Complex Permittivity Model for Time Domain Reflectometry Soil Water Content Sensing: II. Calibration
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Despte numerous applications of time domain reflectometry (TDR), serious difficulties in estimating accurate soil water contents under field conditions remain, especially in fine-textured soils. A complex dielectric mixing model was calibrated for fine-textured soils (24-45% clay) and its accuracy was evaluated and compared with empirical calibrations. The Ap and Bt horizons of two soils were packed into columns and adjusted to volumetric water contents () ranging from air dry to near saturation. Travel time and bulk electrical conductivity ( 0) were measured using TDR at temperatures (7) of 8, 22, and 40C and using three coaxial cables to obtain a range of input spectrum bandwidths ( S). Apparent permittivities (K a) were predicted using the complex permittivity model with measured , T, 0, S, and soil bulk density, and fitted to measured a by optimizing specific surface area (A s), the power-law exponent (a), and an empirical polarization loss factor. Measured K a was best approximated using the power-law dielectric mixing model with a semiempirical effective frequency estimate and a = 0.68. Predicted A s increased with increasing clay content, cation exchange capacity, and measured specific surface areas. The two-parameter power-law calibration removed temperature bias in 0 estimates and reduced the RMSE in 9 estimates by an average of 0.006 m 3 m -3 compared with an empirical calibration. Empirical models predicted field with oscillations of up to 0.022 m 3 m -3 in phase with soil temperatures resulting from permittivity temperature dependencies. In contrast, the calibrated dielectric mixing model removed or dampened in-phase fluctuations to <0.004 m 3 m -3, which permitted the detection of more subtle changes (<0.02 m 3 m -3) in . Soil Science Society of America.
