*2.1. Study Area and Ground Measurements*

The study area was located in the southern United States, covering 26 states with a geographical range of 75◦~122◦W and 30◦~38◦N (Figure 1). The region covers a vast area, with subtropical, temperate continental and tropical climates in the southeast, central and southwest. The southeast is warm and humid, the middle is cold in winter and hot in summer and the southwest has a large annual temperature difference of up to 25 ◦C. The topography of the entire region is high in the west and low in the east, and there is a clear topographic divide (100◦W) in the middle of the region. The western part of this topographic divide is dominated by plateaus and mountains, while the eastern part is plains (Figure 1a). Based on the LC\_Type1 (Scheme of global vegetation classification for IGBP) band of the MCD12Q1 product of MODIS, a map of vegetation cover types in the study area was obtained, as shown in Figure 1b.

**Figure 1.** Location and measured points of the study area. (**a**) Topographic map, (**b**) soil type map.

The measured soil moisture data used in the study area were freely available to the public in terms of volumetric water content (m3/m3) and were derived from the International Soil Moisture Observation Network (ISMN; http://ismn.geo.tuwien.ac.at/ networks, accessed on 6 November 2021). In this study, four monitoring networks, ARM, SCAN, SNOTEL and USCRN in ISMN, with a total of 160 measured monitoring points, were used from July 2019 to December 2021, with a soil moisture depth of 5 cm. Among these measurement stations of soil moisture, meaningless stations with values less than 0 cm3/cm3 or greater than 1 cm3/cm3 were excluded.
