Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/25/2005
Publication Date: 11/16/2005
Citation: Robinson, D.A., Kelleners, T.J., Cooper, J.D., Gardner, C.M., Wilson, P., Lebron, I., Logsdon, S.D. 2005. Evaluation of a capacitance probe frequency response model accounting for bulk electrical conductivity: comparison with TDR and network analyzer measurements. Vadose Zone Journal. 4(4):992-1003. Interpretive Summary: We need to know the water content to measure and predict crop water use, as well as runoff and leaching of chemicals. Some of the techniques used to measure water content have not been accurate for soils high in clays. This study showed that both soil properties and equipment properties contributed to inaccurate measurement of soil water. This information is important primarily for scientists and land managers who measure soil water content, but also for equipment manufactures so they can improve their devices that measure soil water content.
Technical Abstract: Soils ranging in texture from sand to clay were used to compare permittivity measurements using a Surface Capacitance Insertion Probe (SCIP) and time domain reflectometer (TDR). Measurements were made using the same electrodes embedded in each soil, making measurements directly comparable. The objective of the work was to test a model describing the frequency response of the SCIP to both permittivity and electrode conductance, and to compare results with the TDR measurements. The model is tested using liquids of known permittivity. In saline, dielectric solutions, SCIP, and TDR determined permittivity are similar for sandy soils but diverge for loam and clay soils. Using Topp's curve as a reference, the SCIP determined permittivities for loams and clays lay close to the curve at water contents < 0.25 m3/m3, then often rose above the curve with increasing water content. SCIP permittivity correction, using electrical conductivity measured at 1 kHz, improved results in sands a little but not enough in loams and clays for reliable calibration. We propose three possible reasons to account for higher than expected permittivity values observed using the SCIP: 1) Higher than expected real permittivity created by dielectric dispersion; 2) A large contribution of the imaginary permittivity due to relaxation processes assumed to be negligible, and 3) Poor model predictions of permittivity due ot excessive damping of the oscillator circuit with high electrical conductivity and dielectric losses. Results from network analyzer measurements for one of the clay soils were used to aid data interpretation. The TDR measurements were much more consistent, producing apparent relative permittivity values below those of the Topp curve for the finer textured soils.