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United States Department of Agriculture

Agricultural Research Service

Title: The Vertical Distribution of a Dye Tracer in a Layered Soil

Authors
item Schwartz, Robert
item Mcinnes, K - TEXAS A&M UNIVERSITY
item Juo, A S R - TEXAS A&M UNIVERSITY
item Cervantes, C - HEREDIA, COSTA RICA

Submitted to: Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 26, 1999
Publication Date: N/A

Interpretive Summary: Under many cropping systems, leaching can account for significant losses of applied agricultural chemicals. This leads to inefficient use of amendments and contamination of surface and ground water resources. The measurement of the fate of applied amendments under field conditions is difficult and often inconclusive due to the poor spatial resolution of sampling devices. The use of dye tracers can complement direct methods of measurement by providing a means to characterize the spatial structure of solute transport through soils. This study was conducted to assess the influence of tillage and residue cover upon the pattern of water flow as exhibited by a dye tracer. A random walk method is used to evaluate the vertical distribution of stained soil. Results of this study indicate that significant bypassing occurs in these soils under rainfall intensities below steady state infiltration rates. Tillage was found to increase the interaction of the surface soil with the dye and decrease the degree of bypassing as compared to no-till treatments. Under no-till management; however, the time required to leach out nitrate already contained in the surface horizon would be longer than for tilled soils.

Technical Abstract: Measurement of solute transport under field conditions is difficult and often inconclusive due to the poor spatial resolution of sampling devices. Use of dye tracers can complement concentration measurements by providing a means to characterize the spatial structure of solute flow through soils. This study was conducted to assess 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 twelve 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 at a rate of 6.81 cm h-1 to plots using 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 to pre- wetted soil, plots with an initially low antecedent water content exhibited significantly greater spreading of dye within the soil profiles. Based on the results of the random walk simulations, the advective-dispersive equation could not describe the dye staining patterns unless the dispersion coefficients estimated from column experiments were increased by one order of magnitude. Tillage did not significantly influence the spreading of dye as compared to other pre-wetted subplots. However, tillage 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.

Last Modified: 9/23/2014
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