THE USE OF A MICROWAVE BACKSCATTER MODEL FOR RETRIEVING SOIL-MOISTURE OVER BARE SOIL

Most attempts at predicting soil moisture from C-band microwave backscattering coefficients for bare soil are made by fitting experimental calibration relations obtained for limited ranges of incidence angle and soil surface roughness. In this paper, a more general approach is discussed using an inversion procedure to extend the use of a single experimental calibration relation to a wider range of incidence angle and surface roughness. A correcting function is proposed to normalize the backscattering coefficients to the conditions (incidence angle and surface roughness) of the calibration relation. This correcting function was derived from simulated data using the physical optics or Kirchhoff's scatter model using the scalar approximation. Before discussing the inversion procedure, the backscattering coefficients calculated by the model have been compared with experimental data measured in the C-band, HH polarization and three incidence angles (THETA = 15-degrees, 23-degrees, 50-degrees) under a wide range of surface soil moisture conditions (0.02 less-than-or-equal-to H(v) less-than-or-equal-to 0.35 cm3 cm-3) and for a single quite smooth soil surface roughness (0.011 less-than-or-equal-to s less-than-or-equal-to 0.014 m). The model was found to be experimentally validated from 15-degrees to 23-degrees of incidence and for surface soil moistures higher than 0.10 cm3 cm-3. For the inversion procedure, it is assumed to have a wider range of validity (15-degrees less-than-or-equal-to THETA less-than-or-equal-to 35-degrees) for the incidence angle. A sensitivity analysis of the model to errors on roughness parameter and incidence angle was performed in order to assess the feasability and suitability of the described inversion procedure.