Impact of Potential Evapotranspiration Estimation on River Discharge in the Casamance Basin (Southern Senegal)

This study assesses the potential evapotranspiration estimations’ impacts on projected hydrological simulations under global warming scenarios (1.5 and 2.0 °C). These warming values are the targets warming of the Paris Agreement in 2015 during the United Nations Climate Change Conference, COP 21. They are used in this study to investigate their potential impacts on one of the fundamental hydrological processes in a West African basin. This is important because the local effects are usually higher than the global mean values given, particularly in one of the most vulnerable regions to climate change, such as sub-Saharian Africa. Then, we use twenty regional climate simulations driven by six global climate models and three evapotranspiration estimations (Hamon, Hargreaves, and Penman) as input for the rainfall-runoff hydrological model GR2M. The results showed that GR2M was successfully calibrated and validated with Nash–Sutcliffe efficiencies of 70.4% and 69.0%, respectively, at the outlet Kolda. The signals of the flow regimes were well reproduced even though the magnitudes were sometimes underestimated. The variations in the simulated flows as a function of those observed displayed coefficients of determination R2 more significant than 0.7. As for projected river discharge, a general decrease was noted for all simulations from RCMs driven by NCC-NorESM1-M, MPI-M-MPI-ESM-LR, ICHEC-EC-EARTH, CNRM-CERFACS-CNRM-CM5, and CCCma-CanESM2 GCMs for all PET estimation, except those driven by MOHC-HadGEM2-ES that showed a slight increase of streamflow. The discharge decrease was more pronounced with Hamon, Hargreaves, and Penman, respectively. The decline of water with MPI-M-MPI-ESM-LR forcing data was more considerable than with the other input data. Furthermore, the ensemble mean of all forcing data shows that the basin was likely to experience water deficit with all PET estimation and scenarios; however, with Hargreaves estimation under 1.5 °C, we notice a streamflow increase. The changes under 2.0 °C might have the most substantial impact on the water resources of the Casamance basin in the future. The PET estimation and the driven GCM played a considerable role in the simulated discharge. Furthermore, the net irradiance, the theoretical maximum daily insolation, the mean temperature, the temperature range (difference between max and min temperature), and the extra-terrestrial radiation are the main parameters that affect the evapotranspiration processes.