Estimating the long-term daily evapotranspiration in the source region of the Yangtze River based on a universal trapezoid method

Study Region: The source region of the Yangtze River (SRYR) Study Focus: Precise estimation of evapotranspiration (ET) and its spatiotemporal dynamics is critical for assessing the water resource sustainability and future water security in the SRYR. However, accurate simulation remains challenging due to harsh environment and sparse observations. Here, the daily ET at 1 km spatial resolution during 2001–2018 in the SRYR was produced using a universal trapezoid method with parameterization of aerodynamic resistance considering the effect of the atmospheric stability on turbulent transport. In addition, the long-term spatiotemporal variations of ET and its drivers were analyzed using the produced ET dataset. New hydrological insights: Evaluations using ET derived from eddy-covariance systems, catchment water balance, and two existing ET datasets indicated that the ET dataset is reliable for quantifying both the magnitude and variations of ET in the SRYR. Mean annual ET (2001–2008) in the SRYR is 516 mm/yr, showing a spatial decrease from southeast to northwest. A decrease trend in annual ET appears in 71.1 % of the SRYR with a mean rate of −0.81 mm/yr while an increase trend appears in 28.9 % of the SRYR with a mean rate of 0.61 mm/yr. The decreased annual ET is mainly results from the reduction of solar radiation and the increase trend is associated with the greening vegetation and the warming of climate.