An Increasing Effect of Soil Moisture on Semiempirical Water-Use Efficiency Models From Wet to Dry Climate Regions

Water use efficiency (WUE) is a crucial parameter for describing the relationship between carbon and water cycles in plants and ecosystems. Previous studies suggest that an additional intercept is necessary for the empirical WUE model. However, the impact of soil moisture on model performance remains uncertain. In this study, 71 sites of FLUXNET2015 were categorized into energy-limited, transitional, and water-limited climates based on the Budyko dryness index (DI). Modified WUE models were utilized to forecast daytime transpiration based on GPP x VPD0.5 with an intercept term, which evaluated whether soil moisture content regulates the intercept term with increasing dryness levels. Our results demonstrated that the WUE model with an additional net radiation (RN) term effectively predicts transpiration in energy-limited and transitional regions. On the other hand, the WUE model with an additional soil water content (SWC) term performs well in water-limited regions with annual precipitation of less than 400 mm and energy-limited regions with annual mean temperature of less than 5 & DEG;C. The significance of SWC on Nash-Sutcliffe efficiency increased from energy-limited to water-limited regions. The coupling of the RN and SWC terms as intercept terms in the empirical WUE model can enhance the prediction of Et in both energy-limited and transitional regions. With global warming leading to more extreme climates in both humid and arid regions, incorporating the SWC term in the empirical WUE model can enhance the accuracy of modeled daytime transpiration globally, providing an easy-to-use method to simulate global transpiration.