Detecting the future impact of the "Grain for Green" program on land use/ land cover and hydrological regimes in a watershed of the Chinese Loess Plateau

    To increase vegetation coverage and improve ecosystem services, the government has promulgated and implemented the program of "Grain for Green" from 1999 to the present. How and where vegetation cover increases and how land use changes are the determinant factors in regional water resources and hydrological regimes. On Chinese Loess Plateau (LP), an arid and semiarid area with fragmented topography and the transitional vegetation nature, accurate prediction of land use and vegetation change is particularly important. We employed a simple habitat analogy approach, the "Matching Tree Species with Site" principle and the Soil and Water Assessment Tool (SWAT) to predict the potential vegetation restoration and land use/land cover (LULC) change and investigated its impact on the hydrological regime in a watershed of Liujiahe. Results showed that the maximum recoverable vegetation cover of the Liujiahe watershed was 71.1%, where still 9.2% potential for the vegetation cover and the vegetation of 36.4% of the area continued to improve in the future. Under the current status of vegetation restoration, the areas suitable for future afforestation in the watershed are limited in the area of only 46.06 km²; on this basis, cropland will decrease by 47.4%, and grassland and forestland will increase by 15.8% and 0.7%, respectively, in comparison with 2020. However, SWAT results showed that vegetation restoration has already reduced annual mean runoff and soil water content (SW) by 44.2% and 43.9%, respectively, while evapotranspiration (ET) has increased by 12.6%, and the land use changes in the next 20-30 years will further reduce the runoff and SW by 15.6% and 11.1% respectively, and increase ET by 1.2%. Overall, large-scale vegetation restoration has greatly affected amounts and spatial-temporal distribution of hydrological regimes on the Loess Plateau. After 20 years of vegetation restoration, there is still room for further restoration in the area, however, considering the limited rainfall and water carrying capacity, the vegetation restoration of this watershed should be based on natural restoration or low water consumption grasses and shrubs to avoid a water resources crisis. These results is expected to provide a perspective for modelling LULC changes in areas with fragmented terrain and highly influenced by human activities, and to provide an important for underpinning sustainable management of natural resources on the LP under long-term ecological restoration.