Vegetation restoration enhances the regional water vapor content by intensifying the inflow from the lower atmosphere on the Loess Plateau in China

The increase in vegetation coverage resulting from the implementation of the Grain for Green Project (GFGP) on the Loess Plateau (LP) in China has sparked debates on how the GFGP affects the water cycle, due to the limited understanding of the interaction between land cover and the atmosphere. This study investigated spatiotemporal changes in atmospheric water vapor (WV) from 1979 to 2022 using ERA5 dataset and quantified the impacts of the GFGP on the WV budget on the LP through numerical simulations with the Community Earth System Model. The analysis of ERA5 data revealed an increase in WV content (WVC) after the implementation of the GFGP (2000-2022) compared to the pre-program period (1979-1999). This increase was primarily attributed to a greater reduction in WV outflow at the eastern boundary than the reduction in inflow at the western boundary, resulting in greater WV retention. Our sensitivity simulation experiments indicated that the GFGP had positive effects on WVC, with evident increases in the lower atmosphere (700-1000 hPa). The GFGP increased WV inflows at the southern and western boundaries by 3.84 x 104 kg<middle dot>s-1<middle dot>m-1, while WV outflows at the eastern and northern boundaries also increased by 1.40 x 104 kg<middle dot>s-1<middle dot>m-1, leading to a net WV input in the low atmospheric layer due to higher inflows than outflows. These findings suggested that although the GFGP positively impacted WV, the observed post-2000 WV increase cannot be solely attributed to the GFGP, indicating that changes in atmospheric circulation patterns under climate change also play an important role.