A reexamination of soil textural effects on microwave emission and backscattering

Abstract

Measurements of the dielectric properties of moist soils at microwave frequencies made recently by the University of Kansas have brought into question the validity of percent of field capacity as a soil moisture iadicator that is independent of soil texture in terms of microwave sensitivity. This was based on microwave emission and backscattering observations from truck and aircraft platforms; however, the dielectric data strongly suggest that the earlier conclusion is the result of an intermediary relationship between soil bulk density and water retention at field capacity. In this research, candidate soil-moisture indicators (gravimetric, volumetric, and percent of field capacity) were tested for their capacity to reduce the divergence in dielectric behavior between soil textures at frequencies of 1.4 and 5.0 GHz. The most congruent dielectric behavior between soil textures is found to occur when soil moisture is expressed on a volumetric basis (which is proportional to the number of water dipoles per unit volume). In contrast to expectations arising from earlier field investigations, the use of percent of field capacity was shown to overcompensate for the dielectric differences between soils and to increase divergence in some cases. This paper shows that an inadequate characterization of soil bulk density in the field, combined with the dependence of bulk density on water retention at field capacity (as controlled by soil structure), provides the most probable explanation for the earlier conclusions. It is recommended that Increased attention be given to accurate mensuration of bulk density in future field experiments, especially for soils high in clay content in which near-surface bulk density is not independent of moisture content.