The relative contribution of vegetation greening to the hydrological cycle in the Three-North region of China: A modelling analysis

China has implemented a few large-scale afforestation programs in the arid and semi-arid areas, including the north-eastern, northern, and north-western regions, collectively referred to as the Three-North region (TNR), to combat desertification and control dust storms. Although these programs have alleviated environmental problems in the region to a certain extent, the effects of increasing vegetation greenness on the hydrological cycle remain controversial. In this study, the relative effects of afforestation programs on the hydrologic processes in the TNR were identified based on large-scale hydrological modeling and satellite remote sensing data. The model forcing was featured with long-term vegetation dynamics. The study period ranged from 2000 to 2015, during which the greening of vegetation in the TNR was increased after multiple afforestation programs were carried out around 2000. The results indicated that, despite the spatial heterogeneity, the vegetation leaf area index (LAI) exhibited a significant increase across the TNR. Evapotranspiration (ET) increased over the entire region at the rate of 2.9 mm/year from 2000 to 2015, and its spatial pattern was consistent with changes in the LAI and precipitation. However, this does not imply equal contribution by vegetation growth and precipitation change. The results from the simulation scenarios indicated that precipitation change had a more significant influence on the ET, soil moisture and runoff than vegetation growth at the regional scale. Vegetation greening and precipitation increase in the Loess Plateau indicated their approximately equal impacts on ET and soil moisture. Consequently, the hydrological cycle was sensitive to afforestation practices at small-catchment or sub-regional scales where the magnitude of vegetation greening is relatively high. Therefore, the hydrological effect of vegetation greening at large scale may be overestimated by previous studies. To ensure sustainable water resource management at the TNR scale, special attention should be paid to climate change rather than afforestation efforts under the current level of ecological restoration programs.