An Evaluation of Evapotranspiration Products over the Tibetan Plateau
Authors: Meng Xianhong , Deng Mingshan , Shu Lele , Chen Hao , Wang Shaoying , Li Zhaoguo , Zhao Lin , Shang Lunyu 
Published: September 2024

Abstract
In situ observations from 13 sites located over the Tibetan Plateau (TP) are used to evaluate evapotranspiration (ET) products, including remote sensing–based, land surface–modeled, and reanalysis products. It is found that the Global Land Surface Satellite (GLASS) product, the Global Land Evaporation Amsterdam Model (GLEAM) product, and the simulations by the Community Land Model–dynamic global vegetation model [biogeochemical dynamic vegetation (CLM–BGCDV)] are the top-ranked products measured by the percentage bias, root-mean-square error, and correlation coefficient against in situ observations. The evaluated data are then used to examine the consistency in spatial and temporal variability of summer ET and its controlling factors on the TP and the three-river source region (TRSR). All products show consistently that precipitation in the central semiarid part of TP is the dominant factor influencing summer ET, while air temperature plays a certain role in the southeastern and eastern TP. Uncertainties exist in the western TP, possibly due to the lack of observations or gaps in the satellite data. Some suggestions for improving ET product development based on models and satellite retrievals, particularly for the cold and complex surface of the TP, are also given.

Significance Statement
Evapotranspiration (ET) is a crucial water cycle component, but accurately assessing it remains difficult. In situ measurements are scarce, while other products have large uncertainties, such as satellite remote sensing retrievals, land surface model simulations, and reanalyses. This is particularly a problem for the Tibetan Plateau (TP). With some considerable effort, ET fluxes from 13 sites and reasonable length in the TP have become available. These in situ data are used to evaluate gridded ET products. Based on an overall score, they are ranked as the Global Land Surface Satellite (GLASS) product, the Global Land Evaporation Amsterdam Model (GLEAM) product, and the model simulation by the Community Land Model–dynamic global vegetation model [biogeochemical dynamic vegetation (CLM–BGCDV)]. Then, we used these top-ranked products to detect the spatial and trend consistencies and the environmental impacts to further discuss the product conformance. It was shown that all products show precipitation dominated the summer ET variation in the semiarid region in the central TP, while air temperature had more impact on the relatively humid region in the east and southeast of the TP. Uncertainties exist in the west TP possibly due to the lack of observations in the assimilation product or missing gaps in the satellite product. Specifically, we conclude that dynamic vegetation could play an increasing role under the background of climate warming, which should be considered in the ET model–based product generation. The suggestions for ET generation based on models and satellite retrievals, particularly for the special cold and complex surface of the TP, are also provided.