2018 |
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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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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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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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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2017 |
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Nguyen, K. L., Robert, C., Conan, J. M., Mugnier, L., Cohard, J. M., Irvine, M., et al. (2017). Measurement of the spatial distribution of atmospheric turbulence with SCINDAR on a mosaic of urban surfaces. (104250l). SPIE Remote Sensing. Warsaw, Poland.
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2016 |
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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Cohard, J. M. (2016). Contribution a l'étude du cycle de l'eau en milieu complexe : Observation et modélisation de la variabilité spatiale et temporelle de l'évapotranspiration. Habilitation thesis, Université Grenoble Alpes, Grenoble.
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Galle, S., Peugeot, C., Grippa, M., Chaffard, V., Afouda, S., Agbossou, E. K., et al. (2016). AMMA-CATCH : un observatoire hydrologique, météorologique et écologique de long terme en Afrique de l'Ouest : résultats importants et données disponibles. Colloque Risques, Catastrophes, Vulnérabilités et Adaptation dans les Pays de l'Afrique de l'Ouest :. Cotonou (BEN).
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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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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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Paci, A., Staquet, C., Allard, J., Barral, H., Canut, G., Cohard, J. M., et al. (2016). La campagne Passy-2015 : dynamique atmosphérique et qualité de l'air dans la vallée de l'Arve. Pollution atmosphérique, 231-232(8), 271–289.
Abstract: Wintertime anticyclonic conditions, associated with clear sky and cold nights, trigger the formation of persistent layers of stable air over the ground. In an urban area, these persistent layers lead to poor air quality, especially when the terrain is mountainous. This is particularly the case in the Arve River Valley near the city of Passy, located 20 km downstream of Chamonix-Mont-Blanc, where air quality stands among the poorest ones in France.
Beyond the monitoring of air quality, as performed by the Auvergne-Rhône-Alpes air quality agency or within the scientific project DECOMBIO led by the Institute for Geosciences and the Environment (IGE), knowledge of the atmospheric dynamics at the valley scale should be gained to understand how pollutants are dispersed. This is the motivation of the Passy project, which started in 2014. It relies on the Passy-2015 field experiment, whereof presentation, along with the discussion of a few results, is the purpose of the present paper. The objective of this field experiment is to document the atmospheric dynamics in the Arve River Valley during wintertime pollution episodes.
The work conducted during the Passy project and the analysis of the Passy-2015 field experiment will benefit from a several-year long collaboration among the different partners. The knowledge thus gained will contribute to refine weather forecast and air quality prediction in the Arve River Valley and, more generally, in mountain urban areas under stable conditions. From an operational perspective, our goal is to improve our ability to forecast critical events such as low temperatures, ice and fog formation, pollution events or locations subject to high pollutant concentration.
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2015 |
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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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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2014 |
Ago, E. E., Agbossou, E. K., Galle, S., Cohard, J. M., Heinesch, B., & Aubinet, M. (2014). Long term observations of carbon dioxide exchange over cultivated savanna under a Sudanian climate in Benin (West Africa). Agricultural And Forest Meteorology, 197, 13–25.
Abstract: Turbulent CO2 exchanges between a cultivated Sudanian savanna and the atmosphere were measured during 29 months (August 2007-December 2009) by an eddy-covariance system in North-Western Benin, West Africa. The site (Lat 9.74 degrees N, Long 1.60 degrees E, Alt: 449 m) is the one of three sites fitted out by the international AMMA-CATCH program. The flux station footprint area is mainly composed of herbs and crops with some sparse trees and shrubs. Fluxes data were completed by an inventory of dominating species around the tower and the meteorological measurements. Flux response to climatic and edaphic factors was studied. Water was found the main controlling factor of ecosystem dynamics: much larger uptake was found in wet than dry season. During wet season, a very clear answer of net CO2 fluxes to photosynthetic photon fluxes density (PPFD) was observed. A low limitation in response to saturation deficit and soil water variability was however observed. The total ecosystem respiration (TER) was found highly dependent on soil moisture below 0.1 m(3) m(-3), but saturates above this threshold. The average annual carbon sequestration was 232 +/- 27 gC m(-2) with its inter-annual variability mainly controlled by TER. Finally, the ecosystem appeared more efficient during morning and wet season than during afternoon and dry period. (C) 2014 Elsevier B.V. All rights reserved.
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Lohou, F., Kergoat, L., Guichard, F., Boone, A., Cappelaere, B., Cohard, J. M., et al. (2014). Surface response to rain events throughout the West African monsoon. Atmospheric Chemistry And Physics, 14(8), 3883–3898.
Abstract: This study analyses the response of the continental surface to rain events, taking advantage of the long-term near-surface measurements over different vegetation types at different latitudes, acquired during the African Monsoon Multidisciplinary Analysis (AMMA) by the AMMA-CATCH observing system. The simulated surface response by nine land surface models involved in AMMA Land Model Intercomparison Project (ALMIP), is compared to the observations. The surface response, described via the evaporative fraction (EF), evolves in two steps: the immediate surface response (corresponding to an increase of EF occurring immediately after the rain) and the surface recovery (characterized by a decrease of EF over several days after the rain). It is shown that, for all the experimental sites, the immediate surface response is mainly dependent on the soil moisture content and the recovery period follows an exponential relationship whose rate is strongly dependent on the vegetation type (from 1 day over bare soil to 70 days over forest) and plant functional type (below and above 10 days for annual and perennial plants, respectively). The ALMIP model ensemble depicts a broad range of relationships between EF and soil moisture, with the worst results for the drier sites (high latitudes). The land surface models tend to simulate a realistic surface recovery for vegetated sites, but a slower and more variable EF decrease is simulated over bare soil than observed.
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Mamadou, O., Cohard, J. M., Galle, S., Awanou, C. N., Diedhiou, A., Kounouhewa, B., et al. (2014). Energy fluxes and surface characteristics over a cultivated area in Benin: daily and seasonal dynamics. Hydrology And Earth System Sciences, 18(3), 893–914.
Abstract: Latent and sensible heat surface fluxes are key factors of the western African monsoon dynamics. However, few long-term observations of these land surface fluxes are available; these are needed to increase understanding of the underlying processes and assess their impacts on the energy and water cycles at the surface-atmosphere interface. This study analyzes turbulent fluxes of one full year, measured with the eddy covariance technique, over a cultivated area in northern Benin (western Africa). The study site is part of the long-term AMMA-CATCH (African Monsoon Multidisciplinary Analysis-Coupling of the Tropical Atmosphere and Hydrological Cycle) hydrological observatory. The flux partitioning was investigated through the evaporative fraction (EF) and the Bowen ratio (beta) at both seasonal and daily scales. Finally, the surface conductance (G(s)) and the decoupling coefficient (Omega) were calculated and compared with specific bare soil or canopy models. Four contrasting seasons were identified and characterized by their typical daily energy cycles. The results pointed out the contrasting seasonal variations of sensible and latent heat fluxes due to changing atmospheric and surface conditions. In the dry season, the sensible heat fluxes were largely dominant (beta similar to 10) and a low but significant evapotranspiration was measured (EF = 0.08); this was attributed to a few neighboring bushes, possibly fed by the water table. During the wet season, after the monsoon onset, surface conditions barely affected the evaporative fraction (EF), which remained steady (EF = 0.75); the latent heat flux was dominant and the Bowen ration (beta) was about 0.4. During the dry-to-wet and wet-to-dry transition seasons, both EF and beta were highly variable, as they depended on the atmospheric forcing or the response to isolated rains. A complete surface-atmosphere decoupling was never observed in 2008 (0 < Omega < 0.6), which suggests a systematic mixing of the air within the canopy with the atmospheric surface layer, irrespective of the atmospheric conditions and the vegetation height. Modeling approaches showed a good agreement of soil resistance with the Sakaguchi bare soil model. Canopy conductance was also well reproduced with the Ball-Berry stomata model. We showed that the skin surface temperature had a large seasonal and daily amplitude, and played a major role in all the surface processes. Consequently, an accurate modeling of the surface temperature is crucial to represent correctly the energy and water budgets for this region.
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Robert, C., Conan, J. - M., Mugnier, L., & Cohard, J. M. (2014). Results of the CO-SLIDAR Cn2 profiler on the sky and on the ground. IOP conference series. Durham.
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2013 |
Richard, A., Galle, S., Descloitres, M., Cohard, J. M., Vandervaere, J. P., Seguis, L., et al. (2013). Interplay of riparian forest and groundwater in the hillslope hydrology of Sudanian West Africa (northern Benin). Hydrology And Earth System Sciences, 17(12), 5079–5096.
Abstract: Forests are thought to play an important role in the regional dynamics of the West African monsoon, through their capacity to extract water from a permanent and deep groundwater table to the atmosphere even during the dry season. It should be the case for riparian forests too, as these streambank forests are key landscape elements in Sudanian West Africa. The interplay of riparian forest and groundwater in the local hydrodynamics was investigated, by quantifying their contribution to the water balance. Field observations from a comprehensively instrumented hillslope in northern Benin were used. Particular attention was paid to measurements of actual evapotranspiration, soil water and deep groundwater levels. A vertical 2-D hydrological modelling approach using the Hydrus software was used as a testing tool to understand the interactions between the riparian area and the groundwater. The model was calibrated and evaluated using a multi-criteria approach (reference simulation). A virtual experiment, including three other simulations, was designed (no forest, no groundwater, neither forest nor groundwater). The model correctly simulated the hydrodynamics of the hillslope regarding vadose zone dynamics, deep groundwater fluctuation and actual evapotranspiration dynamics. The virtual experiment showed that the riparian forest transpiration depleted the deep groundwater table level and disconnected it from the river, which is consistent with the observations. The riparian forest and the deep groundwater table actually form an interacting transpiration system: the high transpiration rate in the riparian area was shown to be due to the existence of the water table, supplied by downslope lateral water flows within the hillslope soil layer. The simulated riparian transpiration rate was practically steady all year long, around 7.6 mm d(-1). This rate lies within high-end values of similar study results. The riparian forest as simulated here contributes to 37% of the annual hillslope transpiration, and reaches 57% in the dry season, whereas it only covers 5% of the hillslope area.
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2012 |
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Guichard, F., Kergoat, L., Taylor, C. M., Cappelaere, B., Chong, M., Cohard, J. M., et al. (2012). Interactions entre surface et convection au Sahel.
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Guyot, A., Cohard, J. M., Anquetin, S., & Galle, S. (2012). Long-term observations of turbulent fluxes over heterogeneous vegetation using scintillometry and additional observations: A contribution to AMMA under Sudano-Sahelian climate. Agricultural And Forest Meteorology, 154, 84–98.
Abstract: Based on a 3-year period of infrared scintillometry, soil and meteorological measurements, this study presents an analysis of the surface energy balance partitioning over a heterogeneous savannah, in the Sudano-Sahelian region. The site is located in Northern Benin, meso-site of the African Monsoon Multidisciplinary Analyses (AMMA) project. The 3-year period enables an analysis of several alternate dry and wet periods, as well as the intermediate dry-to-wet and wet-to-dry periods. Infrared scintillometry, coupled with measurements of the available energy (net radiation minus ground heat flux) and a careful analysis of the aerodynamic properties of the scintillometer footprint, are employed to provide robust estimates of the turbulent (sensible and latent heat) fluxes over complex terrain, in terms of the topography and in terms of the spatially and temporally heterogeneous vegetation cover. A characterization of the uncertainties on each term of the energy balance is given at the scale of the scintillometer footprint. These uncertainties strongly depend on the season for the residual latent heat flux. Results point out that the climate of the Sudano-Sahelian region is characterized by a strong seasonal cycle and inter-annual variability, related to changing atmospheric and land surface conditions. The evaporative fraction is found to be relatively constant during the wet period (0.67) and more variable during the dry and intermediate periods. In addition, sensible heat flux and net radiation are well correlated during the dry season. The diurnal cycle shows a predominance of evaporation during the wet season and sensible heat during the dry season. Results point a significant latent heat flux during the dry period, signature of persistent vegetation in the Sudano-Sahelian region. Finally, that data set at hourly time step would provide useful information for modelling and the parameterization of the associated processes for this region. (C) 2011 Elsevier B.V. All rights reserved.
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Tanguy, M., Baille, A., Gonzalez-Real, M. M., Lloyd, C., Cappelaere, B., Kergoat, L., et al. (2012). A new parameterisation scheme of ground heat flux for land surface flux retrieval from remote sensing information. Journal of Hydrology, 454, 113–122.
Abstract: The objective of the study was to assess the performance of a new parameterisation scheme of ground heat flux (G) for retrieving surface fluxes from remote sensing data (MODIS-Terra). Formulae that are based on empirical relationships relating G to net radiation, Rn (G=αRn, α being a function of a vegetation index, VI) are currently used, but presented drawbacks, especially in bare or sparse vegetation areas because of the poor adequacy of VI-based relationships to account for changes in soil moisture. In this study, we proposed to link α to the evaporative fraction, EF. In a first step, using a non-dimensional form of the surface energy balance, we demonstrated that α is functionally related to EF and to the ratio γ=G/H (H=sensible heat flux). In a second step, we proposed an EF-based parameterisation of α, using ground fluxes data sets collected throughout the years 2005, 2006 and 2007 at four flux-tower sites in West African countries (Mali, Benin, Niger) that differ in surface conditions and Monsoon influence. The analysis indicated that the average site-specific values of α and EF were well described by a linear relationship of the type α=aEF+b, with a=-0.22 and b=0.23. In a third stage, we investigated whether ET-retrieval from remote sensing information (MODIS-Terra) using the new parameterisation of α perform better than the classical formulation through VI-based relationships. We found that the retrieved values of H using the new parameterisation supplied the best agreement with the observed ground data and significant improvement with respect to estimates from α-VI relationships. Advantages and limitations of the proposed parameterisation scheme were discussed.
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2011 |
Descloitres, M., Seguis, L., Legchenko, A., Wubda, M., Guyot, A., & Cohard, J. M. (2011). The contribution of MRS and resistivity methods to the interpretation of actual evapo-transpiration measurements: a case study in metamorphic context in north Benin. Near Surface Geophysics, 9(2), 187–200.
Abstract: A quantitative budget estimate of actual evapo-transpiration is a key issue for enhanced hydrological modelling in northern Benin. Actual evapo-transpiration is estimated using large aperture scintillometer equipment, devoted to sensible heat flux measurements. However, a previous study reported that the actual evapo-transpiration cycle is not fully understood. Indeed, the actual evapo-transpiration depends strongly on several factors such as climate, vegetation pattern, soil water storage and human activities. The respective contributions of the aquifer and vadose zone to the actual evapo-transpiration budget are not known. When using piezometric variations of the water table, the aquifer contribution is not easy to quantify since the specific yield may vary in the investigated area, located in a metamorphic rock environment. In the present study, we investigate whether significant differences in the aquifer's specific yield could exist within the large aperture scintillometer measurement area, leading to different actual evapo-transpiration water losses. We use joint frequency electromagnetic resistivity mapping, geological surveys and magnetic resonance sounding (MRS) to delineate the effective porosity of the regolith around the scintillometre measurement area. Thirteen MRS soundings implemented in key areas reveal a clear classification of the main geological units on the basis of their water content. The MRS water content varies between 1.5-3% for amphibolite and micaschists formations to more than 12% for quartzitic fractured formations, whereas the MRS relaxation time T-1 is less discriminating (150-250 ms), indicating a small variation in pore size. Then, as a first modelling exercise, we assumed that the MRS water content (the effective porosity) maximizes the specific yield. The actual evapo-transpiration budget given by a previous study (Guyot et al. 2009) is then re-interpreted using geophysical data: we found that a) the measured water table depletion can explain the actual evapo-transpiration value providing enough water for the transpiration process and b) the significant discrepancies in actual evapo-transpiration signals observed between the eastern and western parts of the watershed can be explained by the respective effective porosity of the geological units. Even if further research is needed to link MRS water content to the specific yield and to evaluate a possible role of the deep vadose zone, the hydrogeophysical mapping presented in this study highlights the role of the MRS method for p
roviding relevant information to understand hydrological processes in this complicated geological context of north Benin.
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Seguis, L., Boulain, N., Cappelaere, B., Cohard, J. M., Favreau, G., Galle, S., et al. (2011). Contrasted land-surface processes along the West African rainfall gradient. Atmospheric Science Letters, 12(1), 31–37.
Abstract: We review the main results of land-surface studies obtained in the three sites of the long-term observing system AMMA-CATCH. Runoff in the Sahel enhances the variability of energy partitioning between non-infiltrative areas where sensible heat is dominant and infiltrative areas where soil water availability increases the latent flux. In terms of water resources, an increase in runoff over the past 50 years, already reported for the exoreic Sahel, was revealed in the endoreic Sahel. In the Sudanian domain, the subsurface origin of streamflow could explain its decrease over the same period. Copyright (C) 2011 Royal Meteorological Society
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Uijlenhoet, R., Cohard, J. M., & Gosset, M. (2011). Path-Average Rainfall Estimation from Optical Extinction Measurements Using a Large-Aperture Scintillometer. Journal Of Hydrometeorology, 12(5), 955–972.
Abstract: The potential of a near-infrared large-aperture boundary layer scintillometer as path-average rain gauge is investigated. The instrument was installed over a 2.4-km path in Benin as part of the African Monsoon Multidisciplinary Analysis (AMMA) Enhanced Observation Period during 2006 and 2007. Measurements of the one-minute-average received signal intensity were collected for 6 rainfall events during the dry season and 16 events during the rainy season. Using estimates of the signal base level just before the onset of the rainfall events, the optical extinction coefficient is estimated from the path-integrated attenuation for each minute. The corresponding path-average rain rates are computed using a power-law relation between the optical extinction coefficient and rain rate obtained from measurements of raindrop size distributions with an optical spectropluviometer and a scaling-law formalism for describing raindrop size distribution variations. Comparisons of five-minute rainfall estimates with measurements from two nearby rain gauges show that the temporal dynamics are generally captured well by the scintillometer. However, the instrument has a tendency to underestimate rain rates and event total rain amounts with respect to the gauges. It is shown that this underestimation can be explained partly by systematic differences between the actual and the employed mean power-law relation between rain rate and specific attenuation, partly by unresolved spatial and temporal rainfall variations along the scintillometer path. Occasionally, the signal may even be lost completely. It is demonstrated that if these effects are properly accounted for by employing appropriate relations between rain rate and specific attenuation and by adapting the pathlength to the local rainfall climatology, scintillometer-based rainfall estimates can be within 20% of those estimated using rain gauges. These results demonstrate the potential of large-aperture scintillometers to estimate path-average rain rates at hydrologically relevant scales.
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2009 |
Guyot, A., Cohard, J. M., Anquetin, S., Galle, S., & Lloyd, C. R. (2009). Combined analysis of energy and water balances to estimate latent heat flux of a sudanian small catchment. Journal Of Hydrology, 375(1-2), 227–240.
Abstract: Actual evapotranspiration is one of the major components of both energy and water budgets, but is often difficult to monitor over long period with sufficient accuracy. Within the framework of the “AMMA-CATCH” program, a project dedicated to the study of the West African Monsoon, a large aperture scintillometer has been installed in a small catchment (12 km(2)), located in the North of Benin. a region exposed to sudanian climate. The present study is an attempt to estimate the latent heat flux over this small but heterogeneous catchment based on scintillation and ground observations. The analysis covers the end of the dry season (lasting from February to April 2006). During this period two isolated rainfall events occurred, giving a unique opportunity to study energy and water budgets simultaneously. The comparison between the average sensible heat flux derived from scintillometer observations and the one obtained with conventional eddy correlation shows a relatively good agreement, where the scattering is mainly explained by differences in footprint associated with both instruments. A relevant hourly residual latent heat flux is then obtained through the energy balance equation, with careful attention brought to the net radiation, and the ground heat fluxes. The residual of the energy budget equation is compared to soil water losses from vadose zone and water table, in order to evaluate whether this estimation is consistent with the water budget of the ground. Daily soil water depletion within the first meter of the surface shows a similar dynamic as the one calculated from the energy balance equation, but exhibits a constant 1 mm/day lag. The excess of actual evapotranspiration is supposed to be explained by water table losses and root extraction by trees. Finally, this study shows how combined energy and water budget analysis can help to better understand water transfers at the watershed scale. (C) 2009 Elsevier B.V. All rights reserved.
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Pellarin, T., Tran, T., Cohard, J. M., Galle, S., Laurent, J. P., de Rosnay, P., et al. (2009). Soil moisture mapping over West Africa with a 30-min temporal resolution using AMSR-E observations and a satellite-based rainfall product. Hydrology And Earth System Sciences, 13(10), 1887–1896.
Abstract: An original and simple method to map surface soil moisture over large areas has been developed to obtain data with a high temporal and spatial resolution for the study of possible feedback mechanisms between soil moisture and convection in West Africa. A rainfall estimation product based on Meteosat geostationary satellite measurements is first used together with a simple Antecedent Precipitation Index (API) model to produce soil moisture maps at a spatial resolution of 10x10 km(2) and a temporal resolution of 30-min. However, given the uncertainty of the satellite-based rainfall estimation product, the resulting soil moisture maps are not sufficiently accurate. For this reason, a technique based on assimilating AMSR-E C-band measurements into a microwave emission model was developed in which the estimated rainfall rates between two successive AMSR-E brightness temperature (TB) measurements are adjusted by multiplying them by a factor between 0 and 7 that minimizes the difference between simulated and observed TBs. Ground-based soil moisture measurements obtained at three sites in Niger, Mali and Benin were used to assess the method which was found to improve the soil moisture estimates on all three sites.
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Timouk, F., Kergoat, L., Mougin, E., Lloyd, C. R., Ceschia, E., Cohard, J. M., et al. (2009). Response of surface energy balance to water regime and vegetation development in a Sahelian landscape. Journal Of Hydrology, 375(1-2), 178–189.
Abstract: The West African monsoon interacts strongly with the land surface, yet knowledge of these interactions is severely limited by the lack of observations of surface energy fluxes. Within the framework of the AMMA project, three eddy covariance flux stations were installed to sample the three main surface types near Hombori (Mali) in the central Sahel at 15.3 degrees N, and a fourth station was installed near Bamba in the northern Sahel at 17.1 degrees N to sample semi-desert conditions. Observed land types near Hombori comprised a grassland growing on sandy soil (near the village of Agoufou), a flooded forest in a clay-soil depression (Kelma), and a bare rocky soil (Eguerit). The energy balance closure at the grassland site was satisfactory, but less so at the flooded forest site. Surface water heat storage during the flood and advection probably were responsible for most of the imbalance. The daily sensible heat flux (H) was fairly constant throughout the year at Bamba and Eguerit, with only a slight increase during the monsoon season corresponding to increased net radiation. By contrast, the seasonal cycle of the grassland site was marked, with H decreasing during the monsoon season from 70 W m(-2) in May to 20 W m(-2) in August. The flooded woodland exhibited the strongest contrast between the dry and wet seasons, with daily sensible heat flux close to zero during the flood. During the peak monsoon season, the two vegetated sites had the highest net radiation and the lowest sensible heat flux, as a consequence of the strong evapotranspiration rates caused by both high soil moisture availability and high leaf area index. Lateral fluxes of water were found to be strong drivers of inter-site sensible and latent heat fluxes variability, with water leaving bare rocky soils as surface runoff and ending in the clay depressions (e.g., Kelma), whereas the sandy soils were locally endorheic, with most of the rainfall being rapidly returned to the atmosphere. An attempt was made to scale the sensible heat flux up to the scale of the AMMA northern super-site (60 km x 60 km), following a simple scaling scheme, which accounted for the contrasting surface types and water regimes. The super-site average sensible heat flux proved to be close to the grassland sensible heat flux, in part because grassland occupies 55% of the area. A strong spatial variability was caused by the difference in water regime and vegetation type, at a scale large enough to potentially influence the atmospheric properties such as the boundary layer. (C) 2009 Elsevier B.V. All rights reserved.
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