Solute Transport Dynamics by High‐Resolution Dye Tracer Experiments—Image Analysis and Time Moments

Accurate measurements of solute concentration are needed to conduct studies of solute transport process in unsaturated soil. In this paper we present a method of obtaining accurate measurements in time and space using dye infiltration and image analysis. The soil color was related to the dye concentration in the soil ( C s ) using 74 small calibration samples. The overall root mean square error (RMSE) was 0.057 g dm −3 , however, for C s <0.75 g dm −3 , the RMSE was only 0.032 g dm −3 . Variability of the concentration estimates at the pixel scale could be reduced by using an average filter. We used the calibration relationship during four infiltration experiments in a 0.95 by 0.975 m large Plexiglas Hele‐Shaw cell to calculate dye concentration patterns. Using the first and second order time moments, the dispersivity λ was calculated for nine different artificial column widths, from 0.0014 (local‐scale) to 0.72 m (meso‐scale). The horizontally averaged λ proved to be identical for column widths from 0.0014 to 0.045 m. For larger scales, λ gradually increased. We noticed that the two experiments with higher flow (1 and 2) and the two experiments with lower flow (3 and 4) showed an almost identical variation of meso‐scale λ with depth. We concluded that above a specific critical value of θ (∼0.22 m 3 m −3 ), solute mixing is enhanced, leading to a lower λ, and that solute transport can be described as a convective‐dispersive process. When θ is lower than this critical level, part of the porosity is deactivated and mixing between individual stream tubes decreases, which implies that transport then occurs as a stochastic‐convective process.