Decline of water resources in Northern Hemisphere Mediterranean Climate Regions based on satellite observations.

The Mediterranean Climate Regions (MCRs) share similarities in terms of experienced temperatures and precipitation patterns, resulting in similar vegetation, i.e. farms and agricultural approaches/strategies. Recent findings suggest that groundwater levels in the Euro-Mediterranean may experiencing negative trends, resulting from decreasing precipitation, increasing evapotransipration and/or increasing withdrawal. This suggests a potential dry transition in MCRs, affecting biodiversity and ecosystems. By focusing on the Euro-Mediterranean and California, this work characterizes the dry transition through soil water content analysis from observation. The Total Water Storage (TWS) variable provided by the GRACE/GRACE-FO mission is utilized, carrying information about groundwater, soil moisture, surface water, snow water equivalent, and water stored in biomass. Furthermore, the contribution related to the variability of TWS due to associated drivers, precipitation, and potential evapotranspiration (PET) is quantified. The methodology framework relies on Empirical Orthogonal Function (EOF) analysis and a Multivariate Linear Regression model (MLR) to characterize, respectively, the modes of variability of TWS and the relative influence of the drivers to the total variance of the field. Our results highlight a general drying trend for both regions; furthermore, they suggest that in certain domains, variations in TWS are more sensitive to specific drivers than others: the western Euro-Mediterranean (Spain, Portugal, and parts of North Africa) is more susceptible to precipitation variability than PET, as it is more influenced by Atlantic flows and the effects of the North Atlantic Oscillation; in contrast, the TWS in the eastern Euro-Mediterranean (Greece and Turkey) is predominantly affected by the increase in PET due to the temperature increase; on the other hand he central EuroMed (Italy and Tunisia) exhibit a mixed behaviour with influences of both precipitation and PET on TWS. Regarding California, similar to the Euro-Mediterranean, there is a negative trend in TWS in equatorward regions, where PET is the major source of variability. Conversely, starting from 43°N in the poleward regions, there is a positive trend in TWS, mostly due to the increase in precipitation in that area.