Theory for passive microwave remote sensing of near-surface soil moisture

Abstract

The theory of microwave thermal emission from a nonscattering half-space medium is developed for application to regions with nonuniform subsurface soil moisture and temperature variations. A coherent stratified model is presented which is valid for nonuniform temperature profiles and rapidly varying moisture profiles, under which conditions the commonly used emissivity and radiative transfer approaches become inaccurate. For naturally occurring profiles the stratified model gives more accurate results than the other approaches at frequencies below about 4 GHz. Experimental results from groundbased radiometric observations of a controlled target area compare systematically with brightness temperatures predicted from the theoretical model to within approximately 10øK. Results of dielectric constant measurements of the sand are given at seven frequencies in the microwave range and for moisture contents in the range 0-30% by volume. By using this model the thermal microwave emission spectrum is computed for a number of representative moisture and temperature profiles in the frequency range 0.25-25 GHz. A regression technique is used to show that multifrequency data can be used to obtain moisture and temperature gradients in the soil when an estimate of the surface temperature is available.

Soil water infiltration observation with microwave radiometers

“…There are well-known theories describing the emission from a soil profile with uniform or varying properties [3]- [5]. The computations involve a nonlinear weighting that decays with depth.…”

Effect of surface roughness on the microwave emission from soils

“…Njoku and Kong, 1977;Wilhcit, 1978;Burke et al, 1978;and England, 1977). These models considered the emission from the soil for a range of 1 moisture and temperature profiles and studied the effect of variations of these subsurface properties on the emission from the surface.…”

Microwave remote sensing of soil moisture content over bare and vegetated fields

“…One of the fundamental means to study these factors is to nlOLlllt the microwave radiometers Oil a mobile tower and make radiometric measurements over well-controlled fields. Experiments of this nature had been carried Out prCVi011sly by Newton (1977) and Njoku and Kong (1977). However, the measurements of Njoku and Kong were made only over a bare, sandy field and the results from their report were limited to the frequencies of 10,7 GHz and 31 AGIiz, The measurements of Newton were done at the frequellCies of 1.4GHz and 10.7 GI-Iz over both bare and vegetated fields with a…”