Elevation plays a crucial role in modulating the spatiotemporal distributions of climatic variables in mountainous regions, which affects water and energy balances, among which reference evapotranspiration (ET0) is a key hydrological indicator. However, the response of ET0 to climate change with elevation continues to be poorly understood, especially in the Tibetan Plateau (TP) which has elevation variations of more than 4,000m. The spatiotemporal variations of ET0 with elevation were investigated using long-term (1960-2017) meteorological observations from 82 stations on the TP. The results suggest that the average annual ET0 showed an insignificant increasing trend. A significant negative correlation between ET0 and elevation was found (p<.01). The positive trends of ET0 decreased with elevation, whereas the negative trends of ET0 increased significantly with elevation (p<.05). The magnitude of trends of ET0 becomes smaller at higher-elevation stations. Sensitivity analysis indicated that ET0 was most sensitive to shortwave radiation (R-s). Moreover, the sensitivities of temperature (T) and wind speed (U) significantly decreased with elevation, whereas those of R-s and vapour pressure deficit (VPD) increased slightly with elevation. The contribution and path analyse indicated that increasing VPD was the dominant contributor to the increase in ET0. The effect of elevation on ET0 variation mainly depended on the tradeoff between the contributions of U and VPD. U was the largest contributing factor for the change in ET0 below 2,500m, whereas VPD was the primary contributor to the increase in ET0 above 2,500m. This study provides insights into the response of ET0 to climate change with elevation on the TP, which is of great significance to hydrometeorological processes in high-altitude regions.

Study region: The Tibetan Plateau Study focus: Evapotranspiration (ET) plays a critical role in the water balance, energy budget, and carbon cycle. However, the variations, trends, and controls of ET on the Tibetan Plateau (TP) are poorly understood because of uncertainties in ET estimates and sparse observations. In this study, the variations in ET and its components and their drivers and controls in the TP were analyzed at seasonal and annual scales during 1982-2015. New hydrological insights for the region: Spatially, the multiyear mean annual ET decreased from the southeastern to northwestern TP. Canopy transpiration (Ec) was the main component of ET (52.7%), followed by soil evaporation (Es) (34.4%) and interception (Ei) (10.7%). Regionally, the averaged ET and its components increased significantly at the seasonal and annual scales. Spatially, the controlling factor for ET changed from water to energy as the climatic zones transferred from aridity to humidity. The annual ET was controlled by soil moisture (SM) in arid and semi-arid zones, whereas Ta was the dominant factor in the other regions. The increased annual Es and Ei were primarily caused by SM, while the annual Ec was determined by Ta. In addition, NDVI played a certain role in regulating the annual Ec and Ei variations. This study improves our understanding of hydrological processes and water resource management under global climate change.