How does temporal trend of reference evapotranspiration over the Tibetan Plateau change with elevation?

Considerable efforts have been made to determine spatial and temporal patterns and the driving factors for reference evapotranspiration (RET) over the Tibetan Plateau (TP). Yet, how temporal trends of RET vary with respect to elevation is less clear. By using long-term daily meteorological records of 84 stations from 1971 to 2015, this study investigates the elevation dependence of RET trend over the TP. The results show that both increasing and decreasing trends of RET for the study period exist over the TP. The increasing and decreasing trends of RET have significant correlations with elevation, but the absolute changes become smaller in the higher elevation ranges at both the annual and seasonal scales. This is due to the elevation-dependent changes of climate factors, including vapour pressure deficit, mean air temperature, solar radiation, and wind speed, which demonstrate different relationships with elevation. A multivariate linear regression method was used to determine the contributions of the climate factors to RET changes, which indicates that vapour pressure deficit and solar radiation were the primary contributing factors to the increasing trend of RET and wind speed mainly influenced the decreasing trend. This elevation dependence of RET change-rate has important implications for water resources and ecosystems over the TP as well as other high-elevation regions, where much of the globe's glacier and snow surfaces are stored.