Since the implementation of the Grain for Green Project (GFGP) in the 1990s, the warming and wetting trend in the Loess Plateau is becoming statistically significant in the context of climate change. However, the correlation between precipitation increase and the regional vegetation restoration is still controversial. To explore the main factors influencing the regional precipitation change, this study selected five potential influencing factors including potential evapotranspiration (PET), normalized difference vegetation index (NDVI), precipitable water (PW), surface temperature (ST), and water vapor transport (WVT). We used the statistical methods to analyze the spatial-temporal distribution of precipitation before and after the GFGP and to quantify the relative influence degree of different factors to precipitation change. The results show that: (1) The precipitation increased significantly (95% confidence level) after the GFGP, with an increase rate of 4.96 mm a⁻¹; (2) from the perspective of spatial-temporal distribution, the precipitation in the southern part of the Loess plateau was significantly increasing with an increase rate of 20–50 mm in the period of 2000–2014; (3) the relative influence degree of NDVI to precipitation increased after the GFGP, and the annual precipitation (PRE_A) and summer precipitation (PRE_S) was more influenced by NDVI (relative influence degree of 30.18% and 31.37%, respectively) compared with winter precipitation. In winter, the PW and the PET are the main influencing factors for the precipitation change with relative influence degrees of 30.13% and 27.64%, respectively. Based on this study, we speculate that the warming and wetting trend of the Loess Plateau in recent years is not only closely related to global climate change, but also significantly affected by local climate change brought by vegetation restoration. The above conclusions are important for future ecological restoration and water resources management in the water-scarce Loess Plateau.

With the rapid development of the socio-economic system and the close connection of inter-regional trade, the physical water consumption in production and the virtual water flow associated with inter-regional trade are both have a significant impact on local water systems, especially in megacities. Beijing is the political, economic and cultural center of China, which is a megacity that has severe water scarcity. To evaluate the status-quo of local water consumption and propose the countermeasures, this study quantitatively analyzed the evolution trend of physical water consumption and the virtual water flow in Beijing. The results show that the total physical water consumption in Beijing decreased from 2.43 billion m³ (2002) to 1.98 billion m³ (2017), while the net virtual water input increased from 1.76 billion m³ (2002) to 3.09 billion m³ (2017), which was mainly embedded in agricultural and industrial products. This study also reveals the equal importance of physical water and virtual water in ensuring the regional water security and sustainable economic development. In view of poor water resource endowment, Beijing should conduct the coupled management of physical water and virtual water to alleviate the local water shortage, i.e., to receive more virtual water embedded in agricultural and industrial products, and allocate the limited local water resources to domestic use and high-benefit sectors.

      Since the implementation of Green for Grain Project (GFGP) in the 1990s, the precipitation in the Loess Plateau has increased significantly. The warming and wetting trend in this area is becoming statistically significant. However, the correlation between precipitation increase and regional vegetation restoration is still controversial. To explore the main factors influencing the regional precipitation change, this study selected 6 main contributing factors of the precipitation in the Loess Plateau based on the cloud-precipitation physics and used the statistical methods to analyze the long-term change trend and the spatial and temporal distribution of precipitation in the Loess plateau before and after GFGP and to quantify the contribution rate of different factors to precipitation change in this area. The results show that: 1) the precipitation increased significantly (95% confidence level) after Green for Grain Project since 1999, with an increase rate of 4.96 mm/a; 2) From the perspective of spatial and temporal distribution, the precipitation in the southern part of the Loess plateau was significantly increasing with an increase rate of 20-50mm in the period of 2000-2015 compared with the average annual value in the base period (1985-1999). Among them, the summer precipitation increased significantly, while the winter precipitation changed non-significantly; 3) The contribution analysis shows that the summer precipitation in the Loess Plateau is mainly affected by regional actual evapotranspiration (AET) and vegetation coverage (NDVI) after GFGP, with the contribution rates of 27.1% and 40.0%, which respectively indicates that the summer precipitation in the Loess Plateau increased mainly due to the regional vegetation restoration and the associated ET increase. The winter precipitation in the Loess Plateau is mainly affected by the precipitable water (PW) in the air and water vapor flux (VF), with the contribution rates of 33.5% and 31.7%, which indicates that the winter precipitation is mainly affected by atmospheric circulation and transport of external water vapor. Based on this study, we speculate that the warming and wetting trend of Loess Plateau in recent years is not only closely related to global climate change, but also significantly affected by local climate change brought by vegetation restoration. The above conclusions are important for future ecological restoration and water resources management in the water-scarce Loess Plateau.