Solute Transport Measurements in Different Soil Types using Time Domain Reflectometry

During recent years, time domain reflectometry (TDR) has proved to be a valuable tool for both water content (θ) and bulk electrical conductivity (σ_a) measurements. To allow resident solute concentration (C_r) measurements, a calibration procedure is necessary for the relationship between σ_a and C_r. Two main calibration approaches exist. Direct calibration allows for C_r measurements with varying θ, while the indirect calibration method is used for conditions with constant θ. In this paper, three methods of achieving direct calibration parameters are presented and evaluated in three different soil types. Calibrations are made in both disturbed and undisturbed soil columns as well as in the field. It was shown that there were only small differences between calibration methods in homogeneous sand. In other soils, choosing the correct calibration is important. In clay soils solute transport measurements are difficult to take under conditions with varying θ, therefore it is suggested that only the indirect calibration approach should be used. When using TDR it is important to be aware of the accuracy of the TDR system in order to interpret data correctly. Some error sources are thus also briefly discussed.