Variation in actual evapotranspiration and its ties to climate change and vegetation dynamics in northwest China

Abstract

Actual evapotranspiration (ETa) has significantly increased under recent climate and vegetation changes. Discriminating the primary driver of ETa variation would improve our knowledge of the interaction between regional hydrological and ecological systems. In this study, a reliable ETa dataset was obtained by validating three ETa 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 ETa variation in northwest China (NWC). On the basis of the magnitude and the sensitivity of ETa variation, results revealed that annual ETa 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 ETa, the partial correlation coefficient-based elasticity method was able to explain 83% of the annual variation in ETa. Precipitation, temperature, and NDVI were the most important factors controlling the increase in annual ETa from 1982 to 2015. Regionally, precipitation was the dominant factor and contributed 73% to the variation of annual ETa 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 ETa 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.