Diagnosis of environmental controls on daily actual evapotranspiration across a global flux tower network: the roles of water and energy

Relative contributions from environmental factors to daily actual evapotranspiration (ETa) across a variety of climate zones and ecosystem types is a widely open research question, especially regarding the roles played by soil water content (SWC; water supply) and net radiation (Rn; energy supply) in controlling ETa. Here, the boosted regression tree method was employed to quantify environmental controls on daily ETa using the global FLUXNET dataset. Overall, the SWC impact on daily ETa increased with increasing aridity index (Φ). However, unlike the traditional Budyko theory that is applied at annual and mean annual scales, the daily FLUXNET data revealed that Rn still played a pivotal role at most sites (roughly Φ < 4), indicating that Rn could be a leading control on daily ETa even at water-limited sites. The variations in the relative controls of SWC and Rn also partly depended on factors affecting water availability for daily ETa (e.g., vegetation characteristics, soil texture, and groundwater depth). Especially, leaf area index exerted a stronger influence on ETa at drier sites than at relatively humid sites, suggesting that near-surface hydrological processes are more sensitive to vegetation variations due to their ability to extract deep soil water and enhance ETa, particularly under arid and semi-arid climatic conditions. As a result, the net effect of environmental controls other than SWC and Rn on ETa was more important at drier sites. Our findings illustrate how environmental controls on daily ETa change as climate and ecosystem vary, which has important implications for many scientific disciplines including hydrological, climatic, and agricultural studies.