Vegetation greening intensified transpiration but constrained soil evaporation on the Loess Plateau

Abstract

Global vegetation greenness has likely enhanced evapotranspiration (ET) that has been assumed to be the primary contributor to water consumption in some regions. However, the proportion of each ET component and their changes given vegetation greening are unclear. In this study, we investigated the response of ET components, especially vegetation transpiration (Et) and soil evaporation (Es), to vegetation and climate changes in a typical dryland region, i.e., the Loess Plateau in China, where vegetation is significantly greening. We used a sophisticated hydrological model (i.e., the Variable Infiltration Capacity, VIC) to partition ET, which considers the water-energy balance and vegetation dynamics associated with satellite products of fractional vegetation cover, leaf area index, surface albedo, and radiation forcing. The results showed that during the greening period of 2000–2018, Et had the largest increasing rate among the components and accounted for 31.8% of the ET. In contrast, Es showed a minor increase but occupied a larger proportion of about 60% of ET. Therefore, water consumption was primarily from Es rather than Et despite the increased proportion of Et. Vegetation greening certainly intensified Et, but also substantially constrained Es with a rate of −2.36 mm yr−1. Increasing water availability (i.e., more precipitation) on the Loess Plateau made the largest contribution to the increase in the annual ET compared with vegetation greening and radiation forcing. These findings imply that the effect of vegetation greening on water consumption may be overestimated on the Loess Plateau. Our study highlights the importance of considering water balance and vegetation dynamics when evaluating ecological programs.