Long term changes in hydrometeorological controls on water recharge in a karstic Mediterranean basin

The Mediterranean basin is particularly vulnerable to climate change. These changes lead to disruptions in the hydrological cycle, potentially impacting groundwater resources and recharge. The hydrogeological catchment of the Lez karst Spring, located south of France north of Montpellier city is actively used to supply Montpellier Mediterranée Métropole (MMM), with drinking water. This study aims to assess the long-term hydroclimatic changes over the hydrogeological basin of the Lez Spring with an approach combining climatic and hydrological analyses. The main objective of this research is to assess eventual long-term trends in the climatic (rainfall, temperature, potential evapotranspiration, soil moisture) and hydrological variables (piezometric levels, spring and river flows); a focus on the interrelationship between these different parameters is also performed to understand the hydroclimatic trend and temporal evolution of this highly anthropized aquifer. The analysis combines measurements of local soil and hydrogeological variables, with longer time series of meteorological observations and the French climate reanalysis SAFRAN since 1960, and the high-resolution COMEPHORE radar rainfall product since 2000 providing hourly rainfall intensities at the kilometric scale to investigate the spatial dynamics of rainfall. The trend analysis results indicated a strong increase of temperature but no significant changes in precipitation totals from the different datasets. However, a positive trend in annual maximum hourly rainfall intensities was detected, associated with an increased spatial variability of rainfall fields and flashiness characteristics. There is a sharp increase of potential evapotranspiration, associated with a decline in soil water content throughout the year. The piezometric levels do not exhibit significant trends since 2007, similarly to the water uptake for the consumption of the city of Montpellier. The combination of the different high-resolution datasets allows a deepened analysis of the relative effects of the contribution of extreme rainfall events and the spatiotemporal variability of rainfall patterns on the recharge processes of the aquifer. This study will ultimately provide the keys to more sustainable management of the karst water resource for drinking water supply in the face of climate change, through a better understanding of the functioning of the Lez spring Hydrogeological catchment.