Microwave remote sensing and its application to soil moisture detection

Abstract

Theoretical and experimental studies of the thermal microwave emission from moist soil were performed. The theoretical study was undertaken to provide a physical understanding of the emission from soil and its relationship to soil moisture. It is shown that the soil permittivity is dependent on the soil water metric potential independent of soil texture. Relationships describing the effects of the soil permittivity profile, surface roughness and vegetation cover on the microwave emission from soil are developed. Emission from the soil volume is described using a radiative transfer approach, the effect of surface roughness is modeled using the Kirchhoff approximation and vegetation cover is modeled as a dielectric slab. Two experimental measurement programs were performed in order to verify the theoretical predictions and demonstrate the feasibility of estimating soil moisture remotely using passive microwave sensors. Antenna temperature measurements of bare and vegetated, smooth and rough soil surfaces were obtained at 1.4 GHz and 10.6 GHz. These data demonstrate that a uniform surface roughness decreases the antenna temperature sensitivity to soil moisture at both 1.4 GHz and 10.6 GHz, but that the 10.6 GHz wavelength is sensitive to much smaller scales of surface roughness than is the 1.4 GHz wavelength. The effects of a periodic row structure are also demonstrated. It is also shown that uniform vegetation up to 125 cm tall has a minimal effect on the response of 1.4 GHz emission to soil moisture. However, such vegetation effectively masks the soil moisture response at 10.6 GHz.