Measurement of Local Soil Water Flux during Field Solute Transport Experiments

Accurate measurement of local soil water flux, at the same location as solute concentration, remains a challenge, and the lack of these measurements limits our understanding of field solute transport. The objective of this paper is to develop and test methods for measuring local soil water flux under steady-state constant flux infiltration, typical of field solute transport experiments. The methods use vertically installed time domain reflectometry (TDR) probes to measure local solute mass flux from the change in TDR measured impedance after either a step increase or decrease (flushing) in electrolyte (tracer) concentration in the applied water. The local soil water flux was calculated directly from the local solute mass flux. The methods assume that water flow and solute transport are one-dimensional and that TDR estimates of bulk electrical conductivity are not sensitive to the vertical distribution of the applied tracer in the pore water. Field experiments indicate estimates of local soil water flux from step increase and step decrease of tracer concentration were very similar (R2 =" src="/math/ge.gif" border=0 0.63) and average water flux was similar to the application rate. There was moderate correlation between local water fluxes measured using these steady-state methods, and a previously reported method based on early time measurements of water storage during the initial (transient) states of water infiltration. The methods can be used to estimate field average hydraulic conductivity as a function of soil water content.