The soil moisture databank Moisture content data from British soils

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Modelling the relative permittivity of soils using soil hygroscopic water content

A model describing the increase in the relative permittivity of water with distance from the soil mineral surface is presented. The model assumes an exponential increase in the value of permittivity with increasing distance from the mineral surface; arguments are presented supporting this approach. The volume of bound water (within the bandwidth of time domain reflectometry, (TDR) 0.01–1 GHz) is considered to be equivalent to the soil hygroscopic water content. The refractive index mixing equation is used as a geometric base into which the model is incorporated. The new equation is tested using measurements of permittivity collected from two drying undisturbed soil cores that contained 10% hygroscopic water. The RMSE of predicted permittivity as a function of water content was found to decrease from 3.5 to less than 1. The model was further tested on data previously presented in the literature and found to correspond reasonably well

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. 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

Evaluation of a Capacitance Probe Frequency Response Model accounting for bulk electrical conductivity: Comparison with TDR and network analyzer measurements

Soils ranging in texture from sand to clay were used to compare permittivity measurements made using a Surface Capacitance Insertion Probe (SCIP) and time domain reflectometer (TDR). Measurements were made using the same electrodes embedded in each soil, making the 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 TDR and network analyzer measurements. The model was tested using liquids of known permittivity and in saline, dielectric solutions. Surface Capacitance Insertion Probe and TDR determined permittivity values are similar for sandy soils but diverge for loam and clay soils. Using Topp's values as a reference, the SCIP-determined permittivities for loams and clays lay close to the curve at water contents <0.25 m3 m−3, then often rose above the curve with increasing water content. Surface Capacitance Insertion Probe permittivity correction, using electrical conductivity (EC) measured at 1 kHz, corrected the results in sands reasonably well but not enough in loams and clays for reliable calibration. We propose three possible reasons for the higher than expected permittivity values observed using the SCIP: (i) higher than expected real permittivity created by dielectric dispersion, (ii) a large contribution of the imaginary permittivity due to relaxation processes assumed to be negligible, and (iii) poor model prediction of permittivity due to excessive damping of the oscillator circuit with high EC 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