The vertical distribution of a dye tracer in a layered soil
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abstract
Solute transport under field conditions is often inconclusive and difficult to measure because of the poor spatial resolution of sampling devices. Dye tracers can complement concentration measurements by characterizing the spatial structure of solute flow through soils. This study assessed the influence of antecedent water content, tillage, and residue cover on the pattern of soil water flow in the field as exhibited by a dye tracer. A random walk method for estimating the vertical distribution of the stained soil fraction was used to evaluate the degree to which the advective dispersive equation 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-1was applied to plots at a rate of 6.81 cm h-1using a spray nozzle. Plots were later excavated to record the vertical distribution of stained soil. The dye patterns demonstrated that significant bypassing can occur within the surface horizon under rainfall intensities below the steady state infiltration rate. Compared with pre-wetted soil, plots with an initially low antecedent water content exhibited significantly greater spreading of dye within the soil profiles. The random walk simulations indicated that the advective-dispersive equation could not describe dye staining patterns unless the dispersion coefficients estimated from column experiments were increased by one order of magnitude. Tillage did not significantly influence the mean penetration and spreading 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 stained soil in the Ap horizon.
