Estimation of Land Surface Evapotranspiration and Identification of Key Influencing Factors in the Zoige Forest–Grass Transition Zone

Evapotranspiration (ET) is an important link between the water and energy cycles and directly determines the amount of available regional water resources. The Zoige forest–grass transition zone is a critical water conservation area in the upper reaches of the Yellow River, with high environmental heterogeneity, significant edge effects, and ecological and climatic gradient effects. The changing characteristics and influencing factors of evapotranspiration and its components in the region remain largely unknown. In this paper, the spatial and temporal evolution of evapotranspiration and its components in the Zoige forest–grass transition zone from 2003 to 2021 was investigated using the MOD16-STM ET algorithm, and the effects of environmental factors were analyzed. The results show that the MOD16-STM ET algorithm has good applicability in the Zoige forest–grass transition zone, and its coefficients of determination are 0.85 and 0.90 at the Zoige and Maqu stations, respectively. Vegetation transpiration accounts for 82% of the total evapotranspiration. ET is strongly influenced by the dynamics of the forest and grassland areas. The spatial distribution of evapotranspiration in the region varies considerably, with the forested areas in the east being larger than the grasslands and wetlands. Temperature and vegetation cover are the two most dominant contributors to ET changes among all the model drivers. Among the external environmental factors, altitude, maximum temperature, and minimum temperature are the dominant factors in the variation of ET in the region, and the interactions between the factors have a greater effect on ET than the individual factors. The findings provide a reference to investigate the spatial and temporal pattern of evapotranspiration and its components and the water cycle process in the Zoige forest–grass transition zone.