Mapping daily evapotranspiration (2003-2018) based on a modified universal Ts-VI triangle method in the source region of Yangtze river on the Qinghai-Tibetan Plateau

The source region of Yangtze river has experienced permafrost degradation and ecological deterioration since 1980s. Accurately estimating the evapotranspiration (ET) and analyzing the spatiotemporal variation in the region is crucial for understanding the change in permafrost and ecological environment. The universal Ts-VI model, transforming the Ts-VI feature space from regional to pixel scale, has been performed over a poorly gauged region with arid ecosystems in the Qinghai-Tibetan Plateau with high spatial resolution and daily continuity. However, the aerodynamic resistance formulation in this universal Ts-VI model only holds true under neutral stability conditions. This study proposes a scheme for improving the estimation of ET based on the universal Ts-VI model by integrating an aerodynamic resistance formulation without the assumption of the neutral stability conditions. The daily ET in the source region of Yangtze river on the Qinghai-Tibetan Plateau from 2003 to 2018 is achieved based on the modified universal Ts-VI model by using the MODIS, Daily 1-km all-weather land surface temperature dataset for Western China (TRIMS LST-TP), and China meteorological forcing dataset (CMFD) and is evaluated by comparing it with eddy covariance measurements in two sites located in permafrost, modeled ET from original universal Ts-VI model, and a readily available daily ET product. Our results indicate that incorporating the aerodynamic resistance formulation without requirement of neutral stability conditions into the universal Ts-VI model can improve the estimation of ET in the plateau of cold and arid region and thus provide more accurate ET maps for study of permafrost degradation and ecological deterioration.