Spatiotemporal variability of green water fluxes and their response climate extremes: pinpointing drought- and heatwave-hotspots over the Acheloos River basin, Western Greece

Evapotranspiration (ET), the largest water flux through which water returns to the atmosphere in vapor form, plays a central role in the terrestrial water and energy cycles. In the water-limited Mediterranean basin, in particular, relatively small changes in green water fluxes (i.e., ET), triggered for example by environmental factors and land cover changes, affect water resources, including runoff, groundwater, irrigation needs, and overall ecosystem functioning. Sustainable water resources management in these regions therefore requires accurate spatiotemporal characterization of ET losses. The increasingly available Earth Observations allow us to address some of these challenges by monitoring ecosystem functioning with high spatiotemporal resolution. Here, focusing on the Acheloos river basin (7531 km²), one of the most important hydrological systems of Greece with regards to water supply (domestic and irrigation uses), hydropower, and ecosystem services, we quantified the spatial variability of ET and its temporal dynamics at the seasonal, annual, and inter-annual time scales. We synthesized remotely sensed ET products together with auxiliary geospatial and environmental variables to tackle the following research questions: (1) What land cover contributes the most to the ET losses over the basin? (2) How did ET respond to recent climate extremes (i.e., droughts and heatwaves), and (3) What were the hotspots with the most sensitive land cover? By synthesizing spatially explicit historical estimates of ET across the study area, together with environmental and land use datasets, this study aims to provide constrained estimates of ET for the dominant land cover types at seasonal, annual, and inter-annual time scales. Such estimates could facilitate impact assessments of natural hazards (e.g., droughts, heatwaves, and wildfires) on the water balance via land cover change feedback (i.e., ET losses), providing valuable insights towards sustainable long-term water resources management in the Mediterranean region.