Identifying the conditions amenable to the determination of solute concentrations with time domain reflectometry

On the basis of an analysis of the weighting of the bulk electrical conductivity along time domain reflectivity (TDR) probes we show theoretical limitations to the measurement of solute concentrations with TDR. Simple example calculations demonstrate that there will only be a unique relationship between the TDR-measured electrical conductivity and the average solute concentration in the pore water under one of two conditions. First, if the water content is spatially uniform throughout the sample volume of the probe, TDR may be used to determine solute concentrations. Second, if the water content is spatially variable but the spatial distribution is temporally constant and the solute concentration is spatially uniform throughout the probe's sample volume, solute concentrations can be inferred from the electrical conductivity response. Further complications arise in soils with spatially variable porosities or surface electrical conductivities, making TDR unsuited to determining solute concentrations even if one of these conditions is met.