Potential of C-Band Sentinel-1 Data for Estimating Soil Moisture and Surface Roughness in a Watershed in Western France

Radar remote sensing has shown a high potential for soil surface parameters estimation in different pedo-climatic context. In the present study, we investigated Sentinel-l radar signal in order to analyze its behavior as function of soil moisture and soil roughness. In addition, we evaluated the approach combining the modified Integral Equation Model (IEM-B) and the Water Cloud Model (WCM) for estimating soil moisture in western France. Soil surface parameters were acquired over 4 campaigns during which composite soil samples were collected simultaneously to Sentinel-l acquisition dates. The dates of those campaigns were defined according to the evolution of the soil surface condition, during the agricultural season. The sensitivity of radar signal $\sigma 0$ to soil moisture was studied over the 22 reference fields and over the Thiessen polygons created around the measurement points. Linear relationships are observed between the radar signal and volumetric soil moisture less than 35 vol. % with higher sensitivity for VH polarization (0.41 dB/vol.% in VH against 0.26 dB/vol.% in VV). The best correlation coefficients (R) were observed for the VH polarization with the Zs roughness parameter $(\mathrm{R}={}$ 0.53 and 0.29 for reference fields and Thiessen polygons, respectively). Following that, a comparison of in situ soil moisture with that predicted based on approach proposed by [1], using Neural network algorithm with a training using the two models IEM-B and Water Cloud Model (WCM) allowed an accuracy with an RMSE ranging between 6.1 and 6.5 vol. % for reference fields and Thiessen polygons respectively. These results confirm that the proposed algorithm is accurate to estimate soil moisture.