Measurement of relative permittivity in sandy soils using TDR, capacitance and theta probes: comparison, including the effects of bulk soil electrical conductivity

Measurement of the apparent dielectric permittivity of soils (dielectric constant) is becoming a popular way of estimating soil volumetric water content. This paper focuses on the measurement of apparent permittivity in four sandy soils using; time domain reflectometry (TDR), a surface capacitance insertion probe (SCIP) and a Theta probe. Measurement of the apparent permittivity using the SCIP and Theta probe are compared with the apparent permittivity measured using the TDR. Calibration of such instrumentation has remained relatively empirical following the engineering approximation presented by Topp et al. (Topp, G.C., Davies, J.L., Anan, A.P., 1980. Electromagnetic determination of soil water content: measurements in coaxial transmission lines. Water Res. Research 16, 574–582.). The refractive index model proposed by Whalley (Whalley, W.R., 1993. Considerations on the use of time domain reflectometry (TDR) for measuring soil water content. J. Soil Sci. 44(1), 1–9.) based on that of Birchak et al. (Birchak, J.R., Gardner, C.Z.G., Hipp, J.E., Victor, J.M., 1974. High dielectric constant microwave probes for sensing soil moisture. Proc. IEEE 62(1), 93–98.) is investigated as a means of gaining some physical understanding of the relative contributions of the different dielectric components in soils. Predictions made by the model are tested against results using multiple linear regression. The predictions agree well with the observed measurements. Inter-electrode conductivity is found to contribute significantly to the apparent permittivity measured using the SCIP and to a lesser extent the TDR but not the Theta probe. Inclusion of inter-electrode conductivity in regression analysis improved results. The Theta probe was found to overestimate the apparent permittivity of the soil by ∼1.5 when compared with TDR results. It is suggested that this may be the result of compaction of the soil close to the electrodes, coupled with a strong bias in the sensitivity of the probe to the region very close to the central electrode.