Effect of Texture on Microwave Emission from Soils

The intensity brightness temperature (TB) of the microwave emission from the soil is determined primarily by its dielectric properties. The large difference between the dielectric constant of water (≅80) and that of dry soil (3-5) produces a strong dependence of the soil's dielectric constant on its moisture content. This dependence is effected by the texture of the soil because the water molecules close to the particle surface are tightly bound and do not contribute significantly to the dielectric properties. Since this surface area is a function of the particle size distribution (soil texture), being larger for clay soils with small particles, and smaller for sandy soils with larger particles; the dielectric properties will depend on soil texture. This dependence has been demonstrated by laboratory measurements of the dielectric constant for soils which are briefly summarized in this paper. The dependence of the microwave emission on texture is demonstrated by measurements of TB from an aircraft platform for a wide range of soil textures. The main conclusion of the paper is that the effect of soil texture differences on the observed TB values can be normalized by expressing the soil moisture values as a percentage of field capacity (FC) for the soil.