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Abstract: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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Macropore flow in frozen soils plays a critical role in partitioning snowmelt at the land surface and modulating snowmelt-driven hydrological processes. Previous descriptions of macropore flow processes in frozen soil do not explicitly represent the physics of water and heat transfer between macropores and the soil matrix, and there is a need to adapt recent conceptual and numerical models of unfrozen macropore flow to account for frozen ground. Macropores remain air filled under partially saturated conditions, allowing preferential flow and meltwater infiltration prior to ground thaw. Nonequilibrium gravity-driven flow can rapidly transport snowmelt to depths below the frost zone or, alternatively, infiltrated water may refreeze in macropores and restrict preferential flow. As with unfrozen soils, models of water movement in frozen soil that rely solely on diffuse flow concepts cannot adequately represent unsaturated macropore hydraulics. Dual-domain descriptions of unsaturated flow that explicitly define macropore hydraulic characteristics have been successful under unfrozen conditions but need refinement for frozen soils. In particular, because pore connectivity and hydraulic conductivity are influenced by ice content, modeling schemes specifying macropore-matrix interactions and refreezing of infiltrating water are critical. This review discusses the need for research on the interacting effects of macropore flow and soil freezethaw and the integration of these concepts into a framework of coupled heat and water transfer. As a result, it proposes a conceptual model of unsaturated flow in frozen macroporous soils that assumes two interacting domains (macropore and matrix) with distinct water and heat transfer regimes. Abbreviations: SFC, soil freezing characteristic; SMC, soil moisture characteristic. Citation: Mohammed, A.A., B.L. Kurylyk, E.E. Cey, and M. Hayashi. 2018. Snowmelt infiltration and macropore flow in frozen soils: Overview, knowledge gaps, and a conceptual framework. Vadose Zone J. 17:180084.