| The presence of a vegetation layer on top of a soil surface causes the level of emission intensity to increase, thereby decreasing the radiometric sensitivity to soil moisture variations. This phenomenon is due partly to the screening of soil emission by the vegetation layer and partly to the inherent emission of radiation from the vegetation layer itself. The effect of the vegetation cover on the radiometric sensitivity to soil moisture is studied both theoretically and experimentally. Theoretically, the dielectric properties of the vegetation cover were studied and mixing formulas were established, then a simple radiative transfer model was employed and extended to improve the understanding of the vegetation effect on soil-moisture sensing. Experimentally, the airborne data from the 1978 Colby experiment were analyzed. The test site consisted of bare soil, wheat stubble, and fully mature corn fields. The results of corn indicate that the radiometric sensitivity to soil moisture decreases in magnitude with increasing frequency and with increasing angle of incidence. Such reductions in sensitivity, with respect to bare soil, were found to be 65% for normal incidence at 1.4 GHz, and 89% at 5.0 GHz. The theoretical-model prediction was compared with the measured 1978 Colby data and 1978-79 Barc data and good fits were obtained. |
