Temperature effects on soil bulk dielectric permittivity measured by time domain reflectometry: A physical model

Near-surface measurements of soil water content (theta) using time domain r eflectometry (TDR) may exhibit anomalous behavior in the presence of diurna l temperature (T) fluctuations. Experimental results obtained in a companio n paper led to the hypothesis that the observed bulk dielectric permittivit y (epsilon(b)) is determined by an interplay between two competing phenomen a: (1) the reduction in the dielectric permittivity of bulk water with incr eased T; and (2) the increase in TDR-measured epsilon(b) with increased T d ue to release of bound water. In this study we develop a physically based m odel for the temperature dependency of TDR-measured soil bulk dielectric pe rmittivity and propose practical correction factors. The model considers th e modified properties of water near solid surfaces to define a layer of rot ationally hindered water (within the TDR frequency bandwidth) having a temp erature dependent thickness. Changes in measured epsilon(b)(T) are thus att ributed to variations in the thickness of the rotationally hindered layer w hich has a lower dielectric permittivity than free water and hence is less "visible" to travel-time-based TDR waveform analyses. The model is sensitiv e to the soil specific surface area and the water content, both of which de termine the ratio of bound to bulk soil water. Comparisons with experimenta l data covering a wide range of soils, water contents, and temperatures sho wed good agreement. Further studies are needed to evaluate some of the mode l's critical parameters such as the cutoff frequency below which water is c onsidered bound. A temperature correction approximation is based on analyti cal expressions for TDR-measured bulk dielectric permitivity and requires e stimates of soil specific surface area and bulk density, which may be estim ated from soil texture. The thermodielectric sensitivity of TDR-measured bu lk dielectric permittivity and water content may serve as a basis for estim ating soil specific surface area.