Evaluating hydrological changes in semi-arid West Africa : Detection of past trends in extremes and framework for modeling the future

Thesis defence by Catherine Wilcox, Monday 1 July 2019 at 2pm, salle Lliboutry, Bâtiment Glaciologie, 54 rue Molière, Saint-Martin-d’Hères

Jury :

– Sandrine Anquetin, DR CNRS, IGE, Examinatrice
– Florence Habets, Directrice de Recherche CNRS, laboratoire de Géologie de l’ENS , Rapporteuse
– Michel Lang, Ingénieur de Recherche, IRSTEA Lyon, Rapporteur
– András Bárdossy, Prof. Dr. rer.nat. Dr.-Ing. Institute for Modelling Hydraulic and Environmental Systems, Dept. of Hydrology and Geohydrology , Examinateur
– Théo Vischel, Maître de conférences, Université Grenoble Alpes, Directeur de thèse
– Gérémy Panthou, Physicien-adjoint CNAP, Université Grenoble Alpes, Co-Encadrant de thèse

Abstract :

The semi-arid regions of West Africa are known for their dry conditions which have predominated since the 1970s. In recent years, however, West Africa has witnessed a series of severe flooding events which caused widespread fatalities and socioeconomic damages. The emergence of this new problem demonstrates the sensitivity of the region to changes in the hydroclimatic system and calls for an improved characterization of flood hazard and the mechanisms that generate it. It also signals the need to develop projections for how flood hazard may evolve in the future in order to inform appropriate adaptation measures.
In this context, the following PhD thesis seeks to answer three main questions :

1) Is there a significant trend in extreme streamflow in West Africa, or are the documented flooding events isolated incidences ?
2) How can one model mesoscale convective systems, the primary driver of runoff in the region, in order to explore the properties of precipitation that drive streamflow ?
3) Based on potential climate change in the region, what trends might be observed in streamflow in the future ?

First, changes in extreme hydrological events West Africa over the past 60 years are evaluated by applying non-stationary methods based on extreme value theory. Results show a strong increasing trend in extreme hydrological events since the 1970s in the Sahelian Niger River basin and since the 1980s in the Sudano-Guinean catchments in the Senegal River basin. Return levels calculated from non-stationary models are determined to exceed those calculated from a stationary model with over 95% certainty for shorter return periods (<10 years).

Next, recent developments are presented for a stochastic precipitation simulator (Stochastorm) designed for modeling mesoscale convective storms, the main rainfall source in the Sahel. Developments include a model for storm occurrence, the explicit representation of extreme rainfall values, and an improvement in the modeling of sub-event intensities. Using high-resolution data from the AMMA-CATCH observatory, simulation outputs were confirmed to realistically represent key characteristics of MCSs, showing the simulator’s potential for use in impact studies.
Finally, a modeling chain for producing future hydrological projections is developed and implemented in a Sahelian river basin (Dargol, 7000km2). The chain is original as it is the first attempt in West Africa to encompass the continuum of scales from global climate to convective storms, whose properties have major impacts on hydrological response and as a result local flood risk. The modeling chain components include the convection-permitting regional climate model (RCM) CP4-Africa, the only RCM (to date) explicitly resolving convection and providing long-term simulations in Africa ; a bias correction approach ; the stochastic precipitation generator Stochastorm ; and a rainfall-runoff model specifically developed for Sahelian hydrological processes. The modeling chain is evaluated for a control period (1997-2006) then for future projections (ten years at the end of the 21st century). Hydrological projections show that peak annual flow may become 1.5-2 times greater and streamflow volumes may double or triple on average near the end of the 21st century compared to 1997-2006 in response to projected changes in precipitation.

The results raise critical issues notably for hydrological engineering. Current methods used to evaluate flood risk in the region do not take non-stationarity into account, leading to a major risk of underestimating potential floods and undersizing the hydraulic infrastructure designed for protecting against them. It is also suggested to not only consider rainfall changes but also societal and environmental changes, interactions, and feedbacks in order to better attribute past hydrological hazards and their future trajectories to related causes.