Exploring the contribution of environmental factors to evapotranspiration dynamics in the Three-River-Source region, China

Evapotranspiration (ET) exerts substantial effects on terrestrial ecosystem carbon and water cycles. A deep understanding of the spatial and temporal variability in ET and its dominant controls has crucial implications for future ecological management, especially for the ecologically-fragile Three-River-Source region (TRSR) in China. The study examined the performance of three ET products including Global Land Evaporation Amsterdam Model (GLEAM), Famine Early Warning Systems Network Land Data Assimilation System (FLDAS) and Global Land Surface Satellite (GLASS) based on eddy covariance-based flux observations. On this basis, ET dynamics in the TRSR from 1982 to 2018 were analyzed at two time scales using the GLASS ET product. Spatially, the pattern in ET showed a gradual upward trend from northwest to southeast. Meanwhile, ET in the majority of regions significantly enhanced, with multiannual mean ET and summer ET (June to August) increasing by 0.609 mm/yr and 0.765 mm/yr, respectively. Especially, ET in summer dominated the interannual dynamics of ET in the TRSR. The individual contributions of five environmental factors including Leaf Area Index (LAI), solar radiation (Rs), air temperature (Ta), precipitation (PRE) and vapour pressure deficit (VPD), to the variability in ET were isolated using ridge regression. Significant differences appeared when explaining ET over different time scales. Rs (33.05 %) and LAI (25.29 %) were the dominant control variables in shaping the interannual ET, with VPD contributing the least by 7.19 %. However, the long-term dynamics in summer ET were mainly controlled by LAI (35.73 %) and PRE (31.19 %), and Ta exerted the least effect by 5.73 %. All analyses will strengthen our understanding of the relationships between climate, vegetation and ET in the TRSR area, China.