Fresnel-Based Reflectivity Index for Surface Soil Moisture Retrieval from Multi-Polarization SAR Observations

In this study, we assess the performance of a recently proposed reflectivity index (IR), derived from Fresnel coefficients and implemented through a change-detection approach for soil moisture retrieval (Zribi et al., 2020). Initially developed for Sentinel-1 VV polarization, the IR is theoretically linked to surface dielectric properties and therefore to surface soil moisture (SSM). We extend its evaluation by integrating dual-frequency SAR observations from Sentinel-1 (VV) and EOS-04 (HH) acquired over a three-year period across three monitoring sites in the Berambadi watershed, India. At each site, multi-depth HydraProbe sensors were deployed to provide high-quality in-situ SSM measurements for validation. All satellite acquisitions from both missions were standardized to comparable spatial resolution (30 m) and matched for similar incidence angles to ensure consistency. Results show that the IR exhibits stronger sensitivity to SSM in VV polarization than in HH, yielding improved retrieval performance across heterogeneous land surface conditions. Nonetheless, the effectiveness of the IR decreases with increasing soil moisture, with a more pronounced reduction in sensitivity observed for VV polarization. Overall, the findings demonstrate the suitability of the IR for operational soil moisture monitoring and highlight the important role of polarization and moisture regime in controlling its performance.