Global analysis of watershed characteristics modulating the sensitivity of evapotranspiration to heat and dry extremes

Abstract The rising frequency, intensity, and persistence of temperature and dry-climate extremes such as heatwaves, very hot days and nights, and prolonged dry spells, pose significant challenges to ecosystems and human well-being. Accurately characterizing these extremes is crucial for informing watershed management strategies aimed at building ecosystem resilience and guiding sustainable water resource planning. This study addresses a critical knowledge gap by examining how Climate Extreme Indices (CEIs) influence fractional evapotranspiration (fET) across global watershed systems, specifically quantifying how watershed characteristics modulate these relationships. Methodologically, we applied a Random Forest regression framework integrated with SHAP (Shapley Additive Explanations) to quantify the coupling strength between CEIs and fET and to identify the most influential watershed attributes. Our findings reveal that soil texture and topography distinctly modulate responses to short- versus long-duration dry events. For instance, watersheds with higher silt content exhibit greater water retention capacity, supporting sustained fET during extended dry periods. In terms of temperature extremes, heatwave metrics provide nuanced insights into vegetation stress: Heatwave Amplitude triggers immediate physiological responses, while Heatwave Magnitude captures longer-term adaptive potential. These insights provide a scalable framework for integrating physical watershed thresholds into proactive climate resilience planning.