Soil moisture variation and affecting factors analysis in the Zhangjiakou–Chengde district based on modified temperature vegetation dryness index

As an important water conservation and sand–wind barrier, the Zhangjiakou–Chengde district (ZC) is highly important for ecological protection in the Beijing–Tianjin–Hebei region. The research on the variation in soil moisture and its affecting factors is important for early drought warning and the improvement of environmental protection. Based on the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Global Land Data Assimilation System (GLDAS) datasets, the spatiotemporal variation in surface soil moisture in the ZC was simulated from 2001 to 2021 using the temperature vegetation dryness index (TVDI) model. The optimal parameter geographical detector (OPGD) method was used to identify the contributions of 10 factors affecting soil moisture. The results indicate that soil moisture generally fluctuated during 2001–2021. Six phases were identified. Spatially, the soil moisture was higher in the east and lower in the western part of the study area. Approximately 83.09 % of the district experienced a progressive increase in soil moisture. The future soil moisture dynamics trend indicates that 62.98 % of the ZC would shift from dry to wet conditions. The normalized difference vegetation index (NDVI), precipitation, land use types, slope, elevation, temperature, aspect, sand content, silt content, and clay content were analyzed to determine their effects on the soil moisture variation. The interaction analysis revealed that the effect of multiple factors was higher than that of the individual factors. The synergistic interaction between NDVI and elevation had the highest influence on soil moisture. The results of the risk detector showed that the NDVI, precipitation, elevation, slope, and clay content contributed to soil moisture. Meanwhile, the temperature and sand content contributed to soil moisture in the converse manner. The research on soil moisture variations and its impact factors on the ZC has high significance for the efficient utilization of water resources and eco–environmental protection.