| Summary: | Relative contributions from environmental factors to daily actual evapotranspiration (ET _a ) across a variety of climate zones 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 ET _a . Here, the boosted regression tree method scheme was employed to quantify environmental controls on daily ET _a using the global FLUXNET dataset. Similar to the general trend suggested by the Budyko theory at annual scales, the results showed that the relative control of SWC on daily ET _a increased with increasing aridity index (Φ); however, Rn played a major role at most FLUXNET sites (roughly Φ < 4), indicating that Rn could be a leading control on daily ET _a even at water-limited sites. The variability in the relative controls of SWC and Rn also partly depended on factors affecting water availability for daily ET _a (e.g. vegetation characteristics and groundwater depth). Our study showed that other than SWC and Rn, the net effect of environmental controls (particularly leaf area index) on daily ET _a was more important at drier sites than at relatively humid sites. This suggests that near-surface hydrological processes are more sensitive to vegetation variations due to their ability to extract deep soil water and enhance ET _a , especially under arid and semi-arid climatic conditions. Our findings illustrate how environmental controls on daily ET _a change as the climate dries, which has important implications for many scientific disciplines including hydrological, climatic, and agricultural studies.
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