Remote Sensing of Soil Moisture with Microwave Radiometers

ABSTRACT MICROWAVE radiometers measure the thermal radiation at wavelengths between 1 mm and about 50 cm that is generated within the soil volume and subsequently emitted through the surface. The fraction of this upwelling radiation incident on the soil surface that is transmitted into the air will be determined by the dielectric properties of a transition layer just below the surface. This fraction can be called the emissivity of the soil and is strongly dependent on the moisture content of the soil in this transition layer. This dependence is due to the large contrast between the dielectric properties of liquid water and those of dry soil. The large dielectric constant of water results from the electric dipole moment of the water molecule. The dielectric constant of water at the lower microwave frequencies is approximately 80 compared with 3 to 5 for dry soils; as a result, the dielectric constant of wet soils can reach values of 20 or more. This produces a range of soil emissivity from about 0.95 for dry soils to 0.6 or less for wet soils. Theoretical studies have indicated that this transition layer is on the order of a few tenths of wavelength thick, i.e., 2 to 5 cm for a 21 cm wavelength. This layer determines the primary soil moisture sampling depth, and its approximate thickness has been confirmed by observations. Results from truck, aircraft, and spacecraft platforms are presented which demonstrate the sensitivity of microwave radiometers to soil moisture variations for a wide range of conditions including light to moderate amounts of vegetative cover.

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