Laboratory calibration of time domain reflectometry to determine moisture content in undisturbed peat samples

摘要

Time domain reflectometry (TDR), while widely used to measure volumetric water content (theta) and bulk electrical conductivity (BEC) in unsaturated granular soils, remains less studied in peat than mineral soils. Empirical models commonly used in mineral soils are not applicable to peat for accurate determination of theta from measured apparent dielectric permittivity (epsilon). Past studies for peat report highly variable calibrations, and suggest differences in origin of organic matter, degree of decomposition and bound water to explain such variability. This study shows that bound water appears to have minimal impact on calibration because of its negligible volumetric fraction at the low bulk densities of peat. Increased volumetric air fraction at the same theta values attributed to high porosity of peat makes the epsilon-theta relationships of mineral soils inapplicable. Temperature effects on e resulted in a correction factor for theta. The temperature correction factor decreased with decreasing theta and was determined experimentally to lie between -0.0021 m(3) m(-3) per degrees C for theta >= 0.79 m(3) m(-3) and -0.0005 m(3) m(-3) per degrees C for theta = 0.35 m(3) m(-3). The decreasing value of the correction factor with theta can be explained by dependence of the epsilon-theta relationship on properties of free water alone. Temperature dependence of BEC was close to that of soil solution. Maxwell-De Loor's four-phase mixing model (MDL) based on physical properties of the multiphase soil system can efficiently simulate the effect of increased air volume and varying soil temperature on the epsilon-theta relationship in peat. In addition, linear epsilon-theta calibration in peat can be improved when BEC is included in the calibration equation.