Assessing land surface drying and wetting trends with a normalized soil water index on the Loess Plateau in 2001-2016

Long-term drought may cause severe damage to ecosystems. To assess drought intensity, we introduced a normalized soil water index (NSWI) of land surface, on the basis of rainfall and actual evapotranspiration. Areas undergoing land surface drying on the Loess Plateau in 2001-2016 were assessed on the basis of the values of NSWI combined with rainfall and land surface water storage (LSWS). The extent of soil drying and wetting at depths of 0-0.01 m, 0-0.1 m and 0-2 m was also quantified. Results showed that up to 7.16% of the Loess Plateau was subjected to decreasing LSWS. On an inter-annual time scale, land surface drying intensified starting in 2003, and this pattern was chiefly evident at the soil depth of 2 m. The approach proposed in this study could also be used to identify temporary dry soil layers (DSLs) in arid ecosystems.