Climate change impacts on groundwater simulated using the AquiFR modelling platform

Abstract. In the context of increasing water stress and climate change, the assessment of changes in groundwater resources is a major challenge for water decision-makers. As part of the EXPLORE2 project, the aim of this study is to estimate changes in groundwater levels over France during the 21^st century. We used the hydrogeological modelling platform AquiFR together with 36 regional climate projections from Eurocordex (CMIP5) from three Representative Concentration Pathways (RCPs), bias-corrected according to a state-of-the-art method: RCP2.6, RCP4.5 and RCP8.5. The future evolution of groundwater is assessed using the standardized piezometric level index, a normalized indicator that provides return periods based on the distribution value over a reference period, here 1976–2004. We found significant scatters between regional climate models and RCPs. Overall, a rise in groundwater levels, affecting most of the study area, is the dominant signal, especially in northern France. This result is in contrast to previous studies in this area. Under RCP8.5 (highest greenhouse gas emissions scenario), the evolution of the occurrence of current 10-year return period events shows a significant increase in the risk of high groundwater levels mostly on the northern part of France, together with an increase in the 10-year low groundwater levels mostly observed in South of France, which highlights a North-South differentiation. The increase in high and low flow events is quite common in surface hydrology, but is less common for groundwater, which has a longer residence time. In order to better reflect the uncertainties, 4 storylines based on the RCP8.5 scenario have been selected to be representative of possible futures that can illustrate the impacts of worst-case scenarios and help decision-makers to adopt sustainable groundwater management policies.