Potential Groundwater Recharge at the Scale of France:  Characterization and Future Trends in the Context of Climate Change

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The quantification of present and future groundwater resources at regional scale is necessary for the implementation of national climate change adaptation plans. We present a method to compute the potential groundwater recharge (PGR) by precipitation applied specifically to the scale of France. A simple water balance approach taking into account the maximum soil water content capacity and the land use is first applied to derive the effective rainfall estimation from the SAFRAN national meteorological reanalysis. The BaseFlow Index (BFI) computed over 611 French river basins with minor human influence on discharge is then used to assess the effective rainfall infiltration ratio for watershed with homogeneous geological lithologies. This infiltration ratio is finally applied to convert effective rainfall into potential recharge at the scale of each groundwater body in France. A sensitivity analysis of BFI (to the automated BF separation method, the length of discharge time series, etc.) was performed. The low annual variability and uncertainty on BFI estimates allow us to consider, as an initial approximation, that the infiltration ratio remains constant over time. To validate this global approach, in the framework of the Explore2 project, we compared computed effective rainfall and potential recharge with alternative potential recharge estimates simulated by a set of hydrological models under current condition (1976-2005). Previous computed variables have been compared with SURFEX physical surface model solving energy balance over the entire re-analysis (1958-2020). Additionally, we used Euro-Cordex climatic projections as input of our model to evaluate the future potential groundwater recharge (2021-2100). Future evolution of potential recharge shows contrasting situations between the North and the South of France which were not highlighted by previous assessments.