The effect of topography on the SAR measurements of surface soil moisture

The SIR-B (Shuttle Imaging Radar B) observations have established a good correlation between radar backscattering coefficient and surface soil moisture. Measurements from an airborne synthetic aperture radar (e.g., AIRSAR), on the other hand, have shown only a limited correlation between these parameters, even though a correction to allow for topographical variations was also attempted. This paper presents some new results from the AIRSAR flights over an agricultural area near Chickasha, Oklahoma. It was found that the changes in backscattering coefficients at P-, L-, and C-bands were, respectively, ~1-2 db, 2-3 db, and 5-6 db when soil moisture in the top 5-cm layer changed by ~0.13 g/cm³. The change in L-band backscattering coefficients was at least 3-4 db less than that observed by SIR-B. Two out of three AIRSAR C-band images also gave a higher backscattering coefficient at HH than VV polarization, which is not the case for images at P- and L-bands. These observed features may be related to problems in AIRSAR calibration