## 3 Results
)**} & \multicolumn{4}{c}{**Downscaling Model**} & \multicolumn{2}{c}{**Downscaling Model**} & \multicolumn{2}{c}{**Level 3 SMAP Soil**} \\  & & & **Using TVDI** & \multicolumn{2}{c}{**Based on Original**} & \multicolumn{2}{c}{**Moisture Product in**} \\  & \multirow{2}{*}{**Type**} & **Observations** & **Estimations** & **Error** & **Estimations** & **Error** & **Estimations** & **Error** \\  & & **(m\({}^{3}\)-m\({}^{-3}\))** & **(m\({}^{3}\)-m\({}^{-3}\))** & **(\%)** & **(m\({}^{3}\)-m\({}^{-3}\))** & **(\%)** & **(m\({}^{3}\)-m\({}^{-3}\))** & **(\%)** \\ \hline A1(1) & Wetlands & 0.1580 & 0.1669 & 5.64\% & 0.1707 & 8.06\% & 0.1329 & 15.89\% \\ A1(2) & Wetlands & 0.1771 & 0.1871 & 5.66\% & 0.1912 & 7.98\% & 0.1465 & 17.28\% \\ A1(3) & Wetlands & 0.2019 & 0.2145 & 6.24\% & 0.2181 & 8.03\% & 0.221 & 9.46\% \\ A1(4) & Wetlands & 0.1932 & 0.2050 & 6.12\% & 0.2090 & 8.19\% & 0.164 & 15.11\% \\ A1(5) & Wetlands & 0.1847 & 0.1972 & 6.77\% & 0.1997 & 8.12\% & 0.1642 & 11.10\% \\ A1(6) & Wetlands & 0.1467 & 0.1568 & 6.91\% & 0.1570 & 6.99\% & 0.1202 & 18.06\% \\ A1(7) & Wetlands & 0.1309 & 0.1394 & 6.51\% & 0.1400 & 6.96\% & 0.1055 & 19.40\% \\ A1(8) & Wetlands & 0.1871 & 0.1994 & 6.57\% & 0.2002 & 7.02\% & 0.1599 & 14.54\% \\ A1(9) & Wetlands & 0.2027 & 0.2142 & 5.67\% & 0.2180 & 7.56\% &  
The RMSE was used to measure the deviation between the observed and estimated values. Before the downscaling process, a large deviation was observed between the SMAP product at 9-km resolution and ground observations due to the large scale. The error range of the SMAP soil moisture product and sample point observation value of the four sampling areas was between 0.0260-0.0555 m\({}^{3}\cdot\)m\({}^{-3}\), and the RMSE was 17.23%-25.38% (Table 6). After the downscaling process, the estimated value of soil moisture in the sampling area was closer to the observed value. The error range of the downscaled result was between 0.0081-0.0218, with RMSEs of 4.67%-8.74%. In addition, a comparison of the four sampling areas indicated that the improvement of A2 (25.38%-8.74%) in the sampling area was largest, followed by B1 (21.00%-8.96%), B2 (18.10%-4.67%), and A1 (17.77%-6.28%) with the lowest enhancement effect. The vegetation water content derived from the _NDVI_ of B1 was greater than 0.6, and its percentage error was 8.96%. The roughness of A2 exceeded 0.155, corresponding to a percentage error of 8.74% (Table 6). Analysis of the experimental results suggests that the improved method was more accurate than the original. The corresponding vegetation water content values of A2 and B1 were 0.3281 and 0.6930, respectively, and the corresponding errors were higher than A2 and B2, as _NDVI_ was used to construct the _TVDI_ scaling factor, resulting in an RMSE that was greater than the original method in areas with high vegetation coverage. In addition, the roughness would limit the data quality, A2 and B1 with the high roughness, resulting in an RMSE greater than A1 and B2. To verify the stability of the downscaling model using the _TVDI_, the proposed method was applied to the long time series of soil moisture (2016-2017). The SMAP data of 12 months were selected after quality flags examination, and the downscaling results were obtained by batch processing. The error percentages of the four sample areas were all 6.74%. Among them, A1 had the smallest range of soil moisture (0.10-0.13), and its mean percentage error was 5.55%. B2 had the largest variation range (0.16-0.26), with a mean percentage error of 5.66%. The range of B1 was between 0.16 and 0.24, with an average error of 7.93%. A2 ranged between 0.20-0.28, with an average error of 7.80% (Figure 6). From the experimental performance of a time series of SMAP, the proposed method had certain stability and can be applied to the scale reduction inversion of a long time series. Figure 7 shows the spatial characteristics of soil moisture dynamic changes in the study area. The variation is relatively small in the northwestern part of the study area, whose land cover types is mainly in wetlands.