Revegetation impacts on moisture recycling over Loess Plateau in China

Moisture recycling of local water sources through evaporation allows a region to maintain precipitation in the same region. Many studies have shown that deforestation can reduce evaporation and downwind rainfall, and it has been suggested that reforestation conversely increase evaporation and downwind rainfall. Precipitation has been observed to increase over China’s Loess Plateau over the past two decades, coinciding with the start of the Grain for Green project - the largest active revegetation programme attempted in the world. However, the contribution of revegetation to the increase in precipitation is yet unknown. Here, we aim to quantitatively analyze the relationship between revegetation, evaporation, and locally recycled moisture. Based on the ERA5 reanalysis data, we used the modified Water Accounting model-2 layers (WAM-2layers) to track the recycling moisture over the Loess Plateau. Preliminary results indicate that local recycling moisture (Er) accounted for almost one-tenth of the annual precipitation, and seems to have a decreasing trend, which was more evident after 2000. Meanwhile, the contribution of local evaporation to local precipitation appears to decrease during both 1982-1999 and 2000-2015, while the decreasing trend has been slightly amplified after the revegetation. Spatially, Er over the Loess Plateau showed a decreasing trend from southeast to northwest. Significant increasing trend of Er can be identified in the northern part of the plateau during 1982-1999. However, after the implement of the Green for Grain Project, most area over the Loess Plateau showed a decreasing trend, which is significant in the east. Thus, contrary to popular wisdom, the revegetation appears to have led to a decrease in evaporation and subsequent recycling, and the increase in precipitation seems to have other causes. These results are subject to high data uncertainty, and further research is needed to better understand the hydroclimatic effects of revegetation projects under climate change.