Abstract
Complex dielectric constant (CDC) of bound water determines the accuracy of the complex dielectric constant of wet soil. According to electrical double-layer structure and dielectric properties, the bound water on clay particle surface is divided into strongly bound water and weakly bound water. Based on this classification, models for the complex dielectric constants of bound water and soil are established taking into consideration factors such as temperature, moisture, texture, and microwave frequency. The results show that the fundamental reason why the complex dielectric constant of bound water is between that of ice and free water is the adsorption force which forms the electrical double-layer structure on the surface of clay particles. Low-concentration cationic solution could exist in free soil water and was found as the reason for the higher salinity and conductivity of free soil water, as compared to the measured soil solution. Results of soil CDC model are in good agreement with measured data across a wide range of microwave frequencies and soil temperature, moisture, and texture. The absolute root mean square error analysis also shows that the soil CDC model in this paper compared to the other models is more accurate.
Highlights
The physical process of soil moisture retrieval by microwave remote sensing involves two steps
An inspection of the green curves of Figure 3a1–a5 reveals the maximum content of bound water (Vmaxb)
The slope is almost invariant as moisture increases
Summary
The physical process of soil moisture retrieval by microwave remote sensing involves two steps. The study of soil dielectric property via physical means is crucial for the soil moisture retrieval by microwave remote sensing [1,2,3]. The CDC of air and dry soil are constant, and that of free water can be calculated by the Debye equation. When soil minerals are exposed to water, the exchangeable cations on the surface of clay particles enter the solution, forming cationic solution around the particles. These ions that contribute to conductivity will re-enter soil particles after water is removed [7,9]. Existing methods mostly perform indirect assessment on the results of the CDC model for bound water through the measured soil CDC [5,6]
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