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

The time domain reflectometry (TDR) method measures the soil apparent permittivity (K-a), which is the basis for estimation of soil volumetric water content (theta) via an empirical calibration equation or dielectric mixing model. The relationship between K-a and theta [i.e., K-a (theta)] in soils with significant volumetric fractions of bound water and with bimodal pore-size distributions displays a distinct increase in slope after theta 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 theta-dependent changes in phase configuration. However, K-a 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 K-a (theta) 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 K-a (theta), suggesting that soil- and probe-specific calibrations may be required for soils with significant volumetric fractions of bound water.