Bound Water, Phase Configuration, and Dielectric Damping Effects on TDR-Measured Apparent Permittivity

The time domain reflectometry (TDR) method measures the soil apparent permittivity (Kₐ), which is the basis for estimation of soil volumetric water content (θ) via an empirical calibration equation or dielectric mixing model. The relationship between Kₐ and θ [i.e., Kₐ(θ)] in soils with significant volumetric fractions of bound water and with bimodal pore-size distributions displays a distinct increase in slope after θ exceeds a threshold value. The interpretation of this change in slope has been aided with application of dielectric mixing models through the inclusion of a bound water phase and/or θ-dependent changes in phase configuration. However, Kₐ measured with time-domain reflectometry (TDR) in soils with significant volumetric fractions of bound water has been previously observed to change as a function of the effective frequency of the soil-attenuated bandwidth. Therefore, the main objective of this work was to investigate the influence of bound water and phase configuration in four, high-surface-area Japanese Andisols with bimodal pore-size distributions using dielectric mixing models alone or coupled with a dielectric damping model. Soil-specific Kₐ(θ) relationships were measured in the laboratory using standard methods and were simulated with two frequency-independent, real-valued dielectric mixing models and a complex-valued, frequency-dependent model coupled with a dielectric damping model. The results of the simulations indicate that frequency-dependent dielectric permittivity of the bound water phase significantly influences TDR-measured Kₐ(θ), suggesting that soil- and probe-specific calibrations may be required for soils with significant volumetric fractions of bound water.