Exploring the multifaceted reason for deficits in soil water within different soil layers in China's drylands

Soil water regulates the hydrological cycle and provides the water required for vegetation growth in drylands. However, existing studies have rarely investigated the reason for changes in soil water within different layers and compared differences in the contribution of driving factors from spatial and inter-annual perspectives. This study analyzed the dynamics of soil water content (SWC) at shallow (0-28 cm), intermediate (28-100 cm), and deep (100-289 cm) layers. The individual and interactive effects of different environmental factors on the spatial heterogeneity of SWC were investigated using the geographical detector. By selecting evapotranspiration (ET), precipitation, air temperature, land surface temperature (LST), and the Normalized Difference Vegetation Index (NDVI), we determined the influences of these factors on SWC dynamics for each layer as well as their contributions using ridge regression. Meanwhile, we also analyzed the influence of extreme events and lag effects of climatic factors on SWC. The results showed that SWC in each layer primarily exhibited a downward trend. In different layers, all interactive factors (two factors) had stronger effects on the spatial heterogeneity of SWC compared to any individual factor. Precipitation was the largest contributor (25.03 ± 20.66%) to SWC dynamics in shallow layer but its contribution decreased with increasing soil depth. NDVI mainly controlled the SWC in intermediate and deep layers with the largest contribution and exerted a dual effect on SWC. The contributions of air temperature and LST were the smallest. The lagged months of precipitation and air temperature on SWC increased with soil depth. Precipitation extremes with long duration and warm temperature extremes had large contributions and exceeded that of the average climate changes. These findings can provide a deeper insight into soil water dynamics and their environmental factors and scientific guidance for maintaining the hydrological cycle and sustainable development of vegetation in drylands.