the dry period. However, the CO2 fluxes for the protected forest were always higher than that for the cultivated savannah within each defined period. This was due to the seasonal changes not only in phenology and physiology but also to the acclimation to environmental conditions, especially to the soil water availability. The water use efficiency was influenced by VPD during the day conditions for two ecosystems. However, the VPD response curve of water usage was relatively constant for the protected forest during the transitional and wet seasons. In contrary, for the cultivated savannah the VPD response decreased about 46 % from transitional dry-wet to wet seasons and remained relatively constant until transitional wet-dry season. The close relationships between the net CO2 assimilation and the canopy conductance were found for the two ecosystems. This suggests a regulation of the stomata by a partial stomatal closure besides the radiation control. Finally, the forest sequestered at the annual scale 640 +/- 50 and the savannah 190 +/- 40 g C m(-2) year(-1). We conclude with a strong relation between the carbon fluxes and water in the two investigated ecosystems. Apart from the radiation control, the stomata also play an important role in the regulation of the CO2 assimilation in the two ecosystems.
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Brocca, L., Pellarin, T., Crow, W. T., Ciabatta, L., Massari, C., Ryu, D., et al. (2016). Rainfall estimation by inverting SMOS soil moisture estimates: A comparison of different methods over Australia. Journal Of Geophysical Research-Atmospheres, 121(20), 12062–12079.
Abstract: Remote sensing of soil moisture has reached a level of maturity and accuracy for which the retrieved products can be used to improve hydrological and meteorological applications. In this study, the soil moisture product from the Soil Moisture and Ocean Salinity (SMOS) satellite is used for improving satellite rainfall estimates obtained from the Tropical Rainfall Measuring Mission multisatellite precipitation analysis product (TMPA) using three different bottom up techniques: SM2RAIN, Soil Moisture Analysis Rainfall Tool, and Antecedent Precipitation Index Modification. The implementation of these techniques aims at improving the well-known top down rainfall estimate derived from TMPA products (version 7) available in near real time. Ground observations provided by the Australian Water Availability Project are considered as a separate validation data set. The three algorithms are calibrated against the gauge-corrected TMPA reanalysis product, 3B42, and used for adjusting the TMPA real-time product, 3B42RT, using SMOS soil moisture data. The study area covers the entire Australian continent, and the analysis period ranges from January 2010 to November 2013. Results show that all the SMOS-based rainfall products improve the performance of 3B42RT, even at daily time scale (differently from previous investigations). The major improvements are obtained in terms of estimation of accumulated rainfall with a reduction of the root-mean-square error of more than 25%. Also, in terms of temporal dynamic (correlation) and rainfall detection (categorical scores) the SMOS-based products provide slightly better results with respect to 3B42RT, even though the relative performance between the methods is not always the same. The strengths and weaknesses of each algorithm and the spatial variability of their performances are identified in order to indicate the ways forward for this promising research activity. Results show that the integration of bottom up and top down approaches has the potential to improve the quality of near-real-time rainfall estimates from remote sensing in the near future.
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Casse, C., Gosset, M., Vischel, T., Quantin, G., & Tanimoun, B. A. (2016). Model-based study of the role of rainfall and land use-land cover in the changes in the occurrence and intensity of Niger red floods in Niamey between 1953 and 2012. Hydrology And Earth System Sciences, 20(7), 2841–2859.
Abstract: Since 1950, the Niger River basin has gone through three main climatic periods: a wet period (1950-1960), an extended drought (1970-1980) and since 1990 a recent partial recovery of annual rainfall. Hydrological changes co-occur with these rainfall fluctuations. In most of the basin, the rainfall deficit caused an enhanced discharge deficit, but in the Sahelian region the runoff increased despite the rainfall deficit. Since 2000 the Sahelian part of the Niger has been hit by an increase of flood hazards during the so-called red flood period. In Niamey city, the highest river levels and the longest flooded period ever recorded occurred in 2003, 2010, 2012 and 2013, with heavy casualties and property damage. The reasons for these changes, and the relative role of climate versus land use-land cover (LULC) changes are still debated and are investigated in this paper. The evolution of the Niger red flood in Niamey from 1950 to 2012 is analysed based on long-term records of rainfall (three data sets based on in situ and/or satellite data) and discharge, and a hydrological model. The model is first run with the present LULC conditions in order to analyse solely the effect of rainfall variability. The impact of LULC and drainage area modification is investigated in a second step. The simulations based on the current surface conditions are able to reproduce the observed trend in the red flood occurrence and intensity since the 1980s. This has been verified with three independent rainfall data sets and implies that rainfall variability is the main driver for the red flood intensification observed over the last 30 years. The simulation results since 1953 have revealed that LULC and drainage area changes need to be invoked to explain the changes over a 60-year period.
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Cazenave, F., Gosset, M., Kacou, M., Alcoba, M., Fontaine, E., Duroure, C., et al. (2016). Characterization of Hydrometeors in Sahelian Convective Systems with an X-Band Radar and Comparison with In Situ Measurements. Part I: Sensitivity of Polarimetric Radar Particle Identification Retrieval and Case Study Evaluation. Journal Of Applied Meteorology And Climatology, 55(2), 231–249.
Abstract: The particle identification scheme developed by Dolan and Rutledge for X-band polarimetric radar is tested for the first time in Africa and compared with in situ measurements. The data were acquired during the Megha-Tropiques mission algorithm-validation campaign that occurred in Niger in 2010. The radar classification is compared with the in situ observations gathered by an instrumented aircraft for the 13 August 2010 squall-line case. An original approach has been developed for the radar-in situ comparison: it consists of simulating synthetic radar variables from the microphysical-probe information and comparing the two datasets in a common “radar space.” The consistency between the two types of observation is good considering the differences in sampling illustrated in the paper. The time evolution of the hydrometeor types and their relative proportion in the convective and stratiform regions are analyzed. The farther away from the convection one looks, the more aggregation dominates, riming diminishes, and hydrometeors are less dense. Particle identification based on the polarimetric radar will be applied to a 5-yr African dataset in the future.
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Chikhi, N., Clavier, R., Laurent, J. P., Fichot, F., & Quintard, M. (2016). Pressure drop and average void fraction measurements for two-phase flow through highly permeable porous media. Annals Of Nuclear Energy, 94, 422–432.
Abstract: The modeling of pressure drop for two-phase flows through porous media is a key point to assess the coolability of debris beds resulting from nuclear severe accidents. Models involve several parameters which are non-linear functions of the void fraction, e.g. relative permeabilities. Their identification requires that experimental data include the measurement of void fraction. This paper presents a new technique developed to reach this objective. The method is based on the use of a capacitance probe and has been validated by comparison with a weighing method. The validation has shown that the accuracy is better than 10%. The measurement device has been implemented in the CALIDE facility, at IRSN, which has been designed to-perform air-water flow through debris bed. Tests have been carried out with beds made of single size 4 mm and 8 mm beads. Measurements of pressure drop and average void fraction are reported in the paper, for air and water flow rates representative of flows that would result of either the reflooding of the damaged core or the cooling of corium debris in a stagnant pool of water. Finally, the pressure drop models used in severe accident simulation codes, based on generalizations of the single-phase Ergun law, have been assessed against the new data. It has been observed that generalized Ergun laws including an interfacial drag term accurately predict the pressure drop and the void fraction for flows with a zero net water velocity. (C) 2016 Elsevier Ltd. All rights reserved.
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Dedieu, J. P., Carlson, B. Z., Bigot, S., Sirguey, P., Vionnet, V., & Choler, P. (2016). On the Importance of High-Resolution Time Series of Optical Imagery for Quantifying the Effects of Snow Cover Duration on Alpine Plant Habitat. Remote Sensing, 8(6).
Abstract: We investigated snow cover dynamics using time series of moderate (MODIS) to high (SPOT-4/5, Landsat-8) spatial resolution satellite imagery in a 3700 km(2) region of the southwestern French Alps. Our study was carried out in the context of the SPOT (Take 5) Experiment initiated by the Centre National d'Etudes Spatiales (CNES), with the aim of exploring the utility of high spatial and temporal resolution multispectral satellite imagery for snow cover mapping and applications in alpine ecology. Our three objectives were: (i) to validate remote sensing observations of first snow free day derived from the Normalized Difference Snow Index (NDSI) relative to ground-based measurements; (ii) to generate regional-scale maps of first snow free day and peak standing biomass derived from the Normalized Difference Vegetation Index (NDVI); and (iii) to examine the usefulness of these maps for habitat mapping of herbaceous vegetation communities above the tree line. Imagery showed strong agreement with ground-based measurements of snow melt-out date, although R-2 was higher for SPOT and Landsat time series (0.92) than for MODIS (0.79). Uncertainty surrounding estimates of first snow free day was lower in the case of MODIS, however (+/- 3 days as compared to +/- 9 days for SPOT and Landsat), emphasizing the importance of high temporal as well as high spatial resolution for capturing local differences in snow cover duration. The main floristic differences between plant communities were clearly visible in a two-dimensional habitat template defined by the first snow free day and NDVI at peak standing biomass, and these differences were accentuated when axes were derived from high spatial resolution imagery. Our work demonstrates the enhanced potential of high spatial and temporal resolution multispectral imagery for quantifying snow cover duration and plant phenology in temperate mountain regions, and opens new avenues to examine to what extent plant community diversity and functioning are controlled by snow cover duration.
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Dieppois, B., Lawler, D. M., Slonosky, V., Massei, N., Bigot, S., Fournier, M., et al. (2016). Multidecadal climate variability over northern France during the past 500 years and its relation to large-scale atmospheric circulation. International Journal Of Climatology, 36(15), 4679–4696.
Abstract: We examine secular changes and multidecadal climate variability on a seasonal scale in northern France over the last 500 years and examine the extent to which they are driven by large-scale atmospheric variability. Multiscale trend analysis and segmentation procedures show statistically significant increases of winter and spring precipitation amounts in Paris since the end of the 19th century. This changes the seasonal precipitation distribution from one with a pronounced summer peak at the end of the Little Ice Age to an almost uniform distribution in the 20th century. This switch is linked to an early warming trend in winter temperature. Changes in spring precipitation are also correlated with winter precipitation for time scales greater than 50 years, which suggests a seasonal persistence. Hydrological modelling results show similar rising trends in river flow for the Seine at Paris. However, such secular trends in the seasonal climatic conditions over northern France are substantially modulated by irregular multidecadal (50-80 years) fluctuations. Furthermore, since the end of the 19th century, we find an increasing variance in multidecadal hydroclimatic winter and spring, and this coincides with an increase in the multidecadal North Atlantic Oscillation (NAO) variability, suggesting a significant influence of large-scale atmospheric circulation patterns. However, multidecadal NAO variability has decreased in summer. Using Empirical Orthogonal Function analysis, we detect multidecadal North Atlantic sea-level pressure anomalies, which are significantly linked to the NAO during the Modern period. In particular, a south-eastward (south-westward) shift of the Icelandic Low (Azores High) drives substantial multidecadal changes in spring. Wetter springs are likely to be driven by potential changes in moisture advection from the Atlantic, in response to northward shifts of North Atlantic storm tracks over European regions, linked to periods of positive NAO. Similar, but smaller, changes in rainfall are observed in winter.
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Dupire, S., Bourrier, F., Monnet, J. - M., Bigot, S., Borgniet, L., Berger, F., et al. (2016). Novel quantitative indicators to characterize the protective effect of mountain forests against rockfall. Ecological Indicators, 67, 98–107.
Abstract: Natural hazards are frequent in mountain areas where they regularly cause casualties and damages to human infrastructures. Mountain forests contribute in mitigating these hazards, in particular rockfalls. Assessing the protective effect of a forest against rockfall is a difficult task for both forest managers and rockfall experts. Accurate and simple tools are therefore required to efficiently evaluate the level of protection that results from the presence of forest. This study defines three novel indicators to quantify the protective effect of forests against rockfalls, regarding (1) the reduction of the frequency of rockfalls, (2) the reduction of their maximum intensity, and (3) the combination of the reduction of the frequency and the energy of the rocks. The first two indicators are relevant for rockfall experts whereas the third is mostly interesting for foresters as it summarizes the protective effect of forest. The Rockyfor3D model was adapted and used to simulate rockfalls propagation on 3886 different forest stands located in all the French Alps. The results of the simulations were used to calculate the three indicators for each forest stand. Finally, the relations between the forest structures and compositions and the indicators values were investigated. Our principal result shows that only three forest characteristics are required to accurately predict the indicators and evaluate the protective level of a forest against rockfall. The two first variables correspond to the basal area and the mean diameter at breast height (DBH) of the forest stand which are two parameters commonly used by forest managers. The third characteristic is the length of forest in the maximum slope direction which can be computed with a geographic information system (GIS). The method proposed in this study is easily reproducible and is suitable to evaluate the protective effect of European mountain forests at different scales. At local scale, the proposed indicators can enrich rockfall studies in which forests are usually set aside to simplify the evaluation. Moreover, the indicators may find direct applications with foresters by allowing them to identify the protective level of their forest and consequently to adapt their management. Finally, the indicators are convenient to perform spatial analysis and produce maps of the protective effect of mountain forests that could find many applications in land settlement or evaluation of ecosystem services. (C) 2016 Elsevier Ltd. All rights reserved.
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Dupire, S., Bourrier, F., Monnet, J. - M., Bigot, S., Borgniet, L., Berger, F., et al. (2016). The protective effect of forests against rockfalls across the French Alps: Influence of forest diversity. Forest Ecology and Management, 382, 269–279.
Abstract: The role of forests in the mitigation of natural hazards has been repeatedly demonstrated. The protective effect of mountain forests against rockfalls has especially been pointed out because it can constitute a natural and cost-effective protection measure in many situations. However, this particular ecosystem service may substantially differ according to the structure and the composition of the forest. Until now, the rockfall protection capability has always been studied at a local scale with only few forest types. Moreover, the comparison of the protective effect of the different forest types studied remains difficult because different methods and indicators were used. For the same reasons, it is not possible to draw conclusions about the influence of biological and structural diversities on the protection capabilities of forests from former works. The aims of this study were (1) to quantitatively assess the protective effect of forests at the French Alps scale and build a classification based on the protection capability, (2) to compare the protective effect of the different forest types present in the French Alps and (3) to analyze the relations between the protective effect and the forest diversity in terms of stand structure and tree composition. For this purpose, the model Rockyfor3D was used to simulate the propagation of rocks on 3886 different forest plots spread over the whole French Alps. Quantitative indicators characterizing the protective effect of each forest plot were then calculated from the simulation results and used to perform the different analyses. Our results emphasized the importance of taking into account the length of forest in the maximum slope direction for an accurate assessment of the protective effect. Thus, the minimum length of forest to get a reduction of 99% of the rockfall hazard was chosen as indicator to compare protective effect between forests. Using this indicator, half of the French Alpine forests presented a high level of protection after a short forested slope (190m). A decreasing gradient in the protection capabilities was observed from forest types dominated by broadleaved species to those dominated by conifer species. Moreover, considering an equivalent proportion of conifers, stands dominated by shade-tolerant tree species showed better ability to reduce rockfall hazard. Finally, our study highlighted that a high biodiversity and a structural heterogeneity within the forest have a positive effect on the reduction of rockfalls hazard.
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Gond, V., Dubiez, E., Boulogne, M., Gigaud, M., Peroches, A., Pennec, A., et al. (2016). Forest cover and carbon stock change dynamics in the Democratic Republic of Congo: Case of the wood-fuel supply basin of Kinshasa. Bois Et Forets Des Tropiques, (327), 19–28.
Abstract: To contribute to the development of methods for the sustainable management of forest ecosystems in Central Africa, the following research question was addressed: can analyses of land cover change help to understand and document the spatial organization and mechanisms of forest degradation? To answer this question in the Democratic Republic of Congo, the Makala project mapped the tree and forest resources of Kinshasa's wood-fuel supply basin and attempted to predict how they would evolve in the future. Maps were made for four periods (1984, 2001, 2006 and 2012) using a mosaic of four LANDSAT images. The above-ground biomass was estimated in 2012 using forest inventories in 317 plots distributed across the four types of plant cover found on the Bateke plateau (4,337 trees and 44 species were recorded). This inventory data combined with the satellite data allowed us to make the first comprehensive assessment of above-ground biomass in the study area. Between 2000 and 2012, the average volume of wood-fuel fell by more than 50%. Over the entire period studied (1984 to 2012), carbon stocks fell by 75%. In the wood-fuel supply area for Kinshasa, the drastic loss of forest cover, shortened fallow periods, savannah encroachment and the decline of biomass and carbon stocks are clear signals of degradation. However, these initial estimations were derived from a small sample that was extrapolated to the entire supply area. It would be very useful to increase sampling in order to obtain more accurate and realistic figures. The experience of the Makala project clearly shows that the analysis of land cover change helps to understand and document the spatial organization and mechanisms of forest degradation. However, only with a sound wood-fuel resource policy and sustainable community land management, combined with very dynamic tree reintroduction on agricultural land, will it be possible to initiate a sustainable process of restoration.
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Hector, B., & Hinderer, J. (2016). pyGrav, a Python-based program for handling and processing relative gravity data. Computers & Geosciences, 91, 90–97.
Abstract: pyGrav is a Python-based open-source software dedicated to the complete processing of relative-gravity data. It is particularly suited for time-lapse gravity surveys where high precision is sought. Its purpose is to bind together single-task processing codes in a user-friendly interface for handy and fast treatment of raw gravity data from many stations of a network. The intuitive object-based implementation allows to easily integrate additional functions (reading/writing routines, processing schemes, data plots) related to the appropriate object (a station, a loop, or a survey). This makes pyGrav an evolving tool. Raw data can be corrected for tides and air pressure effects. The data selection step features a double table-plot graphical window with either manual or automatic selection according to specific thresholds on data channels (tilts, gravity values, gravity standard deviation, duration of measurements, etc.). Instrumental drifts and gravity residuals are obtained by least square analysis of the dataset. This first step leads to the gravity simple differences between a reference point and any point of the network. When different repetitions of the network are done, the software computes then the gravity double differences and associated errors. The program has been tested on two specific case studies: a large dataset acquired for the study of water storage changes on a small catchment in West Africa, and a dataset operated and processed by several different users for geothermal studies in northern Alsace, France. In both cases, pyGrav proved to be an efficient and easy-to-use solution for the effective processing of relative-gravity data. (C) 2016 Elsevier Ltd. All rights reserved.
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Kerr, Y. H., Al-Yaari, A., Rodriguez-Fernandez, N., Parrens, M., Molero, B., Leroux, D., et al. (2016). Overview of SMOS performance in terms of global soil moisture monitoring after six years in operation. Remote Sensing Of Environment, 180, 40–63.
Abstract: The Soil Moisture and Ocean Salinity satellite (SMOS) was launched in November 2009 and started delivering data in January 2010. The commissioning phase ended in May 2010. Subsequently, the satellite has been in operation for over six years while the retrieval algorithms from Level 1 (L1) to Level 2 (L2) underwent significant evolutions as knowledge improved. Moreover, other approaches for retrieval at L2 over land were investigated while Level 3 (L3) and Level 4 (L4) were initiated. In this paper, these improvements were assessed by inter comparisons of the current L2 (V620) against the previous version (V551) and new products (using neural networks referred to as SMOS-NN) and 1.3 (referred to as SMOS-L3). In addition, a global evaluation of different SMOS soil moisture (SM) products (SMOS-L2, SMOS-L3, and SMOS-NN) was performed comparing products with those of model simulations and other satellites. Finally, all products were evaluated against in situ measurements of soil moisture (SM). To achieve such a goal a set of metrics to evaluate different satellite products are suggested. The study demonstrated that the V620 shows a significant improvement (including those at L1 improving L2) with respect to the earlier version V551. Results also show that neural network based approaches can often yield excellent results over areas where other products are poor. Finally, global compa'rison indicates that SMOS behaves very well when compared to other sensors/approaches and gives consistent results over all surfaces from very dry (African Sahel, Arizona), to wet (tropical rain forests). RFI (Radio Frequency Interference) is still an issue even though detection has been greatly improved through the significant reduction of RFI sources in several areas of the world. When compared to other satellite products, the analysis shows that SMOS achieves its expected goals and is globally consistent over different eco climate regions from low to high latitudes and throughout the seasons. (C) 2016 Elsevier Inc All rights reserved.
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Legchenko, A., Vouillamoz, J. M., Lawson, F. M. A., Alle, C., Descloitres, M., & Boucher, M. (2016). Interpretation of magnetic resonance measurements in the varying earth's magnetic field. Geophysics, 81(4), WB23–WB31.
Abstract: At the scale of a magnetic resonance sounding (MRS) field setup, the earth's magnetic field in the subsurface may vary laterally with depth and over time. These variations can be caused by different natural factors and generally cannot be compensated for by accurate tuning of the measuring device. The varying geomagnetic field (GMF) causes nonresonance conditions of excitation that affect the amplitude and phase of the MRS signal. Usually, variations of the GMF do not exceed a few hertz and their effect on the amplitude is relatively small, permitting us to assume near-resonance conditions for inversion. However, in some cases, the results may be erroneous if a varying GMF is not taken into account. Motivated by possible improvements in MRS inversion, we have developed a procedure for measuring and interpreting MRS data that considers a varying GMF. Our results showed that it is relatively easy to take a time-varying GMF into account. As a demonstration, we have developed the inversion of MRS data measured in Benin (West Africa). A depth-varying GMF is a more complex problem, and to consider this, we have developed an algorithm of nonlinear inversion. We have tested this approach on synthetic data, which resulted in an improved inversion. Field validation of this procedure awaits the discovery of a suitable test site with known variations of the earth's magnetic field in the subsurface.
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Leroux, D. J., Pellarin, T., Vischel, T., Cohard, J. M., Gascon, T., Gibon, F., et al. (2016). Assimilation of SMOS soil moisture into a distributed hydrological model and impacts on the water cycle variables over the Ouémé catchment in Benin. Hydrol. Earth Syst. Sci., 20(7), 2827–2840.
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Makris, A., Prieur, C., Vischel, T., Quantin, G., Lebel, T., & Roca, R. (2016). Stochastic tracking of mesoscale convective systems: evaluation in the West African Sahel. Stochastic Environmental Research And Risk Assessment, 30(2), 681–691.
Abstract: In this work we apply a recently proposed Bayesian multiple target tracking model to mesoscale convective systems tracking. This stochastic model follows the multiple hypothesis tracking paradigm and can handle a varying number of targets while detecting the target birth, death, split, and merge events. The model is tested experimentally with real MCS targets detected from meteosat IR data over the Sahelian region. The performance of the stochastic tracking is evaluated by comparing it qualitatively and quantitatively with well established deterministic methods.
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Malam-Abdou, M., Vandervaere, J. P., Bouzou-Moussa, I., Descroix, L., Mamadou, I., & Faran-Maiga, O. (2016). Flow production within two small crystalline basement catchments of Western Niger: multiscale standpoint for the hydrodynamical functionning. Geomorphologie-Relief Processus Environnement, 22(4), 363–375.
Abstract: The observed increase of the catchments flow values in the Sahel during the drought years has exacerbated the interest for the studies on the superficial hydrodynamic properties of soils and their impact onto runoff volumes. This work focuses on the processes responsible for the flow production within two experimental catchments situated in the granitic basement of Western Niger. It aims at characterizing the hydrodynamical functioning at three nested spatial scales. At point scale (50 cm(2)), soil surface infiltrability (0-10 cm) is determined for different surface features through hydraulic conductivity (Ks) measurements carried out in situ. At the plot scale, runoff is measured from 10 m(2) experimental plots while at catchment scale (5 ha), stream flow is controlled by stream gauges at the exutories. Measurements show low values of hydraulic conductivity ranging from 10 mm.h(-1) (minimal value measured on erosion crusts ERO) to 40 mm.h(-1) (maximal value measured on cultivated sites in seasonal average). Runoff coefficients are inversely ranked, from 0.60 on ERO to 0.25 on cultivated surface. Catchment scale runoff coefficients are 0.41 and 0.28 for the not cultivated and the cultivated basins, respectively. Results obtained at these three scales are consistent and reveal the strong runoff production ability of these granitic catchments resulting from both low point scale infiltrability and small re-infiltration of runoff water within the stream network. This last component represents only 4% of annual rainfall while it may reach more than 50% for sedimentary catchments of similar size. Within these catchments, thus, a 2-4 mm rainfall is sufficient to generate a flow at the outlet due to rather small (less than 20 cm) sand deposits covering the stream beds. Finally, both at the plot and at the catchment scales, hydrodynamical functioning was found quasi independent on the initial water content.
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Mamadou, O., Galle, S., Cohard, J. M., Peugeot, C., Kounouhewa, B., Biron, R., et al. (2016). Dynamics of water vapor and energy exchanges above two contrasting Sudanian climate ecosystems in Northern Benin (West Africa). Journal Of Geophysical Research-Atmospheres, 121(19), 11269–11286.
Abstract: Natural ecosystems in sub-Saharan Africa are experiencing intense changes that will probably modify land surface feedbacks and consequently the regional climate. In this study, we have analyzed water vapor (Q(LE)) and sensible heat (Q(H)) fluxes over a woodland (Bellefoungou, BE) and a cultivated area (Nalohou, NA) in the Sudanian climate of Northern Benin, using 2years (from July 2008 to June 2010) of eddy covariance measurements. The evaporative fraction (EF) response to environmental and surface variables was investigated at seasonal scale. Soil moisture was found to be the main environmental factor controlling energy partitioning. During the wet seasons, EF was rather stable with an average of 0.750.07 over the woodland and 0.700.025 over the cultivated area. This means that 70-75% of the available energy was changed into actual evapotranspiration during the investigated wet seasons depending on the vegetation type. The cumulative annual actual evapotranspiration (AET) varied between 73050mmyr(-1) at the NA site and 104070mmyr(-1) at the BE site. With similar weather conditions at the two sites, the BE site showed 30% higher AET values than the NA site. The sensible heat flux Q(H) at the cultivated site was always higher than that of the woodland site, but observed differences were much less than those of Q(LE). In a land surface conversion context, these differences are expected to impact both atmospheric dynamics and the hydrological cycle.
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Mazzilli, N., Boucher, M., Chalikakis, K., Legchenko, A., Jourde, H., & Champollion, C. (2016). Contribution of magnetic resonance soundings for characterizing water storage in the unsaturated zone of karst aquifers. Geophysics, 81(4), WB49–WB61.
Abstract: Understanding the role of the unsaturated zone in aquifer recharge and contaminant attenuation processes is a major challenge for the protection and management of karstic water resources. We present the potential of the magnetic resonance soundings (MRS) geophysical method for characterizing the vadose zone of karst aquifers composed of epikarst and infiltration layers. To investigate the hydraulic functioning of the Durzon karst system located on the Larzac plateau (southern France), we used the MRS method at 16 sites. TheMRS results have been compared with available geologic information and to core water content measurements. The remarkable spatial variability of the MRS response observed in the study area makes it possible to determine ranges of water storage properties in relation to the lithology of the investigated carbonate formations (dolomite, marly, and siliceous limestone). All soundings found either constant or increasing MRS water content with depth, which demonstrates that the infiltration zone might be the major water storage entity for permanent water storage, with important consequences for recharge quality and quantity. These results show the feasibility and potential of the MRS method for the characterization of the karst unsaturated zone and for understanding the vertical distribution of water content, which impacts the overall functioning of karst systems.
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Mourre, L., Condom, T., Junquas, C., Lebel, T., Sicart, J. E., Figueroa, R., et al. (2016). Spatio-temporal assessment of WRF, TRMM and in situ precipitation data in a tropical mountain environment (Cordillera Blanca, Peru). Hydrology And Earth System Sciences, 20(1), 125–141.
Abstract: The estimation of precipitation over the broad range of scales of interest for climatologists, meteorologists and hydrologists is challenging at high altitudes of tropical regions, where the spatial variability of precipitation is important while in situ measurements remain scarce largely due to operational constraints. Three different types of rainfall products – ground based (kriging interpolation), satellite derived (TRMM3B42), and atmospheric model outputs (WRF – Weather Research and Forecasting) – are compared for 1 hydrological year in order to retrieve rainfall patterns at timescales ranging from sub-daily to annual over a watershed of approximately 10 000 km(2) in Peru. An ensemble of three different spatial resolutions is considered for the comparison (27, 9 and 3 km), as long as well as a range of timescales (annual totals, daily rainfall patterns, diurnal cycle). WRF simulations largely overestimate the annual totals, especially at low spatial resolution, while reproducing correctly the diurnal cycle and locating the spots of heavy rainfall more realistically than either the ground-based KED or the Tropical Rainfall Measuring Mission (TRMM) products. The main weakness of kriged products is the production of annual rainfall maxima over the summit rather than on the slopes, mainly due to a lack of in situ data above 3800 ma.s.l. This study also confirms that one limitation of TRMM is its poor performance over ice-covered areas because ice on the ground behaves in a similar way as rain or ice drops in the atmosphere in terms of scattering the microwave energy. While all three products are able to correctly represent the spatial rainfall patterns at the annual scale, it not surprisingly turns out that none of them meets the challenge of representing both accumulated quantities of precipitation and frequency of occurrence at the short timescales (sub-daily and daily) required for glacio-hydrological studies in this region. It is concluded that new methods should be used to merge various rainfall products so as to make the most of their respective strengths.
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Pellarin, T., Mialon, A., Biron, R., Coulaud, C., Gibon, F., Kerr, Y., et al. (2016). Three years of L-band brightness temperature measurements in a mountainous area: Topography, vegetation and snowmelt issues. Remote Sensing Of Environment, 180, 85–98.
Abstract: L-band passive measurements (1.4 GHz) over continental areas are known to be related to surface soil moisture. Two satellite missions were recently launched to measure land surface emissions at this frequency band (SMOS-Soil Moisture and Ocean Salinity in 2009 and SMAP-Soil Moisture Active/Passive in 2015). In order to improve soil moisture retrievals from satellite data, ground-based radiometer systems operating at the same frequency were deployed over specified areas to investigate the L-band emission of various land covers under various climatological conditions. In this study, three years of L-band passive measurements from a radiometer installed on top of a steep mountain in the French Alps were analyzed and compared to L-band passive simulations. The innovative radiometer location led to large footprints due to the distance between the radiometer and the area under study. This experiment also produced microwave measurements affected by various potential difficulties typically encountered in SMOS/SMAP satellite missions: topography, heterogeneous footprints, dry/wet snow events, dew and vegetation litter. Based on in situ and modeling data, this paper investigates the potential of a radiative transfer model (L-band Microwave Emission of the Biosphere, L-MEB) to simulate L-band measurements and analyzes the differences with ELBARA observations. First, it was found that the topography generated a mixing of the horizontal and vertical polarizations. In addition, a large positive bias was found on ELBARA measurements (31 K and 12 K in horizontal and vertical polarizations respectively). Investigations showed that the sky reflection measured by the radiometer was partially substituted by land reflection coming from the surrounding topography. Second, the low-vegetation emission was investigated and highlighted the inability of the MODIS NDVI product to correctly represent the vegetation dynamics. Finally, dry snow conditions were found to have non-negligible impact at L-band and a particular signature was found during snow melting periods, with potential applications at the SMOS/SMAP spatial scales (similar to 40 km). (C) 2016 Elsevier Inc. All rights reserved.
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Philippon, N., de Lapparent, B., Gond, V., Seze, G., Martiny, N., Camberlin, P., et al. (2016). Analysis of the diurnal cycles for a better understanding of the mean annual cycle of forests greenness in Central Africa. Agricultural And Forest Meteorology, 223, 81–94.
Abstract: Central Africa hosts the world's second largest tropical forest after the Amazonian basin. However, as compared to its Amazonian counterpart, the Central Africa forests receive much less rain (similar to 1500 mm/year in mean spread over two rainy seasons in March-May and September-November, and two dry seasons). They also experience a slower deforestation rate, so that the main threat for the next decades might come from climate variations. Nonetheless, their response to the annual cycle of solar radiation and rainfall/clouds is still poorly known. Analysing high resolution remote sensing data of Enhanced Vegetation Index, rainfall, cloudiness, and solar radiation for a target region located between 0 and 5 degrees N and 12-19 degrees E, we explore the climatic drivers of the forests greenness mean annual cycle. Three main points emerge; first, the diurnal cycle is a key-scale for understanding the mean annual cycles of rainfall and incoming solar radiation at surface, then how climate shapes the greenness mean annual evolution; second, neither the two dry seasons nor the two rainy seasons resemble each other in terms of cloud cover, solar radiation and rainfall, and their links with greenness levels; third, whereas the first rainy season (March-May) appears optimal for greenness especially because of favorable light conditions, water availability is the main controlling factor during the main dry season and at the start of the first vegetative season (February). Regarding the little dry season (mid-June-mid-August) and the second rainy season (September-October), light availability might be the main limiting factor. These findings pave the way for further studies of the climate interannual variability and change impacts on the Central Africa forests, taking into account time-scale interactions. (C) 2016 Elsevier B.V. All rights reserved.
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Ringard, J., Dieppois, B., Rome, S., Diedhiou, A., Pellarin, T., Konare, A., et al. (2016). The intensification of thermal extremes in west Africa. Global And Planetary Change, 139, 66–77.
Abstract: This study aims in filling the gap in understanding the relationship between trend and extreme in diurnal and nocturnal temperatures (Tx and Tn) over the Gulf of Guinea area and the Sahel. Time-evolution and trend of Tx and Tn anomalies, extreme temperatures and heat waves are examined using regional and station-based indices over the 1900-2012 and 1950-2012 periods respectively. In investigating extreme temperature anomalies and heat waves, a percentile method is used. At the regional and local scales, rising trends in Tx and Tn anomalies, which appear more pronounced over the past 60 years, are identified over the two regions. The trends are characterized by an intensification of: i) nocturnal/Tn warming over the second half of the 20th century; and ii) diurnal/Tx warming over the post-1980s. This is the same scheme with extreme warm days and warm nights. Finally annual number of diurnal and nocturnal heat waves has increase over the Gulf of Guinea coastal regions over the second half of the 20th century, and even more substantially over the post-1980s period. Although this trend in extreme warm days and nights is always overestimated in the simulations, from the Coupled Model Intercomparison Project Phase 5 (CMIP5), those models display rising trends whatever the scenario, which are likely to be more and more pronounced over the two regions in the next 50 years. (C) 2016 Elsevier B.V. All rights reserved.
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Vouillamoz, J. M., Valois, R., Lun, S., Caron, D., & Arnout, L. (2016). Can groundwater secure drinking-water supply and supplementary irrigation in new settlements of North-West Cambodia? Hydrogeology Journal, 24(1), 195–209.
Abstract: Since the end of the Cambodian Civil War in 1998, the population of the Oddar Meanchey province has drastically increased despite the lack of adequate infrastructure, including basic amenities such as drinking-water supply. To improve the access to drinking water, governmental and aid agencies have focussed on drilling shallow boreholes. The use of groundwater for irrigation is also a growing concern to cope with the occasional late arrival of the rainy season or to produce food during the dry season. Since the groundwater resource in the province has not been documented, a 4-year study was undertaken (2011-2014), aiming to estimate the capability of groundwater to supply domestic needs and supplementary irrigation for rice production. Aquifer properties were estimated by combined use of hydrogeological techniques with the geophysical magnetic resonance sounding method. Groundwater storage and recharge were estimated based on new developments in the application of the geophysical method for quantifying specific yield. The median groundwater storage of the targeted sandstone aquifer is 173 mm, the recharge is diffuse and annually ranges from 10 to 70 mm, and the transmissivity is low to medium. Simulations of pumping indicate that the aquifer can easily supply 100 L of drinking water per capita daily, even considering the estimated population in 2030. However, the shallow aquifer can generally not deliver enough water to irrigate paddy fields of several hectares during a 2-month delay in the onset of the monsoon.
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Abdou, M. M., Vandervaere, J. P., Descroix, L., Moussa, I. B., Maiga, O. F., Abdou, S., et al. (2015). Evolution of the hydraulic conductivity of a cultivated sandy soil in West Niger. Biotechnologie Agronomie Societe Et Environnement, 19(3), 270–280.
Abstract: Description of the subject. The present study aims to present and analyze the evolution of the hydraulic conductivity of soils in cultivated and fallow areas in the loamy-sand superficial soils within the granitic basement region of West Niger. To this end, experimental plots considered representative of the area were chosen in the Sahelian cultivated area. Objectives. Our objective was to evidence the effect of cultural practices and of both wet hoeing and dry hoeing on this evolution and their consequences in terms of soil infiltrability and runoff. Method. A tension disc infiltrometer was used together with a pair of minitensiometers in one-dimensional flow geometry. Results. Measurements were carried out and variations in total rainfall were calculated, allowing us to determine and quantify the effects of the following on the evolution of conductivity: (i) hoeing, (ii) the level of soil moisture prior to this operation and (iii) the lack of cultivation in fallow areas. Hydraulic conductivity was consistently found to be minimal at the surface. Conductivity in the fallow field was stable at 20 mm. h(-1). In the cultivated zones, conductivity was very high (120 mm. h-1) after hoeing in wet conditions, decreasing to the fallow value after 70 mm of rain and even down to half of this value after 230 mm of rain. Hoeing in dry conditions showed poor efficiency, both in terms of conductivity and the duration of the effect. Conclusions. The benefits of hoeing were found to be only short-lived, with the task needing to be repeated after 100 mm of rain.
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Amogu, O., Esteves, M., Vandervaere, J. P., Abdou, M. M., Panthou, G., Rajot, J. L., et al. (2015). Runoff evolution due to land-use change in a small Sahelian catchment. Hydrological Sciences Journal-Journal Des Sciences Hydrologiques, 60(1), 78–95.
Abstract: Land-use changes have been significant these last decades in West Africa, particularly in the Sahel region; in this area, climatic and demographic factors have led to a rise in cropped areas in recent decades causing strong changes in the water cycle and in river regimes. This study compares the rainfall-runoff relationships for two periods (1991-1994 and 2004-2011) in two small and similar neighbouring Sahelian catchments (approx 0.1 km(2) each). This allows identification of the different hydrological consequences of land-use/land-cover change, particularly the fallow shortening and the consequent degradation of topsoil. The main land surface change is a 75% increase in crusted soil area. Runoff increased by more than 20% on average between the two periods while flood duration decreased by 50% on average. However, runoff values remained largely constant in the lower part of the northern basin due to a strong increase in in-channel infiltration. Editor D. Koutsoyiannis; Associate editor T. Wagener
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Boucher, M. A., Perreault, L., Anctil, F., & Favre, A. C. (2015). Exploratory analysis of statistical post-processing methods for hydrological ensemble forecasts. Hydrological Processes, 29(6), 1141–1155.
Abstract: Despite many recent improvements, ensemble forecast systems for streamflow often produce under-dispersed predictive distributions. This situation is problematic for their operational use in water resources management. Many options exist for post-processing of raw forecasts. However, most of these have been developed using meteorological variables such as temperature, which displays characteristics very different from streamflow. In addition, streamflow data series are often very short or contain numerous gaps, thus compromising the estimation of post-processing statistical parameters. For operational use, a post-processing method has to be effective while remaining as simple as possible. We compared existing post-processing methods using normally distributed and gamma-distributed synthetic datasets. To reflect situations encountered with ensemble forecasts of daily streamflow, four normal distribution parameterizations and six gamma distribution parameterizations were used. Three kernel-based approaches were tested, namely, the best member' method and two improvements thereof, and one regression-based approach. Additional tests were performed to assess the ability of post-processing methods to cope with short calibration series, missing values or small numbers of ensemble members. We thus found that over-dispersion is best corrected by the regression method, while under-dispersion is best corrected by kernel-based methods. This work also shows key limitations associated with short data series, missing values, asymmetry and bias. One of the improved best member methods required longer series for the estimation of post-processing parameters, but if provided with adequate information, yielded the best improvement of the continuous ranked probability score. These results suggest guidelines for future studies involving real operational datasets. Copyright (c) 2014 John Wiley & Sons, Ltd.
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Drigeard, E., Fontaine, E., Wobrock, W., Schwarzenbock, A., Duroure, C., Williams, E. R., et al. (2015). A Comparison of Airborne In Situ Cloud Microphysical Measurement with Ground-Based C-Band Radar Observations in Deep Stratiform Regions of African Squall Lines. Journal Of Applied Meteorology And Climatology, 54(12), 2461–2477.
Abstract: This study addresses clouds with significant ice water content (IWC) in the stratiform regions downwind of the convective cores of African squall lines in the framework of the French-Indian satellite Megha-Tropiques project, observed in August 2010 next to Niamey (13.5 degrees N, 2 degrees E) in the southwestern part of Niger. The objectives included comparing the IWC-Z reflectivity relationship for precipitation radars in deep stratiform anvils, collocating reflectivity observed from ground radar with the calculated reflectivity from in situ microphysics for all aircraft locations inside the radar range, and interpreting the role of large ice crystals in the reflectivity of centimeter radars through analysis of their microphysical characteristics as ice crystals larger than 5 mm frequently occurred. It was found that, in the range of 20-30 dBZ, IWC and C-band reflectivity are not really correlated. Cloud regions with high IWC caused by important crystal number concentrations can lead to the same reflectivity factor as cloud regions with low IWC formed by a few millimeter-sized ice crystals.
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Gascon, T., Vischel, T., Lebel, T., Quantin, G., Pellarin, T., Quatela, V., et al. (2015). Influence of rainfall space-time variability over the Ouémé basin in Benin. Proceedings of the International Association of Hydrological Sciences, 368, 102–107.
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Genthon, P., Hector, B., Luxereau, A., Descloitres, M., Abdou, H., Hinderer, J., et al. (2015). Groundwater recharge by Sahelian rivers-consequences for agricultural development: example from the lower Komadugu Yobe River (Eastern Niger, Lake Chad Basin). Environmental Earth Sciences, 74(2), 1291–1302.
Abstract: In the Sahel region, temporary rivers and ponds constitute green spaces of welfare where sustainable development requires parsimonious management of water resources. The Komadugu Yobe valley in Eastern Niger is presented here as an example case of recent agricultural development based on irrigated pepper cropping. Piezometric maps indicate there that the river recharges the upper Quaternary aquifer. A simplified numerical model allows to quantify the exchanges between the river and the aquifer, based mainly on exploration geophysics data and three piezometric records, covering between 1 and 3 years of our 4 years modeling period. Recharge at the valley axis is modeled with a linear river coefficient constant for each hydrological year. The main findings are that: (i) during dry years, the river is disconnected from the aquifer and separated from it by a layer of unsaturated medium, (ii) however, this effect can be reversed, such as during the 2010-2011 wet year or after the Sahelian drought event of the 1980s and 1990s, (iii) the infiltration rate from the river amounts to 30-40 % of its total discharge and to at least four times the abstraction for pepper irrigation along its 150 km lower course at the Niger-Nigeria boundary, which implies that neither the aquifer recharge nor the river discharge is at risk due to the present agricultural development. Similar modeling near temporary river axes could provide some help in water resource management in the Sahel.
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Hector, B., Seguis, L., Hinderer, J., Cohard, J. - M., Wubda, M., Descloitres, M., et al. (2015). Water storage changes as a marker for base flow generation processes in a tropical humid basement catchment (Benin): Insights from hybrid gravimetry. Water Resources Research, 51(10), 8331–8361.
Abstract: In basement catchments of subhumid West Africa, base flow is the main component of annual streamflow. However, the important heterogeneity of lithology hinders the understanding of base flow generation processes. Since these processes are linked with water storage changes (WSCs) across the catchment, we propose the use of hybrid gravity data in addition to neutron probe-derived water content and water levels to monitor spatiotemporal WSC of a typical crystalline basement headwater catchment (16 ha) in Benin. WSC behaviors are shown to provide insights into hydrological processes in terms of water redistribution toward the catchment outlet. Hybrid gravimetry produces gravity change observations from time-lapse microgravity surveys coupled with gravity changes monitored at a base station using a superconducting gravimeter and/or an absolute gravimeter. A dense microgravity campaign (70 surveys of 14 stations) covering three contrasted years was set up with a rigorous protocol, leading to low uncertainties (< 2.5 mu Gal) on station gravity determinations (with respect to the network reference station). Empirical orthogonal function analyses of both gravity changes and WSCs from neutron probe data show similar spatial patterns in the seasonal signal. Areas where storage and water table show a capping behavior (when data reach a plateau during the wet season), suggesting threshold-governed fast subsurface redistribution, are identified. This observed storage dynamics, together with geological structures investigated by electrical resistivity tomography and drill log analysis, make it possible to derive a conceptual model for the catchment hydrology.
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Issoufou, H. B. A., Rambal, S., Le Dantec, V., Oi, M., Laurent, J. P., Saadou, M., et al. (2015). Is the WBE model appropriate for semi-arid shrubs subjected to clear cutting? Tree Physiology, 35(2), 197–208.
Abstract: It is crucial to understand the adaptive mechanisms of woody plants facing periodic drought to assess their vulnerability to the increasing climate variability predicted in the Sahel. Guiera senegalensis J.F.Gmel is a semi-evergreen Combretaceae commonly found in Sahelian rangelands, fallows and crop fields because of its value as an agroforestry species. We compared canopy leafing, and allometric measurements of leaf area, stem area and stem length and their relationships with leaf water potential, stomatal conductance (g(s)) and soil-to-leaf hydraulic conductance (KS-L), in mature and current-year resprouts of G. senegalensis in Sahelian Niger. In mature shrubs, seasonal drought reduced the ratio of leaf area to cross-sectional stem area (A(L) : A(S)), mainly due to leaf shedding. The canopy of the current-year resprouts remained permanently leafed as the shrubs produced leaves and stems continuously, and their A(L) : A(S) ratio increased throughout the dry season. Their KS-L increased, whereas g(s) decreased. West, Brown and Enquist's (WBE) model can thus describe allometric trends in the seasonal life cycle of undisturbed mature shrubs, but not that of resprouts. Annual clear cutting drives allometric scaling relationships away from theoretical WBE predictions in the current-year resprouts, with scaling exponents 2.5 times greater than those of mature shrubs. High KS-L (twice that of mature shrubs) supports this intensive regeneration process. The adaptive strategy described here is probably common to many woody species that have to cope with both severe seasonal drought and regular disturbance over the long term.
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Lee, J. H., Pellarin, T., & Kerr, Y. H. (2015). EnOI Optimization for SMOS Soil Moisture Over West Africa. Ieee Journal Of Selected Topics In Applied Earth Observations And Remote Sensing, 8(4), 1821–1829.
Abstract: In land surface or numerical weather prediction (NWP) models, a soil moisture initialization scheme is important not to drift the prognostic variables to errors. We propose a novel approach for a stationary data assimilation scheme of ensemble optimal interpolation (EnOI) effective for soil moisture and ocean salinity (SMOS) soil moisture initialization. For the optimization of EnOI, the satellite retrieval error specification was conducted rather than ensemble evolution. As combining two ensembles generated from a satellite retrieval and a land surface model, this approach is termed as “two-step EnOI” in this study: (first step) the SMOS soil moisture retrieval ensembles (i.e., errors in brightness temperature, landscape, and geophysical parameters) were merged with SMOS L3 data; (second step) the data assimilation result from the first step was further used for the observations of the EnOI. This two-step EnOI was compared with a sequential ensemble Kalman filter (EnKF) evolving model state ensembles over time but assuming global constant a priori random errors for the SMOS observations. The point-scale comparison results showed that two-step EnOI was better matched with the field measurements than the SMOS L3 data and a sequential ensemble KF scheme. On meso-scale, a spatial average of two-step EnOI reached that of a sequential ensemble KF with the significantly reduced ensemble size. These results suggest that the performance of two-step EnOI is comparable to a sequential ensemble KF but computationally more effective. From this, it is illustrated that appropriate error specification of satellite retrieval is more important than a sequential evolution of model state ensembles, and brightness temperature ensemble mean can reduce the SMOS retrieval biases without sequential evolution.
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Louvet, S., Pellarin, T., al Bitar, A., Cappelaere, B., Galle, S., Grippa, M., et al. (2015). SMOS soil moisture product evaluation over West-Africa from local to regional scale. Remote Sensing Of Environment, 156, 383–394.
Abstract: This paper assessed the SMOS soil moisture values from Level 3 (SMOS L3SM) product provided by the French CNES-CATDS. The evaluation was conducted at the local scale through comparison with ground-based soil moisture measurements acquired in Mali, Niger and Benin from 2010 to 2012. The SMOS L3SM product was compared to three other satellite-based soil moisture products. It was found that, in average over the three sites, the SMOS L3SM product provided the best coefficients of correlation and the lowest root mean square errors (RMSE). The second part of the paper is devoted to retrieve soil moisture estimates between successive SMOS soil moisture measurements in order to increase the temporal resolution. The result of the methodology allows obtaining 3-hour soil moisture mapping over West Africa with a coefficient of correlation greater than 0.82, and an RMSE lower than 0.030 m(3) m(-3) in Niger and Mali and lower than 0.044 m(3) m(-3) in Benin. (C) 2014 Elsevier Inc. All rights reserved.
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Mamadou, I., Gautier, E., Descroix, L., Noma, I., Moussa, I. B., Maiga, O. F., et al. (2015). Exorheism growth as an explanation of increasing flooding in the Sahel. Catena, 131, 130–139.
Abstract: For two decades, the Niamey area, in Niger, has undergone the creation of several new wadis (“koris” in hausa, the most spoken language in West Africa). The significant runoff increase in the Sahelian reach of Niger river makes us interested in the behavior of the basins of the tributary koris of Niger River in the Niamey area, in Niger. These koris generally formed during a single storm event, within depressions previously occupied by ponds; these ponds are overflown creating a new “kori”. This study examines in detail the causes of this new exorheism mechanism. The main explanation of this evolution has been determined as being the strong runoff increase, related to an extension of crusted soils due to agricultural practices, mostly the reduction of fallow duration. The degradation of their structural stability leads to crusting and a strong reduction of their hydraulic conductivity. This is linked to water and sediment balance at the catchment scale. (C) 2015 Elsevier B.V. All rights reserved.
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Mialon, A., Richaume, P., Leroux, D., Bircher, S., Al Bitar, A., Pellarin, T., et al. (2015). Comparison of Dobson and Mironov Dielectric Models in the SMOS Soil Moisture Retrieval Algorithm. Ieee Transactions On Geoscience And Remote Sensing, 53(6), 3084–3094.
Abstract: The Soil Moisture and Ocean Salinity (SMOS) mission provides global surface soil moisture over the continental land surfaces. The retrieval algorithm is based on the comparison between the observations of the L-band (1.4 GHz) brightness temperatures (TB) and the simulated TB data using the L-band Microwave Emission of the Biosphere (L-MEB) model. The L-MEB model includes a dielectric model for the computation of the soil dielectric constant. Since the beginning of the mission, the Dobson model has been used in the operational SMOS algorithm. Recently, a new model of the soil dielectric constant has been developed by Mironov et al. and is now considered. This paper is the first evaluation of these two models based on the actual SMOS observations. First, both Dobson and Mironov models were modified to ensure that the SMOS retrieval algorithm converges to realistic soil moisture retrievals (symmetrization for negative soil moisture values was applied). Second, soil moisture was retrieved over several sites using both Dobson and Mironov models to compute the soil dielectric constant and were compared with in situ measurements. At a global scale, the use of the Mironov model leads to higher retrieved soil moisture than when using the Dobson model (0.033 m(3)/m(3) on average). However, the comparisons of the two model output with in situ measurements over various test sites do not demonstrate a superior performance of one model over the other.
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Panthou, G., Vischel, T., Lebel, T., Blanchet, J., Quantin, G., & Ali, A. (2015). Estimation de cartes d'aléa pluviométrique en Afrique de l'Ouest : comparaison de différentes approches. La Houille Blanche, (6), 42–48.
Abstract: La Houille Blanche – Revue internationale de l eau, est une publication de la Société Hydrotechnique de France
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Panthou, G., Vischel, T., Lebel, T., Blanchet, J., Quantin, G., & Ali, A. (2015). Mapping rainfall return level in West Africa: comparison of different approaches. Houille Blanche-Revue Internationale De L Eau, (6), 42–48.
Abstract: In a world of increasing exposure of populations to natural hazards, the mapping of extreme rainfall remains a key subject of study. Such maps are required for both flood risk management and civil engineering structure design, the challenge being to take into account the local information provided by point rainfall series as well as the necessity of some regional coherency. Two approaches based on the extreme value theory are compared here, with an application to extreme rainfall mapping in West Africa. The first approach is a local fit and interpolation (LFI) consisting of a spatial interpolation of the generalized extreme value (GEV) distribution parameters estimated independently at each station. The second approach is a spatial maximum likelihood estimation (SMLE); it directly estimates the GEV distribution over the entire region by a single maximum likelihood fit using jointly all measurements combined with spatial covariates. Five LFI and three SMLE methods are considered, using the information provided by 126 daily rainfall series covering the period 1950-1990. The methods are first evaluated in calibration. Then the predictive skills and the robustness are assessed through a cross-validation and an independent network validation process. The SMLE approach, especially when using the mean annual rainfall as covariate, appears to perform better for most of the scores computed. Using the Niamey 104 year time series, it is also shown that the SMLE approach has the capacity to deal more efficiently with the effect of local outliers by using the spatial information provided by nearby stations.
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Panthou, G., Vischel, T., Lebel, T., Quantin, G., & Ali, A. (2015). Caractérisation de la structure spatio – temporelle des pluies extrêmes : estimation de courbes IDSF pour la région de Niamey. La Houille Blanche, (3), 58–63.
Abstract: La Houille Blanche – Revue internationale de l eau, est une publication de la Société Hydrotechnique de France
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Panthou, G., Vischel, T., Lebel, T., Quantin, G., & Ali, A. (2015). Space-time structure characterization of extreme rainfall: IDAF curves estimation in the Sahel. Houille Blanche-Revue Internationale De L Eau, (3), 58–63.
Abstract: West Africa is known for having experienced major drought events, but during the last decades numerous floods and exceptional inundations have also struck the region. The flood management is now a major concern for West African countries. Floods can occur at different temporal and spatial scales associated either with meso-scale convective systems that can generate exceptional rainfall totals over a small surface area (a few tens of km(2)) during a short period of time (a few hours), or with 5 to 20 days rainfall accumulations over a large part of the region that cause unusual flooding over large scale watersheds. Intensity-Duration-Area-Frequency (IDAF) curves are interesting tools for two reasons: they are useful for hydraulic structures design as they provide estimates of the return level of heavy rains for several temporal and spatial aggregations and they are helpful to characterize the severity of storms. Obtaining such curves from rainfall networks requires long series, high spatial density and high time-frequency of records. In West Africa, such characteristics are provided by the AMMA-CATCH Niger network. This network is located in the Niamey region where 30 recording rain-gauges (5 minutes series) have operated since 1990 over a 16000 km(2) area. In this study, the IDAF curves are obtained by separately considering the time (IDF) and the spatial (ARF) scales. Annual maxima intensities are extracted for different spatial and temporal resolutions. The IDF model used is based on the concept of scale invariance (simple scaling) to normalize the different temporal resolution of maxima series to which a global GEV is fitted. This parsimonious framework allows using the concept of dynamic scaling to describe the ARF. The IDAF curves obtained describe the distribution of extreme rainfall for time resolutions ranging from 1 hour to 24 hours and space scales between 1 km2 and 2500 km(2).
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Robert, C., Conan, J. M., Mugnier, L. M., & Cohard, J. M. (2015). Near ground results of the CO-SLIDAR C 2 n profiler. Journal of Physics: Conference Series, 595(1), 012030.
Abstract: CO-SLIDAR jointly uses the slopes and scintillation indices from a double source recorded with a Shack-Hartmann (SH) wavefront sensor which leads to a robust C 2 n profile restoration. This technique coupled to a 0.35-m telescope in the mid-IR has provided horizontal C 2 n profiles over 2.7 km with a 300 m resolution. The SH wavefront sensor data revealed a very good fit of the spatial spectra of the phase to the Kolmogorov model, and of the scintillation distributions to the small perturbation regime as well. Quantitatively the inversion of the measurement covariances allows us to retrieve the C 2 n profile with small error bars and stable structure. These C distributed estimations are comparable with the averaged C 2 n measurements from scintillometers set in parallel.
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Vouillamoz, J. M., Lawson, F. M. A., Yalo, N., & Descloitres, M. (2015). Groundwater in hard rocks of Benin: Regional storage and buffer capacity in the face of change. Journal Of Hydrology, 520, 379–386.
Abstract: Groundwater plays a major role in supplying domestic water to millions of people in Africa. In the future, the ability to increase reliable water supplies for domestic and possibly irrigation purposes will depend on groundwater development. Groundwater storage is a key property because it controls the buffering behavior of the aquifer as it is subjected to time-varying conditions such as increased pumping or land-use change. However, quantitative knowledge of groundwater storage in Africa is very limited. This lack of knowledge is a major concern in hard rocks, which cover about 40% of the surface area of Africa. This paper presents a unique quantitative assessment of groundwater storage in different types of hard rocks and a first estimate of the capacity of hard rock aquifers to buffer changes in climatic and anthropogenic conditions. Our study area in Benin (West Africa) is composed of various grades of metamorphic rocks. We used the latest developments in the application of the magnetic resonance geophysical method to confront the methodological difficulty of quantifying groundwater storage. We successfully conducted 38 magnetic-resonance measurements in eight (8) different geological units; each measurement was quantitatively interpreted in terms of groundwater storage. We determined the groundwater storage of our study area to be 440 mm 70 mm (equivalent water thickness). To assess the buffer capacity of aquifers, we compared groundwater storage to groundwater discharge. Groundwater discharge is the sum of natural discharge plus human abstraction. We estimated natural discharge (i.e. deep drainage plus evapotranspiration) from water table fluctuations monitored in six (6) piezometers. Human abstraction was calculated based on the number of operating boreholes and their average daily abstraction. We found that human abstraction (0.34 mm/year +/- 0.07 mm) is far less than natural discharge (108 mm/ year 58 mm). We conclude that increased abstraction due to population growth will probably have a smaller impact on storage than observed land-use change, which may lead to a change in the evapotranspiration rate. We calculated buffer capacity as the ratio of current storage to total discharge, and obtained a result of 6 years +/- 47 months. This buffer capacity confirms groundwater's ability to buffer changes. Finally, our study is intended to promote a more quantitative approach to assessing groundwater resources in Africa and to support our ability to adapt to current and future changes. (C) 2014 Elsevier B.V. All rights reserved.
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