Field study of macropore flow processes using tension infiltration of a dye tracer in partially saturated soils

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Macropores are important preferential pathways for the migration of water and contaminants through the vadose zone. The objective of this study was to examine small-scale preferential flow processes during infiltration in macroporous, low permeability soils. A series of tension infiltration tests were conducted using Brilliant Blue dye tracer at two field sites in southwestern Ontario, Canada. The maximum applied pressure head was varied for each test and the resulting dye stain patterns and macropore networks were characterized by excavation, mapping, photography, and image analysis. Worm burrows were the dominant macropore type, with average macropore densities exceeding 400 m−2 and peak densities of more than 750 m−2 at 30 cm depth at both sites. Flow in macropores became significant at infiltration pressures >−3 cm, with corresponding increases in infiltration rate, soil water content variability (spatially and temporally), and depth of dye staining. The results demonstrated clear evidence for partially saturated macropore flow under porewater tension conditions and the associated importance of macropore-matrix interaction in controlling this flow. Field observations of transient infiltration showed that film and rivulet flow along macropores yielded vertical flow velocities exceeding 40 m d−1. Simple calculations showed that film flow along the walls and corners of irregularly shaped macropores could explain the observed results.

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