The vertical distribution of a dye tracer in a layered soil

Solute transport under field conditions is often inconclusive and difficult to measure because of the poop spatial resolution of sampling devices. Dye tracers can complement concentration measurements by characterizing the sp atial structure of solute flow through soils. This study assessed the influ ence of antecedent water content, tillage, and residue cover on the pattern of soil water now in the field as exhibited by a dye tracer. A random walk method for estimating the vertical distribution of the stained soil fracti on was used td evaluate the degree to which the advective dispersive equati on corresponded to field plot transport. The dye-tracer study was conducted on 12 0.9-m-diameter plots within a 2-ha field in southern Costa Rica. A 4 -cm depth of Brilliant Blue FCF solution at 5 g L-1 was applied to plots at a rate of 6.81 cm h(-1) using a spray nozzle. Plots were later excavated t o record the vertical distribution of stained soil. The dye patterns demons trated that significant bypassing can occur within the surface horizon unde r rainfall intensities below the steady state infiltration rate. Compared w ith pre-wetted soil, plots with an initially law antecedent water content e xhibited significantly greater spreading of dye within the soil profiles. T he random walk simulations indicated that the advective-dispersive equation could not describe dye staining patterns unless the dispersion coefficient s estimated from column experiments were increased by one order of magnitud e. Tillage did not significantly influence the mean penetration and spreadi ng of dye within the entire soil profile, compared with other undisturbed, pre-wetted subplots, but it increased the interaction of the soil with the dye near the surface as indicated by a significantly greater fraction of st ained soil in the Ap horizon.