The Effects of Soil Moisture, Surface Roughness, and Vegetation on L-Band Emission and Backscatter

Measurements with Shuttle Imaging Radar B (SIR-B) at 1.28 GHz and an airborne multiple-beam push-broom radiometer at 1.4 GHz were made over a number of agricultural fields near Fresno, California during October 7-10, 1984. These measurements provided a unique data set for studies of microwave emission and backscatter from surfaces of various characteristics. The effects of surface roughness and vegetation (alfalfa and lettuce) were analyzed with respect to the responses of microwave emission and backscatter to soil-moisture variations. A theoretical model (Kirchhoff approximation) was employed to assess these effects. It was found that for microwave emission, the effect of surface roughness is less significant compared to that of vegetation. On the other hand, the surface roughness was shown to play a dominant role compared to the vegetation cover in the microwave backscatterve backscatter. The two roughness parameters in the theoretical model calculations were the surface correlation length and the standard deviation of surface height. These parameters were found to be affected strongly by the soil-texture effect in the emissivity calculations. A disagreement was found between the calculated and the observed scattering coefficients if the measured surface correlation length and standard deviation of surface height were input to the model. Either one of these two parameters had to be modified appreciably to bring a comparability between the measured and calculated scattering coefficients.