Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/30/2008
Publication Date: 5/1/2009
Citation: Schwartz, R.C., Evett, S.R., Bell, J.M. 2009. Complex permittivity model for time domain reflectometry soil water content sensing: II. Calibration. Soil Science Society of America Journal. 73(3):898-909.
Interpretive Summary: Time-domain reflectometry (TDR) is the most widely used and accurate electromagnetic technique for estimating water content in the soil. Because of its ability to automatically acquire measurements at multiple locations and times, TDR is a promising technique for monitoring soil water. However, there are serious difficulties in estimating accurate soil water contents using TDR under field conditions. The greatest obstacles are its sensitivity to soil clay content, electrical conductivity, and to fluctuations in temperature. Our objectives were to calibrate a physically-based model and evaluate its accuracy compared with previous empirical approaches. We also examined its use under field conditions and with fine-textured soils. The physically-based model removed temperature bias under field conditions which led to more accurate water content estimation. These improvements in water content estimation permitted the detection of small increases in soil water content associated with small (< 0.4 inches) rainfall events.
Technical Abstract: Despite numerous applications of time domain reflectometry (TDR), serious difficulties in estimating accurate soil water contents under field conditions remain, especially in fine-textured soils. Our objectives were to calibrate a complex dielectric mixing model described by Schwartz et al. (this issue) for fine-textured soils, evaluate its accuracy compared with previous empirical approaches, and examine its use under field conditions. The Ap and Bt horizons of two fine-textured soils (24 – 45% clay) were packed into columns and adjusted to volumetric water contents ranging from air-dry to near saturation. Travel time and bulk electrical conductivity were measured using TDR at 8, 22, and 40°C and using three coaxial cables to obtain a range of bandwidths. Measured apparent permittivities, Ka, were best approximated using the power law dielectric mixing model with a decoupled, semi-empirical effective frequency estimate. The power law exponent was fixed at 0.68 to avoid nonuniqueness problems associated with its positive correlation with the fitted specific surface area, As. Predicted As increased with increasing clay content and measured specific surface areas. The two-parameter mixing model calibration removed temperature bias in volumetric water content estimates and reduced the RMSE in volumetric water content estimates by an average of 0.006 m**3 m**-3 compared with an empirical square root of permittivity calibration. Empirical calibration models predicted field ' with diel oscillations of up to 0.022 m**3 m**-3 in phase with measured soil temperatures. In contrast, the calibrated dielectric mixing model removed or dampened in-phase volumetric water content fluctuations to < 0.004 m**3 m**-3, which permitted the detection of more subtle changes (< 0.02 m**3 m**-3) in volumetric water content.