Comparison of soil moisture penetration depths for several bare soils at two microwave frequencies and implications for remote sensing

Microwave brightness temperature measurements were made over three different bare soils at frequencies of 5 GHz (X = 6 cm) and 1.67 GHz (X = 18 cm) to compare differences in penetration depth according to texture, soil moisture, and wavelength. The soil plots were wetted, and circularly polarized microwave measurements were made during the dry-down cycle. Soil profile temperature and profile moisture content were monitored continuously for the duration of the experiment. Laboratory analyses of soil physical properties such as texture and bulk density were conducted to aid theoretical calculations of the soil dielectric constants. Laboratory measurements of the soil dielectric constants were also made. Two commonly used soil dielectric models were compared. Theoretical values of soil dielectric constant were calculated with the Dobson model and the Wang-Schmugge model, and both compared reasonably well with laboratory measurements and also to values reported in the literature. Calculated soil emissivities derived from the field measurements also compared well with emissivities calculated from both the modeled dielectrics and those measured in the laboratory. Emissivities were compared with the average soil moisture for surface profiles of varying thickness. It was found that the observed effective penetration depth appeared to exceed the theoretically defined values.