Topsoil structure influencing soil water retrieval by microwave radiometry

Many remote sensing applications, including those of future space missions, require accurate knowledge of the influence of topsoil structure on the water content as measured using L-band radiometry. We report on field-measured L-band (1.4 GHz) microwave emission from bare soil. Of special interest in this work is the procedure used to remotely sensed data to soil water content and its comparability with time domain reflectometer (TDR) in situ measurements. Surface roughness of the soil was characterized on a millimeter scale using an optical measurement technique. Different models for interpreting the microwave signals in terms of the water content were investigated. The agreement between in situ water contents and surface water contents estimated with radiometry data using the Fresnel equation was found to be poor. A coherent layer model, with and without considering roughness effects, was tested to compare radiometrically measured and modeled soil reflectivities. The correspondence remained unsatisfactory, even when we considered a dielectric gradient fitted to the TDR profiles and surface roughness represented by a scattering model. We developed a new air-to-soil transition model, which includes dielectric mixing effects due to small-scale surface structures. This model considerably improved agreement between measured and modeled results. We conclude that small-scale structures of the topsoil cannot be neglected in interpreting L-band measurements.