*4.2. Estimation of Soil Moisture*

Based on the study area and data, data from July 2019 to December 2020 (modeling time) were used for modeling, and data from 2021 (validation time) were used to retrieve the soil moisture (Figure 7). Accuracy was evaluated in terms of unbiased root-mean-square error (ubRMSE) and Pearson correlation coefficient (R) based on soil moisture data from measured sites. Figure 7 provides the retrieval maps of soil moisture for any time in the four seasons: spring, summer, fall and winter. It can be seen that the western region showed a significantly lower soil moisture phenomenon in four seasons in 2021, while the eastern part was relatively much wetter. Based on the occurrence of the phenomenon of annual drought in the western United States in 2021 (US Drought Monitor, USDM), the retrieval results of soil moisture in this study were more consistent with the actual situation.

**Figure 7.** Soil moisture maps retrieved using the method of this study for different periods and represent (**a**) 4 January 2021, (**b**) 2 March 2021, (**c**) 4 August 2021 and (**d**) 26 November 2021, respectively.

The sites where the retrieved soil moisture was in good agreement with the measured data are shown in Figure 8. It can be seen that the retrieval results of soil moisture at these four stations were dense, and all of them could capture the changes in measured soil moisture and reflect the changes in low and high values of soil moisture.
