Weekly VPD and Monthly Precipitation as Contrasting Dominant Drivers of Soil Moisture in the Yangtze River Basin

Abstract

Soil moisture is a key indicator of agricultural and hydrological drought, but its meteorological controls vary across temporal scales. Using ESA CCI soil moisture products from 2010 to 2022, this study investigated soil moisture variability in the Yangtze River Basin at weekly, monthly, and annual scales. The product was first validated using ground observations and ERA5 reanalysis data, and a generalized additive model (GAM) was then applied to quantify the relative contributions of precipitation, temperature, wind speed, and VPD under normal conditions and during three extreme drought events. The validation showed that ESA CCI soil moisture captured basin-scale variations well, with mean absolute deviations of 0.0460, 0.0434, and 0.0072 m3/m3 in the upper, middle, and lower reaches, respectively, and a basin-wide RMSE of 0.0127 m3/m3 against ERA5. The attribution results revealed a clear scale-dependent shift in soil moisture controls. At the weekly scale, VPD dominated soil moisture variability, with a basin-wide average contribution of 47.04% and a maximum contribution of 55.92% in the lower reaches, indicating that short-term soil drying is mainly driven by VPD. At the monthly scale, precipitation became the primary control, with a basin-wide average contribution of 36.62% and a maximum contribution of 44.65% in the upper reaches, reflecting the role of accumulated rainfall recharge in maintaining soil moisture storage. During extreme drought events, the monthly-scale dominance of precipitation weakened, and precipitation, VPD, temperature, and wind speed each contributed approximately 20–30%, suggesting that drought development results from the combined effects of reduced water input and enhanced atmospheric water loss. These findings indicate that precipitation-based drought monitoring may underestimate rapid soil drying risks, whereas incorporating atmospheric demand indicators such as VPD can improve drought early warning and water resource management under a warming climate.