Separating meteorological condition and soil moisture controls on the variation in stand evapotranspiration of a larch plantation during three hydrological years

Abstract Evapotranspiration (ET) is an essential component of the forest stand water budget, and the main factors controlling ET variations are meteorological conditions and soil moisture. A deep understanding of their impacts on ET is crucial for forest water management and sustainable development in changing environments. However, the independent effects of meteorological conditions and soil moisture on ET are not clear because their impacts occur concurrently under natural environmental conditions. In this study, a relatively simple but mechanism-based forest stand ET model that couples the effects of the reference ET (ETo) and relative extractable water (REW) in the 0–60 cm soil layer was developed to quantify the independent impacts of ETo and REW on ET. The model was established based on observed stand transpiration, forest floor ET, meteorological conditions, and soil water content obtained on non-rainfall days of the 2015, 2016, and 2019 growing seasons from a larch (Larix principis-rupprechtii) plantation in Northwest China. The following key results were obtained. (1) At this sub-humid site, the weather-dependent ET increased linearly with rising ETo; daily ET initially increased rapidly with rising REW, and then gradually reached a threshold; thereafter, it tended to be stable. The response could be described by an exponential saturation function. (2) The daily stand ET model that coupled these response functions to individual factors was determined as ET = 0. 785 × [ETo × (1 – EXP(–13.567REW))]. (3) The contributions of ETo and REW were further determined using the new ET model, and the results show that ETo was the main driving factor affecting the ET variations in three different hydrological years compared with REW and the interaction of ETo and REW. Compared with those in the normal year 2015, REW and ETo limited the ET variation in the dry year 2016 by 13.1% and 7.3%, respectively, and ETo limited the ET variation in the wet year 2019 by 8.8%. Our study provides a simple and feasible method for separating the meteorological conditions (i.e., ETo) and soil moisture controls on ET variations, and can improve the understanding of the impacts of varying environments on ET.