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

Abstract

Core Ideas Frequency‐dependent bound water permittivity influences TDR‐measured Ka. Predictive power of frequency‐independent dielectric mixing models is limited. TDR measurements of Ka should be paired with effective frequency measurements. 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.

A review of time domain reflectometry (TDR) applications in porous media

“…(1.3%) and could be calibrated to a lower level (1%) for a specific soil type. However, this equation was found to be restricted to unfrozen soils and soils with low specific surface areas (Dirksen and Dasberg, 1993;Dobson et al, 1985;Dyck et al, 2019). A special calibration was required for partially frozen soil (Stein and Kane, 1983).…”

“…This discovery significantly boosted the applications of TDR in soil science and opened a new era for soil water measurements. Many subsequent investigations, including TDR cable testers, TDR probe designs (Zegelin et al, 1989), TDR waveform recording or analysis tools (Wang et al, 2016), and TDR calibration curves (Dirksen and Dasberg, 1993;Dyck et al, 2019;Friedman, 1998;He and Dyck, 2013;He et al, 2016;Malicki et al, 1996;Ponizovsky et al, 1999;Robinson et al, 2005a;Roth et al, 1990) led to improvements in TDR capabilities. Detailed history of TDR developments in soil science studies is reported by Topp et al (2003).…”

Dielectric Spectroscopy and Application of Mixing Models Describing Dielectric Dispersion in Clay Minerals and Clayey Soils

“…Soils are well known to contain “bound” water, often estimated as the amount of hygroscopic water [ 14 ], though to what extent this impacts dielectric measurements remains poorly understood and open to investigation [ 15 , 16 , 17 , 18 , 19 , 20 ]. The impact of “bound” water on dielectric measurements has support based on experimental evidence mostly from work conducted on mineral and non-mineral surfaces, e.g., in food [ 21 ].…”

Forest restoration methods, seasonality, and penetration resistance does not influence aboveground biomass stock on mining tailings in Mariana, Brazil

“…The dielectric permittivity is sensitive to changes in temperature (Seyfried & Grant 2007). In some soils, as it does in the water, the molecular vibrations increases with the temperature and, these vibrations, with the presence of an electric field applied; hinder the rotational dipole moment (Dyck et al 2019, Mohamed & Paleologos 2018. In practical terms, the dielectric permittivity decreases as the temperature increases.…”