This paper is the second in a series evaluating the microwave dielectric behavior of soil-water mixtures as a function of water content and soil textural composition. Part II draws upon the data presented in Part 1 [13] to develop appropriate empirical and theoretical dielectric mixing models for the 1.4-to 18-GHz region. A semiempirical mixing model based upon the index of refraction is presented, requiring only easily ascertained soil physical parameters such as volumetric moisture and soil textural composition as inputs. In addition, a theoretical model accounting explicitly for the presence of a hydration layer of bound water adjacent to hydrophilic soil particle surfaces is presented. A four-component dielectric mixing model treats the soil-water system as a host medium of dry soil solids containing randomly distributed and randomly oriented disc-shaped inclusions of bound water, bulk water, and air. The bulk water component is considered to be dependent upon frequency, temperature, and salinity. The soil solution is differentiated by means of a soil physical model into 1) a bound component and 2) a bulk soil solution.The performance of each model is evaluated as a function of soil moisture, soil texture, and frequency, using the dielectric measurements of five soils ranging from sandy loam to silty clay (as presented in Part I [13]) at frequencies between 1.4 and 18 GHz. The semiempirical mixing model yields an excellent fit to the measured data at frequencies above 4 GHz. At 1.4 GHz, the model does not fully account for the dielectric properties of bound water at low moisture contents, and hence the soil-dependent differences in the observed dielectric behavior are underpredicted. This simple model, however, should be adequate for most applications requiring estimated soil dielectric properties for use in emission and scattering calculations. Application of the theoretical model shows that it is an appropriate formulation and that the inclusions do in fact behave as discs. The theoretical model yields values that correlate well with the observed effects of soil type and frequency; however, it also produces systematic prediction errors caused by uncertainty about the dielectric properties of bound water. The model shows that bound water is dielectrically dissimilar to either ice or free water, supporting the hypothesis that bound water is not completely irrotational and therefore may exhibit a dielectric relaxation at microwave frequencies.

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Because the microwave dielectric constant of dry vegetative matter is much smaller (by an order of magnitude or more) than the dielectric constant of water, and because a vegetation canopy is usually composed of more than 99% air by volume, it is proposed that the canopy can be modeled as a water cloud whose droplets are held in place by the vegetative matter. Such a model was developed assuming that the canopy “cloud” contains identical water droplets randomly distributed within the canopy. By integrating the scattering and attenuation cross‐section contributions of N droplets per unit volume over the signal pathlength through the canopy, an expression was derived for the backscattering coefficient as a function of three target parameters: volumetric moisture content of the soil, volumetric water content of the vegetation, and plant height. Regression analysis of the model predictions against scattering data acquired over a period of four months at several angles of incidence (0°–70°) and frequencies (8–18 GHz) for HH and VV polarizations yields correlation coefficients that range from .7 to .99 depending on frequency, polarization, and crop type. The corresponding standard errors of estimate range from 1.1 to 2.6 dB.

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