The large difference between the dielectric constant of water and dry soils produces a strong dependence of the dielectric properties, and thus emissivity, of wet soils on their moisture content in the microwave (50 > X > 1 cm) region of the electromagnetic spectrum. This change in emissivity with soil moisture content can be measured remotely with microwave radiometers. The variation of emissivity with soil moisture is dependent on the wavelength (A) of observation, soil type, surface roughness and vegetative cover. These dependencies are discussed both theoretically and experimentally. Results obtained from aircraft and spacecraft platforms are presented which give a positive indication of the utility of this remote measurement technique.
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