Evapotranspiration Partitioning Based on Leaf and Ecosystem Water Use Efficiency

Partitioning evapotranspiration (ET) into evaporation (E) and transpiration (T) is essential for understanding the global hydrological cycle and improving water resource management. However, ecosystem-level ET partitioning remains challenging. Here we proposed a novel ET partitioning method that uses the unified stomatal conductance model to estimate T:ET by calculating the ratio of the ecosystem water use efficiency (WUEeco) to leaf WUE (WUEleaf) using half-hourly flux data. The WUEleaf values estimated by the unified stomatal conductance model agree with an independently measured ratio of hourly photosynthetic rate to T rate (R-2 = 0.69). The sensitivity of T:ET to the key parameter g(1) varied among different plant functional types (PFTs), but the T:ET variations for each PFT were all controlled within 20% when g(1) altered within its 95% confidence interval. The mean annual T:ET was highest for evergreen broadleaf forests (0.63), followed by deciduous broad forests (0.62), grasslands (0.52), evergreen needleleaf forests (0.43) and woody savannas (0.40). C-3 croplands had higher T:ET (0.65) than C-4 croplands (0.48). Seasonal variations in T:ET varied across PFTs and the leaf area index explained about 50% of the variation in seasonal T:ET. Our method is not only consistent with other three EC-based methods: Z16, N18, and L19 (R = 0.92, 0.94, and 0.68), but also shows high correlations to sap flow-based T (R = 0.70) at three different forest sites. The method developed in this study provides a feasible and universal approach for ET partitioning of global EC sites, improving the understanding of ecosystem T characteristics across climates and PFTs.