Dynamics of deep soil drought triggered by revegetation across a semiarid watershed

A distinct greening trend is evident in Asia, especially on the Chinese Loess Plateau (CLP), which is driven by climate change and human-induced revegetation projects, such as the Grain for Green (GFG) project launched in 1999. However, revegetation may cause below-ground soil drought via excessive consumption of deep soil moisture (SM). To ascertain the contributions of revegetation projects to the greening trend on the entire CLP, and then evaluate the spatial-temporal variations of soil drought, as indicated by the dried soil layer (DSL), we collected multisource satellite datasets from 1982 to 2019 on the CLP and measured SM to a depth of 500 cm on 20 occasions at 73 locations from 2013 to 2016 at a typical watershed. We found that the revegetation project failed initially to make a positive contribution in the first few years because of the drought conditions in 1999-2005; after 2005, the increasing trend of vegetation change on the CLP indicated that the revegetation project, as a type of external disturbance, began to improve vegetation growth, meanwhile the increased precipitation played a critical role. The contribution of the revegetation projects increased quickly until 2013, after which it remained stable and reached average values of 58.8%±19.34% in the representative areas that conducted the GFG project. The DSLs occurred at > 90% of the sampling sites within the watershed, and the spatially and temporally averaged DSL thickness (DSLT) and soil water content within the DSL (DSL-SWC) were 257 cm and 10.4%, respectively, which suggests that 51.4% of the 500-cm-profile is drying out below 125 cm. The DSLT and DSL-SWC demonstrated a moderate degree of variability (20% < CV < 84%) in space, and showed a moderate and weak temporal variability, in time, respectively. The temporal series of the mean spatial DSLT significantly correlated with climatic variables. The spatial variation of the mean temporal DSL-SWC differed significantly among the land uses and between shaded and sunlit aspects. Our results highlighted that the meteorological processes, land use, and topography played an essential role in shaping DSL variation and distribution pattern. Understanding this information is helpful for vegetation construction, soil and water conservation, and soil drought meditation via the best management practices in the CLP and other water-limited regions with deep soils.