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Topsoil Structure Influencing Soil Water Retrieval by Microwave Radiometry

^a Institute of Terrestrial Ecology, ETH Zurich, Grabenstrasse 11a, 8952 Schlieren, Switzerland
^b Institute of Terrestrial Ecology, ETH Zurich, Grabenstrasse 11a, 8952 Schlieren, Switzerland
^c EAWAG, Ueberlandstrasse 13, 8600 Duebendorf, Switzerland
^d Centre d'Etudes Spatiales et de la Biosphere (CESBIO) 18 av. Edouard Belin, bpi 2801, 31401 Toulouse Cedex 4, France
^e Institute of Applied Physics, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland

Correspondence: ^* Corresponding author (mike.schwank{at}env.ethz.ch)

Received for publication 6 June 2004. 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 a bare soil. Of special interest in this work is the procedure used to transform 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.

Abbreviations: DSM, digital surface model • TDR, time-domain reflectometer

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