A DIELECTRIC MODEL OF THE VEGETATION EFFECTS ON THE MICROWAVE EMISSION FROM SOILS

A layer of vegetation over the soil surface absorbs some of the radiation emitted from the soil and emits at its own temperature. This results in a reduction of the information in the microwave radiation about the soil surface. To study this problem further we use the Ulaby and El-Rayes model for the dielectric constant of vegetation to estimate the absorption loss and optical depth, tau, of plant canopies for frequencies between 1 and 40 GHz. We treated tau as the product of a vegetation parameter b and vegetation water content, VW. We compared both the linear and square root (refractive) mixing models with the observed data in terms of the b parameter. These data were obtained from published reports on the values of tau and VW for crops ranging from prairie grass to corn and soybeans. The data fit the curve for the refractive model quite well. For the refractive model the value of b was independent of VW, while for the linear model there was some dependence on VW. For both models b is roughly proportional to the frequency.