A Soil Semi-Empirical Dielectric Model Based on the Dielectric Variation Characteristics of an Electrical Double-Layer Solution

The complex permittivity of bound water are uncertain factors in the modeling of soil permittivity. Based on the electrical double-layer theory, this study divides soil bound water into strongly and weakly bound components. These strongly and weakly bound water are assumed to be cation solutions in the adsorption and diffuse layers, respectively. Characteristic functions of the permittivity of weakly bound water were proposed to describe the variation in the complex permittivity of weakly bound water with changes in moisture, and a soil semi-empirical dielectric model (SEM) was established based on these characteristic functions. Through further fitting, this SEM only requires the soil temperature, moisture, clay content, and microwave frequency to obtain the soil complex permittivity. Measured data from 20 soils and four SEMs were used for model validation. The results show that this SEM can accurately calculate the complex permittivity of sand, silt, clay, and soils with low organic matter and low salinity in the range of 0 to 0.5 volume moisture content at 1.4–18 GHz. The characteristic functions link the electrical properties of clay with the complex permittivity of bound water. The two constituent formulas of the characteristic functions (the cation concentration and electric field strength) are almost independent of soil texture. Soil texture affects the soil complex permittivity by changing the contents of strongly and weakly bound water. This phenomenon suggests that the dielectric algorithm of bound water is minimally affected by soil texture. The dielectric algorithms of bound water utilized in existing SEMs are reasonable.