SIMPLE LINEAR-MODEL FOR CALIBRATION OF TIME-DOMAIN REFLECTOMETRY MEASUREMENTS ON SOLUTE CONCENTRATION

The lack of an adequate method for calibration of time domain reflecto metry (TDR) measurement on bulk soil electrical conductivity is a limi ting factor, especially for field-scale solute transport study. This s tudy investigated an appropriate calibration method that can be applie d to horizontally positioned TDR probes in situ. A nonlinear model (RM ) has been proposed assuming a two-pathway conductance. Using the quad ratic form, the RM equated electrical conductivity in the immobile pha se of soil solution (ECws) in a series-coupled pathway with electrical conductivity in the mobile phase of soil solution (ECwc) in a continu ous pathway. We related bulk Soil electrical conductivity (ECa) to ele ctrical conductivity of soil water (ECw), assuming constant ECws durin g breakthrough. This resulted in a linear relationship between ECa and ECw. The linear relation, termed here simple linear model (SLM), was tested using experimental data obtained from soil columns. Laboratory breakthrough experiments were performed on short and large columns by applying a pulse input of solute. At the end of breakthrough, separate calibration experiments were performed using a step input of solute. Both RM and SLM were compared with the step input calibration method ( SIM) as a reference method. Results show that the SLM is better than R M in terms of mass recovery. In addition, parameters of the solute tra nsport model were not affected within 20% of uncertainty in the slope coefficient of the SLM when compared with the reference method. Advant ages of the SLM are that it has an identical equation form to the SIM but less effort is required, especially for soil columns showing prefe rential now, and it can be readily applied to field conditions.