*4.3. Validation of Soil Moisture*

The data from the measured sites were based on the results of point measurements, and the soil moisture results were obtained in this study for a 36 × 36 km grid. It is reasonable to use the average value of measured soil moisture in a large area grid as the true value, but there is a lack of such a design for a soil moisture station. Therefore, related studies have used the values of a single measured site to verify the accuracy of soil moisture retrieval [11,14,20]. The accuracy validation of soil moisture results using the retrieval method of this study was executed based on the data measured in the study area. Tables 2 and 3 provide statistics on the accuracy comparison between the SMAP and the soil moisture retrieval results using the method of this study, based on the data of each measured site within the experimental area.

**Figure 8.** Examples of soil moisture retrieved by this study that agree well with the measured data at the measured sites (CYGNSS are the retrieval results of this study, and in situ represents measured soil moisture data; SM is soil moisture).


**Table 3.** Correlation comparison of soil moisture for SMAP and this study based on measured site data.


The average ubRMSE for the retrieval results of soil moisture in this study was 0.061 cm3/cm3 at 160 measured stations, with an average correlation of 0.4. However, SMAP products presented a better retrieval performance for the soil moisture (ubRMSE = 0.056 cm3/cm3, R = 0.55). In ubRMSE, the median and mean values of soil moisture retrieved by the method used in this study in the four monitoring networks were worse than those of SMAP, with relatively large standard deviations. The same was true in the correlation. Areas with low or no variation in soil moisture during the validation time

resulted in a lower correlation, possibly because the effect of random noise was amplified. The soil moisture values retrieved by CYGNSS may have had more noise than those of SMAP due to the lack of complete coverage of the soil moisture time series in part of the grid.

In order to show the effectiveness of the method in this study, the accuracy indexes for the CYGNSS product (i.e., UCAR/CU) and the results of this study at the common measured sites were systematically counted [19] (Tables 4 and 5). Since the UCAR/CU product was only updated until August 2020, a comparison of the accuracy metric results from this study and the accuracy metric results from the paper by Chew et al. [19] was performed.


**Table 4.** Accuracy comparison for the results of this study and the UCAR/CU product.

**Table 5.** Correlation comparison for the results of this study and the UCAR/CU product.


Compared with the UCAR/CU product, the accuracy of the soil moisture results obtained by this study was significantly improved, with the average ubRMSE of SCAN, SNOTEL and USCRN improved by 3.5%, 19% and 10%, respectively. The average ubRMSE at 88 measured sites was improved by 10%. The standard deviation and median had smaller values relative to those of UCAR/CU, indicating that the accuracy of this study remained more stable. In terms of correlation, the results of this study were the same as those of UCAR/CU. Furthermore, the median of the results of two measured networks (SNOTEL and USCRN) was relatively high, and the overall standard deviation was larger.
