Climate change impacts on groundwater simulated using the AquiFR modelling platform

In the context of increasing water stress and climate change, the assessment of changes in groundwater resources has become a major challenge for water decision-makers. As part of the EXPLORE2 project, this study aims to estimate changes in groundwater levels on French aquifers during the 21st century. We use the hydrogeological modelling platform AquiFR, forced by 36 regional climate projections from Eurocordex (CMIP5), bias-corrected according to a state-of-the-art method, based on three Representative Concentration Pathways (RCPs): RCP2.6, RCP4.5 and RCP8.5. To better reflect uncertainties, four storylines based on the RCP8.5 scenario have been selected to represent possible futures that illustrate the impacts of worst-case scenarios, to help decision-makers to adopt sustainable groundwater management policies. The future evolution of groundwater is assessed using the standardized piezometric level index, a normalized indicator of groundwater level that provides return periods based on the distribution value over a reference period – in this case 1975–2004. The analyses revealed significant variability between regional climate models. The predominant signal is an increase in groundwater levels, affecting most of the study area, particularly in northern France. This result is in contrast to previous studies in this area. Under the 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, primarily on the northern part of France, which is consistent with 3 out of the 4 storylines. This is accompanied by an increase in the 10-year low groundwater levels mostly observed in the southern part of the French aquifer systems represented by AquiFR. This pattern is shared by all four storylines. This finding underscores a discernible North-South distinction. The increase in the severity of events is a well-known consequence of climate change on surface hydrology, but is less prevalent in groundwater hydrology, which is characterised by a longer residence time.