Assessing the impact of large-scale afforestation on the atmospheric water cycle of the Loess Plateau in China

Afforestation has been regarded as an appropriate way to mitigate climate change and enhance ecosystem services. How afforestation affects the availability of water resources is a hot topic in the science community. Most current studies investigate the impact of afforestation on water resources through offline modeling or observation on a small spatial scale. However, the atmospheric water cycle (AWC) is also an important aspect that can alter the availability of water resources, especially on a large spatial scale. With an investment of about US$54.57 billion, the Chinese government implemented the world’s largest afforestation project, the Grain for Green Program (GFGP), to curb the severe soil erosion over the Loess Plateau (LP) since 1999. Here we focused on this ideal platform, the LP, to explore the impact of large-scale afforestation on the processes related to the atmospheric water cycle. We adopted two different approaches to discern the hydroclimatic effect of the GFGP. This first approach used the reanalysis dataset to compare the hydroclimatic states before (1982–1998) and after (1999–2018) the GFGP. Since the reanalysis dataset cannot separate the impact of climate change and afforestation, this study also applied a regional climate model (the Weather Research & Forecasting Model, WRF) to isolate the net hydroclimatic effect of the GFGP by controlled experiments. In particular, the WRF model was driven by two land surface conditions with/without the implementation of the GFGP. We found both approaches reached similar conclusions. Results show the vegetation coverage fraction over the LP increased by 3.15% decade⁻¹ induced by the GFGP. The climatological precipitation and evapotranspiration (ET) increased by 54.62 and 22.56 mm, respectively, after starting the GFGP in 1999. The large-scale afforestation intensifies the atmospheric water cycle over the LP. In addition, based on the dynamic precipitation recycling model, we also found the precipitation recycling ratio approximately increased by 1%. The GFGP alters the regional circulation by influencing diabatic heating, and moisture convergence, resulting in more moisture being advected from the south boundary, thus more atmospheric moisture was retained in the LP. Additionally, the internal branch of the AWC contributes to about 15% of the increased precipitation, while the contribution of the external branch is about 85%. Moreover, the GFGP remotely affects the water vapor budget in the downwind areas. Our work enriched the current understanding of how afforestation affects the water cycle from a precipitation recycling perspective and can help policy-makers to make science-informed afforestation strategies.