Effect of Cable Length on Time Domain Reflectometry Calibration for High Surface Area Soils

Abstract

Time domain reflectometry (TDR) has been used by soil scientists to determine soil water content (θ). A waveform analysis determines an apparent dielectric number (ϵa) which can often be empirically related to ϵ. “Bound water” near colloid surfaces has different properties than free water. At the gigahertz frequencies used for TDR, free water has a negative temperature effect but bound water has a positive temperature effect on dielectric number. Long coaxial cables reduce the higher frequencies of the TDR equipment, which can influence the frequency dependent ϵa of bound water. The objective of this study was to determine the effect of coaxial cable length and temperature on apparent dielectric properties for samples with and without large amounts of bound water. Two undisturbed columns of Okoboji mucky silty clay loam (fine, smectitic, mesic cumulic Endoaquoll) with a specific surface area of 286 m2 g−1 and two packed sand samples with calculated surface areas of 0.01 m2 g−1 were used for the experiment. The ϵa was determined at four cable length combinations, three temperatures, and a range of θ. The temperature correction factors for Okoboji ranged from 0.008 to 0.012 θ/°C, depending on cable length. Long cables increased the rise time 41%, which decreased the frequency bandwidth. The Okoboji samples had a bulk electrical conductivity as high as 0.14 S m−1, which hampered determination of the final part of the waveform. In summary for Okoboji, cable length and temperature had a greater effect on ϵa than did θ. High surface area samples should be calibrated using the same cable length used for measurements, and the temperature effect should be incorporated.