Since the Grain for Green Project was launched in the Loess Plateau in 1999, the vegetation restoration effect has been significant. However, a high degree uncertainty in changes and predictions of regional water and carbon cycles caused by vegetation changes hinders the development of vegetation restoration assessment and prediction. Water use efficiency (WUE) is an important indicator for assessing the water-carbon cycle in ecosystems. Based on the simulation results of regional scale actual evapotranspiration (ET) by the PT-JPL model, combined with the GPP of the Loess Plateau, this paper analyzes the temporal and spatial trends of WUE on the Loess Plateau after the Grain for Green Project, and further uses CMIP6 to predict the WUE's of Loess Plateau under three scenarios. The results show that vegetation in the Loess Plateau increased significantly from 2001 to 2015, and that GPP and ET increased at rates of 3.59 g C·m^-2 and 4.39 mm per year, respectively. WUE increased in 72.68% of the regions with an average annual 0.003 g C·mm^-1·m^-2 increase. In the three scenarios from 2015 to 2100, ET showed an increasing trend while GPP and WUE didnot change much in SSP126 and increased significantly in SSP245 and 370 scenarios. WUE increased with the increase of GPP. WUE combines "consumption" and "use" of water resources to illustrate the effectiveness of the Project. Although the revegetation increases regional water consumption, it has significantly improved the vegetation cover, effectively enhanced the carbon sequestration capacity and water use efficiency of the vegetation, and increased the drought tolerance of the entire Loess Plateau vegetation. The WUE's of different tree species need to be further analyzed to screen out the tree species with higher drought resistance for vegetation restoration. The ecological construction and water conservation of the whole region is an eternal topic, and the healthy development of water-carbon cycle should be improved in current climate context in order to provide a better theoretical guarantee for the synergistic development of economic construction and ecological civilization of the Loess Plateau.

Loess Plateau  /  water use efficiency  /  Grain for Green Project  /  vegetation restoration