Sensitivity to soil moisture by active and passive microwave sensors

The backscatter measured by a radar and the emission measured by a radiometer are both very sensitive to the moisture content m, of bare-soil surfaces. Vegetation cover complicates the scattering and emission processes, presumably masking the soil surface and reducing soil-moisture sensitivity. Although researchers generally agree with the preceding statement, numerous claims and counterclaims have been voiced, espousing the superiority of the radiometric technique over the radar, or vice versa. The discussion often reduces to disagreements over the answer to the following question “Which of the two sensing techniques is less impacted by vegetation cover?” This paper is an attempt to answer that question. Using realistic radiative transfer models for the emission and backscatter, calculations were performed for three types of canopies, all at 1.5 GHz. The results lead to two major conclusions. First, the presumption that vegetation cover reduces the soil moisture sensitivity, is not always true. Over certain ranges of the optical depth τ of the vegetation canopy and the roughness of the soil surface, vegetation cover can enhance, not reduce, the radar sensitivity to soil moisture. The second conclusion is that under most vegetation and soil-surface conditions, the radiometric and radar soil moisture sensitivities decrease with increasing τ and the rates are approximately the same for both sensors, suggesting that, at least as far as vegetation effects are concerned, neither sensor can claim superiority over the other