The role of soil pipes as a slope runoff mechanism, Subarctic Yukon, Canada

Pipeflow in subarctic slopes provides a preferential runoff mechanism that bypasses the soil matrix, rapidly conveying water to the stream. Extensive soil piping occurs in the central Wolf Creek basin, Yukon, at the interface of the organic and mineral horizons. Flow in these pipes are ephemeral, transmitting water only when the water table is within or above the zone where pipes occur. During snowmelt, pipeflow began several days after the onset of surface runoff. Pipeflow contribution increased until ground thaw lowered the water tables, leaving matrix flow within the organic layer as the dominant mode of runoff. Pipeflow accounted for 21% of runoff during the 15day melt period of 1997. Following melt, pipeflow recurred only during two intense summer rainstorms, each yielding different pipeflow response characteristics. During melt, pipeflow closely followed the daily snowmelt cycles and responded earlier than the integrated slope runoff. In the summer, pipeflow hydrographs rose and fell much quicker than direct storm runoff from the entire slope, which was dominated by fast matrix flow within the organic layer. Pipeflow contributing areas were relatively small, but their extent changed with hillslope wetness. Analysis revealed that the Manning flow formula can be combined with contributing areas to simulate pipeflow discharges. Unlike temperate environments where frozen ground is not a factor, the frost table position significantly controls the position of the phreatic surface, and must be considered in any models of pipeflow in permafrost slopes.