Dielectric Permitivity And Its Relationship With Water Content For Several Soils In China

Time Domain Reflectometry (TDR) is becoming a widely used method to measure soil volumetric water content. The accuracy of soil water content measured by TDR depends on two key factors: soil dielectric constant and the calibration model of soil volumetric water content vs. dielectric constant. The purposes of this study are: a) to study dielectric properties of different types of Chinese soil, and to analyze the reason of variance in soil dielectric constant, and b) to compare the most representative θ_v-K a calibration models on different types of soil and their performance. In this study repacked soil columns were applied to get a range of soil water content from oven-dry to saturation and then to measure dielectric constant and water content by TDR.The results showed the difference of dielectric constant resulted from bulk density for -different types of oven-dry soil. The linear relationship between dielectric constant and bulk density was obtained. The dielectric constant related to organic matter negatively.Topp (1980) equation, Hook-Livingston (1996) equation and three-phase mixing dielectric model can be applied to calculate water content of eight types of soil basically, with RMSE 0.018 cm³ cm^-3, 0.014 cm³ cm^-3, 0.015 cm³ cm^-3 respectively. To improve the accuracy we proposed a θ_v-K a^0.5 calibration for most of agricultural soils through fitted experimental data, which further agree well with tested data in the laboratory and field examination respectively.Moreover, the relationship between dielectric constant and soil water content for two types of soil different from three empirical models due to organic matter or bulk density. We compared several empirical equations on these soils and obtained the θ_v-K a^0.5 calibration including bulk density. High clay and ferralic mineral content in Krasnozem of southern China have complicated effects on TDR measurement, which resulted in different θ_v-K a curve from those of any other soils. We provided specific calibration equation for Krasnozem and the tested results indicted that specific calibration equation fitted better than three empirical models by 3-5 times.