Correction of TDR-based soil water content measurements in conductive soils

Abstract

Time Domain Reflectometry (TDR) is a widespread technique for measurement of soil water content (SWC). The main assumption behind the use of Time Domain Reflectometry (TDR) is of negligible losses, therefore assuming that only the real part determines the value of the TDR-measured apparent dielectric permittivity. This assumption does not hold for soils where surfaces are conductive (clay soils) or where high concentrations of electrolyte are present in the soil solution (saline soils) because under these conditions the contribution of the imaginary part becomes important. One of the main effects of dielectric losses on the TDR measurement is overestimation of SWC. In this study we present a methodology for separating the real and the imaginary part from the measurement of the apparent dielectric permittivity. This approach allows correction of the SWC overestimation, by using the TDR-measured electrical conductivity as indicator of dielectric losses. Oven-dry gravimetric soil water content was used as an independent method for soil water content assessment. The original SWC overestimation (in respect to the oven-dry gravimetric based measurement) reached values of up to 20% of total soil saturation, after the correction the differences were reduced to a 3–5%. The methodology can be applied based on knowledge of measured permittivity and electrical conductivity only, making it readily applicable to field experiments.