The role of climate change and vegetation greening on the variation of terrestrial evapotranspiration in northwest China's Qilian Mountains.

Terrestrial evapotranspiration (ET) reflects the complex interactions of climate, vegetation, soil and terrain and is a critical component in water and energy cycles. However, the manner in which climate change and vegetation greening influence ET remains poorly understood, especially in alpine regions. Drawing on the Global Land Evaporation Amsterdam Model (GLEAM) ET data, the interannual variability of ET and its ties to precipitation (P), potential evaporation (ET) and vegetation (NDVI) were analysed. The Budyko framework was implemented over the period of 1982 to 2015 to quantify the response of ET to climate change's direct (P and ET) and indirect (NDVI) impacts. The ET, P, ET and NDVI all showed significant increasing trends from 1981 to 2015 with rates of 1.52 mm yr, 3.18 mm yr, 0.89 mm yr and 4.0 × 10 yr, respectively. At the regional level, the positive contribution of increases in P and NDVI offset the negative contribution of ET to the change in ET (∆ET). The positive ∆ET between 1982 and 2001 was strongly linked with the concomitant increase in NDVI. Increases in vegetation contributing to a positive ∆ET differed among landscape types: for shrub, meadow and steppe they occurred during both periods, for alpine vegetation between 1982 and 2001, and for desert between 2002 and 2015. Climate change directly contributed to a rise in ET with P as the dominant factor affecting forested lands during both periods, and alpine vegetation between 2002 and 2015. Moreover, ET was a dominant factor for the desert between 1982 and 2001, where the variation of P was not significant. The contributions of factors having an impact on ∆ET are modulated by both the sensitivity of impact factors acting on ET as well as the magnitudes of factor changes. The greening of vegetation can influence ET by increasing vegetation transpiration and rainfall interception in forest, brush and meadow landscapes. These findings can help in developing a better understanding of the interaction of ecosystems and hydrology in alpine regions.