A Field Study Using Dyes to Characterize Preferential Flow of Water

Abstract Characterization of preferential flow in soils through normal water tracer studies are, in most cases, inconclusive because of the inability of most solute sampling devices to detect the spatial pattern of preferential flow pathways. In this study, a number of field‐plot experiments were performed to provide direct observation of the preferential flow of water and solutes by adding soluble dye to stain the flow pathways. In addition to characterizing the spatial structure of preferential flow pathways, the dye trace patterns were used to investigate the interactive role of soil structure and irrigation methods on the preferential flow patterns of water and solutes. A narrow pulse of an anionic water‐soluble dye, Acid‐Red 1, was applied to the surface of a number of 1.5 by 1.5 m field plots, some of which had the top layer completely disturbed to the depth of 30 to 40 cm. The distribution patterns of the dye in these plots were examined after the plots received 100 mm of irrigation water, under either ponding or daily sprinkler irrigation. The observed vertical and horizontal distribution patterns of the water‐soluble dye clearly indicated the preferential flow patterns for each experimental condition. These include vertical fingering of dye traces 5 to 20 cm wide, which extended more than twice as deep as the mean displacement of the dye, as well as isolated patches of dye indicating lateral flow.