One of the world's largest revegetation programs, the Grain for Green Project (GfGP), has been taking place on the Loess Plateau of China since 1999. Such massive revegetation causes changes in the region's hydrological cycle, water availability, and ecological sustainability through enhanced evapotranspiration (ET). Here we quantify effects of the GfGP's revegetation on ET over this water-stressed region. Our approach involves use of a modified Priestley-Taylor Jet Propulsion Laboratory (PT-JPL) model, incorporating vegetation dynamics as a new component. The original PT-JPL model has been expanded from site scale to regional scale, thereby allowing its application to the Loess Plateau. The modified PT-JPL model was calibrated and validated against flux tower-measured and water balance-based ET observations. The model performed well at a regional scale with the incorporation of vegetation dynamics. To quantify the net effect of revegetation on evaporative water consumption after the GfGP, we compared scenarios with and without revegetation. We find the revegetation has led to a significant increase in ET across the Loess Plateau, of 4.39 mm/yr averaged over the past 15 years (mean annual precipitation was 464 mm). Compared with the no revegetation scenario, the GfGP revegetation appreciably enhanced evaporative water consumption across the Loess Plateau, by approximately 31 × 10 8 m 3 /yr (or 4.90 mm/yr). Our findings suggest that to maintain ecologically sustainable restoration and rational use of water resources, factors including the strength of revegetation and the relationship between evaporative water consumption and revegetation type should be considered. Key Points:• The PT-JPL model has been upgraded to allow incorporation of vegetation dynamics at a regional scale • The extended PT-JPL model performs well at regional scale • Compared with a no-revegetation scenario, revegetation increases evaporative water consumption byapproximately 31 × 108 m3/yr (4.90 mm/yr) across theLoess Plateau, China Supporting Information:• Supporting Information S1

Estimating long-term average annual water balance at the catchment scale has been an important scientific problem in hydrology and a reliable method for long-term estimates of evapotranspiration (D. Wang, 2012). Water and vegetation in the catchments have reached equilibrium through long-term evolution, so that there is an inextricable relationship between water balance and vegetation change (Gerten et al., 2004). Changes in vegetation affect the elements of water balance, thus vegetation coverage plays an influential role in regulating regional water balance (Heimann & Reichstein, 2008; Seddon et al., 2016). Precipitation (P), evapotranspiration (ET), streamflow (Q), and water storage changes (ΔS) are important components of water balance estimates. Previous studies (e.g., Shao et al., 2019) have demonstrated that vegetation restoration could lead to increases in regional ET over the Loess Plateau, thus changing the water availability. This could potentially exacerbate tensions between water supply and demand in water-stressed regions. Thus, it can be further hypothesized that the vegetation changes may influence the catchment-scale water balance. In addition, the relationship between ΔS and vegetation may be scale dependent and remains unclear, thus studying the time scale of ΔS in vegetation recovery areas is particularly important. The Loess Plateau, a typical arid and semi-arid region, accounts for 6.6% of China's total land area and supports 8.5% of the population (Fu et al., 2011). It is one of the most water scarce regions with the most fragile ecosystems in the world (Y. Wang et al., 2011; B. Zhang et al., 2016). In the past, the Loess Plateau, with its sparse vegetation coverage, frequent summer rains and intensive agricultural practices, suffered unprecedented water scarcity, soil erosion and fragile ecosystems (C. Wang et al., 2016; Z. Yang et al., 2016). These ecological and environmental problems significantly affect ecological environment and socioeconomic

Upstream inland rivers provide most of water resources for midstream and downstream in arid and semiarid regions. From the perspective of ecological rehabilitation, it is necessary to simultaneously ensure upstream streamflow while still maintaining economy-ecosystem balance in upstream. However, vegetation dynamics effects caused by ecological rehabilitation cannot be well evaluated in previous studies due to the sparseness of observations at regional scales. With increased availability of vegetation and hydrological data, it is now possible to explore the relationship between water yield and ecological balance affected by vegetation dynamics. This study takes Upper Hei River Basin, part of the Asia's water tower-the Qinghai-Tibet Plateau, as an example and uses the Eagleson's ecohydrological model to explore the effects of vegetation dynamics on streamflow and ecology in growing seasons during 1992-2015. We compare scenarios with and without vegetation changes and find that there is limited evidence that ecological rehabilitation could enhance water yield. Approximately 51.31% of Upper Hei River Basin vegetation cover (M) tends to deviate from ecological optimization state compared to equilibrium vegetation cover (M eq). In order to explore ways to ensure streamflow and maintain ecology, different vegetation scenarios are simulated. Results show that on average, grazing in area where M is larger than M eq could increase streamflow by approximately 7.3%/year and planting trees in other area could increase streamflow by approximately 2.9%/year. Therefore, effective control on vegetation dynamics can simultaneously ensure streamflow and maintain ecological balance, which can also provide guidance and theoretical support for upstream inland river planning.

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