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

Abstract 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.

[1]  T. Gish,et al.  Soil Moisture Determination in Gravelly Soils with Time Domain Reflectometryd , 1989 .

[2]  T. J. Dean.  The IH capacitance probe for measurement of soil water content , 1994 .

[3]  O. H. Jacobsen,et al.  A laboratory calibration of time domain reflectometry for soil water measurement including effects of bulk density and texture , 1993 .

[4]  A. P. Annan,et al.  Electromagnetic determination of soil water content: Measurements in coaxial transmission lines , 1980 .

[5]  R. Plagge,et al.  Empirical evaluation of the relationship between soil dielectric constant and volumetric water conte , 1992 .

[6]  S. Dasberg,et al.  Time Domain Reflectometry Measurements of Water Content and Electrical Conductivity of Layered Soil Columns , 1991 .

[7]  M. G. Hodnett,et al.  The dielectric calibration of capacitance probes for soil hydrology using an oscillation frequency response model , 1998 .

[8]  G. C. Topp,et al.  Comments on ‘Considerations on the use of time‐domain reflectometry (TDR) for measuring soil water content’ by W.R. Whalley , 1994 .

[9]  Willem Bouten,et al.  Frequency domain analysis of time domain reflectometry waveforms: 2. A four‐component complex dielectric mixing model for soils , 1994 .

[10]  Christian Roth,et al.  Improving the calibration of dielectric TDR soil moisture determination taking into account the solid soil , 1996 .

[11]  J. Maxwell.  A Treatise on Electricity and Magnetism , 1873, Nature.

[12]  W. R. Whalley.  Considerations on the use of time‐domain reflectometry (TDR) for measuring soil water content , 1993 .

[13]  S. Zegelin,et al.  Improved field probes for soil water content and electrical conductivity measurement using time domain reflectometry , 1989 .

[14]  J. Miller,et al.  Measurement of Soil Water Content Using a Simplified Impedance Measuring Technique , 1996 .

[15]  Timo J. Heimovaara,et al.  Frequency domain analysis of time domain reflectometry waveforms: 1. Measurement of the complex dielectric permittivity of soils , 1994 .

[16]  C. G. Gardner,et al.  High dielectric constant microwave probes for sensing soil moisture , 1974 .

[17]  T. Hassard,et al.  Applied Linear Regression , 2005 .

[18]  Guido Wyseure,et al.  Measurement of volumetric water content by TDR in saline soils , 1997 .

[19]  T. J. Dean,et al.  Soil water content measurement with a high-frequency capacitance sensor , 1998 .

[20]  P. J. Gregory,et al.  Use of Time Domain Reflectometry (TDR) to Measure the Water Content of Sandy Soils , 1995 .

[21]  F. Ulaby,et al.  Microwave Dielectric Behavior of Wet Soil-Part II: Dielectric Mixing Models , 1985, IEEE Transactions on Geoscience and Remote Sensing.

[22]  R. Carmichael.  CRC handbook of physical properties of rocks , 1982 .

[23]  T. J. Dean,et al.  SOIL MOISTURE MEASUREMENT BY AN IMPROVED CAPACITANCE TECHNIQUE, PART I. SENSOR DESIGN AND PERFORMANCE , 1987 .

[24]  J. Knight.  Sensitivity of time domain reflectometry measurements to lateral variations in soil water content , 1992 .

[25]  R. Schulin,et al.  Calibration of time domain reflectometry for water content measurement using a composite dielectric approach , 1990 .

[26]  A. Thomsen,et al.  HIGH-RESOLUTION TIME DOMAIN REFLECTOMETRY: SENSITIVITY DEPENDENCY ON PROBE-DESIGN , 1995 .

[27]  C. Dirksen,et al.  IMPROVED CALIBRATION OF TIME DOMAIN REFLECTOMETRY SOIL WATER CONTENT MEASUREMENTS , 1993 .