Regional Variations in Drivers of Extreme Reference Evapotranspiration Across the Contiguous United States

Extremely high reference evapotranspiration indicates an abnormal atmospheric water demand, with significant implications for regional water resource management. Despite its importance, our understanding of extreme reference evapotranspiration remains limited, particularly regarding the contributions of its underlying drivers. This study focuses on all 339 hydrological unit code 6 basins across the Contiguous United States (CONUS) and, using two different methods for identifying extreme events, analyzes the meteorological contributions in various regions. We evaluate how factors such as meteorological data sets, meteorological variables, temporal and spatial scales, and the severity of extreme events introduce uncertainty into the analysis. The results reveal distinct regional patterns in the dominant drivers of extreme daily reference evapotranspiration across the CONUS. Air temperature is the dominant driver in the northern US, while solar radiation primarily drives extreme events in the southeastern US. Wind speed is the main driver in the southwestern US, with its influence increasing at higher severity levels or with finer temporal resolution. The air temperature and humidity jointly dominate the central US. All contributions from meteorological forcings vary with selection of temporal scales and severity levels, and utilizing multiple data sets enhances the robustness of extreme event identification.