Explaining Anomalies in SAR and Scatterometer Soil Moisture Retrievals From Dry Soils With Subsurface Scattering

This article presents the results of a laboratory investigation to explain anomalously high soil moisture estimates observed in retrievals from SAR and scatterometer backscatter, affecting extensive areas of the world associated with arid climates. High-resolution C-band tomographic profiling was applied in experiments to understand the mechanisms underlying these anomalous retrievals. The imagery captured unique high-resolution profiles of the variations in the vertical backscattering patterns through a sandy soil with moisture change. The relative strengths of the surface and subsurface returns were dependent upon both soil moisture and soil structure, incidence-angle, and polarization. Copolarized returns could be dominated by both surface and subsurface returns at times, whereas crosspolarized returns were strongly associated with subsurface features. The work confirms suspicions that anomalous moisture estimates can arise from the presence of subsurface features. Diversity in polarization and incidence angle may provide sufficient diagnostics to flag and correct these erroneous estimates, allowing their incorporation into global soil moisture products.