Exploring the utility of GRACE measurements for characterizing large regional karst systems

In the face of climate change and human activities, Groundwater Storage (GWS) in karst aquifers of the Euro-Mediterranean region has experienced a significant decline, particularly across 60-80 percent of its terrain. We undertook a spatiotemporal characterization of karst aquifer dynamics in this region by integrating satellite observations (GRACE) and the process-based model VarKarst that simulates potential groundwater recharge in karst terrain. Utilizing the GLDAS and ERA5-Land datasets, we independently obtained Groundwater Storage Anomalies (GWSA) and Subsurface Water Storage Anomalies (SWSA) by decomposing the terrestrial water storage anomaly (TWSA) detected by GRACE. GWSA focuses explicitly on the saturated part of aquifers, while SWSA considers both the saturated and unsaturated parts. This approach is adopted to better understand the role of soil and epikarst in the groundwater recharge processes in large karst regions. Comparisons of GRACE-derived GWSA and the potential groundwater recharge calculated by the model VarKarst for the period 2002-2019 revealed the steepest GWS declines in the polar(tundra) climate zone of the Alpine region. Recharge trends were mixed, with the strongest decline in polar climates (-3.0mm/year) linked to rising temperatures (evapotranspiration). Incorporating gridded sectoral water withdrawal information is imperative for interpreting the observed spatial patterns of GWSA/SWSA. The temperate climate zones show a strong correlation between SWSA and lagged recharge, which should be due to the flow-regulating role of soil/epikarst. In addition, the current study incorporates spring discharge data from selected karst catchments to evaluate the previous analysis. Spatial scale limitations were identified for small karst catchments, as evidenced by poor correlations with spring discharge, while larger karst catchments show stronger correlations. This research highlights the importance of considering groundwater recharge processes and the epikarst dynamics when using GRACE data to assess regional karst hydrogeology. Such an integrated method will provide a clearer picture of the impacts on GWS under current climatic and anthropogenic stressors.