Field Measurement of Solute Travel Times Using Time Domain Reflectometry

Abstract A limiting step in our understanding of solute transport through unsaturated field soils is the lack of experimental data. Field solute transport characteristics such as the travel‐time density functions, F L (T) , for different soil depths are often very difficult and time‐consuming to measure. The objective of this study was to develop a method to measure F L (T) for a conserving tracer added as a pulse under conditions of constant surface water flux density. The method uses vertically installed time domain reflectometry (TDR) probes. At constant soil volumetric water content, ϑ, the TDR estimate of bulk electrical conductivity, EC E , is related to the average pore water concentration of an added solute tracer, which in turn is related to the total specific mass (g m ‐2 ) of the tracer between the TDR probes. The change in EC* E with time is shown to be linearly related to the mass flux of the solute past the ends of the TDR transmission rods. Solute mass flux out of a laboratory column was very similar to that measured using the TDR method. Excellent agreement was also obtained between solute‐mass‐flux measurements in the field (0.2‐m depth), using ceramic solution samplers, and the new TDR method. The method measures mass flux of the solute at a particular depth and not solute concentration. Thus, no assumptions about local water flux density are necessary in estimating F L (T) . The method also has the advantage that ϑ can be measured at the same location (soil volume) where the solute mass flux is measured. The method is limited to soils where addition of a solute pulse will cause a measurable reduction in the reflected signal. This depends on steady‐state soil water content, EC E at steady state, and the depth of measurement.