Search

• A significant rise in ET a occurred over 75% of the vegetated area in NWC. • Results of ET a variation attribution by different methods were compared. • Partial correlation coefficient-based elasticity method was robust in capturing ET a variation. • Precipitation was the dominant variable driving the variation in ET a in NWC. Actual evapotranspiration ( ET a) has significantly increased under recent climate and vegetation changes. Discriminating the primary driver of ET a variation would improve our knowledge of the interaction between regional hydrological and ecological systems. In this study, a reliable ET a dataset was obtained by validating three ET a products from different sources with in-situ observations. We modified the elasticity-coefficient method using partial correlation, and compared it with the traditional regression-based method in estimating ET a variation in northwest China (NWC). On the basis of the magnitude and the sensitivity of ET a variation, results revealed that annual ET a significantly increased (2.32 mm/yr, p ≤ 0.05) in NWC over 1982–2015 with over 75% of the vegetated area showing an increasing trend. Robust in capturing spatial variation patterns in ET a , the partial correlation coefficient-based elasticity method was able to explain 83% of the annual variation in ET a. Precipitation, temperature, and NDVI were the most important factors controlling the increase in annual ET a from 1982 to 2015. Regionally, precipitation was the dominant factor and contributed 73% to the variation of annual ET a in the vegetated NWC. However, the contribution rates of precipitation varied across the different land cover types and ranged from 31% in irrigated cropland to 81% in the steppe. Potential uncertainty in the attribution of causative factors could arise in selecting the separation method and potential driving factors. Establishing a reliable relationship between ET a and potential driving factors should be confirmed with observations. The present study's results can guide sustainable water resources management and ecological restoration in water limited regions. [ABSTRACT FROM AUTHOR]

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., (Crown Copyright © 2020. Published by Elsevier B.V. All rights reserved.)