2.2.3. Sentinel-2

Sentinel-2 (S2) is a high-resolution imaging satellite that carries a multi-spectral imager (MSI). The revisit period for one satellite is 10 days, and the revisit period for two satellites is 5 days [69,70]. In this study, cloud-free S2 TOA Level-1C data one week before and after the S1 acquisition is selected to calculate vegetation and water indices using the green band (B3: Green), red band (B4: Red), near-infrared band (B8: NIR), and short-wave infrared (B11: SWIR) [71].

### 2.2.4. SRTM DEM

Shuttle Radar Topography Mission (SRTM) data is mainly measured jointly by the National Aeronautics and Space Administration (NASA) and the National Imagery and Mapping Agency (NIMA). Interferometric radar data is captured using dual radar antennas and converted into digital terrain data. This study uses the "NASA SRTM Digital Elevation 30-m" elevation dataset provided by the GEE platform [64].

### 2.2.5. SM Data Products

In this study, we also compared the retrieval SM with three SM data products, such as the fifth generation of the land component of the European Centre for Medium-Range Weather Forecasts atmospheric reanalysis (ERA5-Land) [13], the Noah land surface model driven by Global Land Data Assimilation System (GLDAS-Noah) [15], and the European Space Agency Climate Change Initiative (ESA CCI) [14]. The specific information of the three datasets and the first layer of the depth range are shown in Table 2. In this study, the SM product data for 2 July and 19 August 2018 were selected for comparing the spatial distribution and average SM content with the retrieval results.

**Table 2.** Information on SM products.

