Abstract
Although the empirical Topp's equation is widely used as a calibration curve of time domain reflectometry (TDR) method, it has been known that the equation is not valid for soils having higher organic matters content, mineral materials with high dielectric constant and clayey soils. The purpose of this study was to apply and examine the Maxwell-De Loor model and the semi-empirical mixing model (α model) to measured data of six textured soils (two Ando soils, Reddish-yellow soils, surface soil and subsoil of Brown forest soils, and Toyoura Sand) collected in Kyushu Island, Japan. Both models involve four components as solid phase, bound water, free water, and air, and are expected to have higher fitness of between volumetric water content (θ) and apparent dielectric constant (ε) than Topp's equation. Relations between θ and ε were measured using a small vessel with TDR probe of two parallel wave guides under a drying process of soil water. The results showed that empirical Topp's equation underestimates soil water content for mineral soils of low bulk densities. Since dielectric mixing models take into account of the effect of bulk density on θ-ε relation, they were more suitable than the Topp's equation for the soils used in this study. Judging from the fitness for wide range of soil water content, the third-degree polynomial regression curve and α model were superior to the Maxwell-De Loor model. However, the Maxwell-De Loor model was found to be so flexible because it reasonably predicted measured values for different types of soil without fitting parameters.
