Microwave Dielectric Model for Aggregated Soils

In order to understand the interactions of soil properties and microwave emission better, a series of field experiments were conducted in 1984. Small plots were measured with a truck-mounted passive microwave radiometer operating at 1.4 GHz. The microwave data were collected concurrently with ground observations of soil moisture and bulk density. Treatment effects included different soil moisture contents and bulk densities. Evaluations of the data showed that commonly used models of the dielectric properties of wet soils could not explain the observations. This conclusion was based on the fact that the roughness parameters determined through optimization were significantly larger than those observed in similar other investigations. These discrepancies are most likely due to inadequate characterization of the effects of soil structure. Commonly used models assume a homogeneous three phase mixture of soil solids, air, and water. Under tilled conditions the soil is actually a two phase mixture of aggregates and voids. Appropriate dielectric models for this tilled condition were evaluated and found to match the observed data. These results indicate that previous conclusions concerning the effects of surface roughness in tilled fields may be incorrect, and the improved model may explain some of the inconsistencies encountered in roughness modeling.