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 (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. (letter)