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

Terrestrial evapotranspiration (ET _a ) 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 _a remains poorly understood, especially in alpine regions. Drawing on the Global Land Evaporation Amsterdam Model (GLEAM) ET _a data, the interannual variability of ET _a and its ties to precipitation (P), potential evaporation (ET _p ) and vegetation (NDVI) were analysed. The Budyko framework was implemented over the period of 1982 to 2015 to quantify the response of ET _a to climate change's direct (P and ET _p ) and indirect (NDVI) impacts. The ET _a , P, ET _p and NDVI all showed significant increasing trends from 1981 to 2015 with rates of 1.52 mm yr ^-1 , 3.18 mm yr ^-1 , 0.89 mm yr ^-1 and 4.0 × 10 ^-4  yr ^-1 , respectively. At the regional level, the positive contribution of increases in P and NDVI offset the negative contribution of ET _p to the change in ET _a (∆ET _a ). The positive ∆ET _a between 1982 and 2001 was strongly linked with the concomitant increase in NDVI. Increases in vegetation contributing to a positive ∆ET _a 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 _a, with P as the dominant factor affecting forested lands during both periods, and alpine vegetation between 2002 and 2015. Moreover, ET _p 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 _a are modulated by both the sensitivity of impact factors acting on ET _a as well as the magnitudes of factor changes. The greening of vegetation can influence ET _a 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.
Competing Interests: Declaration of competing interest The authors declare no competing financial interests.
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