Fig 1.
Location of the study area and the experimental region.
Fig 2.
Distribution of sampling sites (a. Sentinel-2 imagery in RGB band (R = band; G = band; B = band; b. Sentinel-1 imagery in VV polarization). Sentinel-1 and Sentinel-2 images are made freely available to the general public by the European Space Agency (ESA) (https://dataspace.copernicus.eu/).
Fig 3.
Surface roughness parameters measurement.
Fig 4.
Conceptual scattering of SAR signal over vegetation covered surfaces described by WCM.
Table 1.
Formulas of optical vegetation index.
Fig 5.
The technology roadmap.
Fig 6.
Correlation relationships between different vegetation indices and VWC.
Fig 7.
Estimation accuracy of VWC with the multiple regression model.
Table 2.
Formulas and accuracies of the VWC estimation models based on different vegetation indices.
Fig 8.
Comparison of microwave backscatter coefficients before and after vegetation correction.
Fig 9.
Scatter diagram of SAR backscattering coefficient to soil moisture before and after vegetation correction.
Fig 10.
Performance of the direct retrieval method in soil moisture retrieval under different SAR polarization modes.
(a. VV polarization; b. VH polarization).
Table 3.
Input parameters and step size of the backscattering coefficients simulation database.
Fig 11.
Comparison of the predicted and measured soil moisture under different SAR polarization modes (a. VV polarization; b. VH polarization).