Evaluation of Two Surface Scattering Models Within the Water Cloud Model Over an Agricultural Area in Mexico and Synergistic Use of Sentinel-1 and Sentinel-2 Images

Soil moisture is a critical parameter that is relevant to several activities such as agriculture and disaster management (e.g., drought, floods, water stress periods, etc.) [1]. Thus, it has a considerable impact on life on Earth. Recently, researchers are counting on the advanced development of remote sensing (RS) technologies for soil moisture retrieval. RS techniques provided amounts of free high, spatial, and time series images that serve soil moisture retrievals at the agricultural field scale such as Synthetic Aperture Radar (SAR). Therefore, this work aims at evaluating the impact of different formulations to represent the soil contribution using synergistically both Sentinel-1 and Sentinel-2 images. This evaluation will reveal the impact of the different soil formulations to represent the backscatter from the ground within the Water Cloud Model (WCM). The formulations representing the soil contribution in this work are the linear equation and the Oh model. The evaluation experiments are set based on in-situ data collected from different fields in Huamantla, Central Mexico. The statistical evaluation of the different formulations of the soil contribution in the total backscatter from the WCM is obtained by comparing the simulations with satellite observations over the complete period. The best results of the evaluated formulations are recorded by the Oh model in the Alvaro site with bias and root mean squared difference (RMSD) values equal -0.0002 dB and $ - {1.49_{x{{10}^{ - 6}}}}$ dB for VV polarization and, 0.0011dB, and ${8.1452_{x{{10}^{ - 6}}}}$ dB for VH polarization. In contrast, the best results obtained by the linear equation are obtained in the Macario site with bias and RMSD values equal to 0.6622 dB and -0.724dB, for VV polarization and 1.2990 dB, and 1.2994 dB for VH polarizations. On the other hand, the total backscatter from the WCM combined with the Oh model outlined the highest accuracy during the vegetated period. In the Palafox-2 site, the combined model obtained a correlation coefficient (r), bias, and RMSD up to 0.965 dB and, -0.107 dB and 0.494 dB for VV polarization, respectively, and 0.916 dB, 0.572 dB, and 1.655 dB for VH polarization, respectively. The slight differences between the two surface scattering models within the WCM suggest that both formulations could be suitable to be implemented in a retrieval process, depending upon the available ancillary information.