Bohl, H. and K. Roth. 1994.
Evaluation of Dielectric Mixing Models to Describe the Water Content vs Dielectric Number Relation.
Proceedings of the Symposium on Time Domain Reflectometry in Environmental, Infrastructure, and Mining Applications, Evanston, Illinois, Sept 7-9, U.S. Bureau of Mines, Special Publication SP 19-94, NTIS PB95-105789, pp. 309-319
(posted August 28, 1996)

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The accuracy and applicability of three dielectric mixing models for calibration of TDR soil water content measurements was studied. The data on which this investigation was based in part on a data base that was collected jointly by the Technical University of Berlin, Germany, and the Institute of Agrophysics, Lublin, Poland. A comparison of the standard error of the tested models shows that they lead to comparable results even though simplifying assumptions about model parameters were made. The residuals (difference between gravimetric water content and TDR-determined water content) exhibit a large scattering for organic soils with no clear trend. This hints at the experimental uncertainties. In contrast, the residuals for mineral soils are dominated by a rather clear trend, under-prediction by TDR for water content from 0.05 to 0.2 and over-prediction by TDR for water content greater than 0.2, indicating that the curvature of the water content vs dielectric number relation produced by the mixing models is too small. A higher curvature in the models may be achieved either by choosing a smaller value of "alpha", thereby implying that the medium is not isotropic and homogeneous, or by allowing a smooth transition of the dielectric number of water between the bound and free states.