Soil moisture was very important for agricultural, meteorological and hydrological research. This paper focused on
monitoring soil surface moisture using multi-incidence angle ENVISAT-ASAR data, by adapting a semi-empirical
polynomial relationship between backscattering coefficient and soil moisture. As SAR signals at low and high incidence
angles differently responded to soil moisture and surface roughness, soil moisture could be estimated with a higher
accuracy by using two incidence angle SAR data to remove the effect of surface roughness.
The AIEM (Advance Integral Equation Model) model was employed to analyze the effect of soil moisture and roughness
on backscattering coefficient at low and high incidence angles. Based on low and high incidence angle data simulated by
AIEM, firstly, backscattering parameter model was built. Secondly, a roughness inversion model was built based on two
incidence angle data. Thirdly, the roughness inversion model was inputted into backscattering parameter model, and
Taylor approximation was employed to develop a polynomial inversion model for soil moisture. In the soil moisture
estimation model, the formula coefficients were obtained by least square method. Finally, the inversion model was used
to derive soil moisture from the simulated data, results showed that there was a significant correlation (R=0.97) between
the estimated and inputted soil moisture, and the RSM error was 0.051. The inversion methodology was also applied to
the ASAR data, a good agreement was observed between the estimated and measured volumetric soil moisture. The
correlation was 0.556, and the root mean square error was 0.0757. Compared with an experience model, the
semi-experience model improved soil moisture estimation accuracy.
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