Partition of daily evapotranspiration using stable water isotope method and a modified Shuttleworth-Wallace Model for urban forest area

Quantification of the contribution of transpiration (T) to evapotranspiration (ET) is important to understand the impact of climate change on the hydrological cycle and guide precision irrigation. So far, few studies have examined seasonal variability of T/ET and its drivers under urban area. In this study, we applied a modified Shuttleworth-Wallace (S-W) model to partition ET for a locust tree forest in jinnan district of Tianjin city. The new model considers the impact of carbon dioxide emissions on vegetation transpiration and significantly improves the performance of the original S-W model. The Eddy Covariance (EC) and stable water isotope method was used to monitor and partition ET in locust tree forest. Isotope composition of ET (δ_ET), soil evaporation (δ_E) and vegetation transpiration (δ_T) were determined using the Keeling-plot method, Craig-Gordon model and Steady-state assumption model (SSA), respectively. The verification result suggest the modified S-W model could provide reliable prediction for ET and its components. The modified S-W model estimated T/ET ranges from 0 to 1, with a near continuous increase over time in the early growing season when leaf area index (LAI) is small and then convergence towards a stable value when LAI is larger. The results show seasonal change in T/ET can be described well as a function of LAI, implying that LAI is a first order factor affecting ET partitioning, and soil moisture also influence the ET partitioning. This study reveals the change in T/ET patterns and its controlling factors in urban woodland areas. Understanding the impact of urbanization and human activities on the urban water cycle will allow more effective water use in urban environments.