Solute transport measurements in different soil types using time domain reflectometry

During recent years, time domain reflectometry (TDR) has proved to be a val uable tool for both water content (theta) and bulk electrical conductivity (sigma (a)) measurements. To allow resident solute concentration (C-r) meas urements, a calibration procedure is necessary for the relationship between sigma (a) and C-r. Two main calibration approaches exist. Direct calibrati on allows for C-r measurements with varying theta, while the indirect calib ration method is used for conditions with constant theta. In this paper, th ree methods of achieving direct calibration parameters are presented and ev aluated in three different soil types. Calibrations are made in both distur bed and undisturbed soil columns as well as in the field. It was shown that there were only small differences between calibration methods in homogeneo us sand. In other soils, choosing the correct calibration is important. In clay soils, solute transport measurements are difficult to take under condi tions with varying theta, therefore it is suggested that only the indirect calibration approach should be used. When using TDR it is important to be a ware of the accuracy of the TDR system in order to interpret data correctly . Some error sources are thus also briefly discussed.