*2.1. Study Area*

The study area is located in the southern United States, characterized by diverse terrains and rich ecosystems. The region experiences a subtropical humid climate, with an average annual precipitation of approximately 834.45 mm, contributing to the area's rich biodiversity and thriving ecosystems. Geographically, the study area exhibits significant variations in altitude, ranging from −88 m to 4277 m, with an average altitude of 1778 m. The terrain generally features higher elevations in the west and lower in the east. This variation in terrain provides excellent conditions for studying the relationship between SM and environmental factors such as terrain and climate. Ecologically, the primary land cover types in the study area are grasslands and tropical savannas, collectively accounting for 55% of the total area. Additionally, a considerable portion of the western region is covered by open shrublands, making up 13% of the total area. The elevation and land cover types of the study area are shown in Figure 1.

**Figure 1.** DEM and land cover type map of the study area.
