Slope runoff processes and flow generation in a subarctic, subalpine catchment

Hillslope runoff was studied in a subarctic, subalpine environment to improve understanding of runoff generation processes and the mechanisms whereby water moves from hillslopes to the stream. Runoff characteristics of four hillslopes were examined between 1997 and 1999, each with distinct soils, frost, topography and vegetation. Lateral fluxes were confined to hillslopes with porous organic soils overlying less permeable mineral substrates, setting up a two-layer flow system whereby most drainage occurs as quickflow in the porous organic layer as matrix flow and/or as preferential flow in pipes, rills and interconnected surface depressions. During snowmelt, meltwater infiltrated and percolated the porous frozen organic layers with little resistance. Percolation ceased at the organic/mineral interface due to the impermeable nature of frozen mineral soils, forming a perched saturated zone and initiating runoff. Snowmelt runoff was greater on slopes with greater snow water equivalent and reduced organic layer thickness. In summer, runoff was greatest on slopes where wet conditions were sustained by inflow. Stormflow hydrographs responded rapidly to rainfall while exhibiting extended recessions compared with temperate regions. Where sustained inflow occurred, the recession limb showed two segments to reflect different source areas of stormflow production. Recession analysis was used to quantify contributing areas, which were highly variable and controlled largely by hillslope wetness and organic-layer properties. Results indicate that the concept of variable source area for runoff generation applies to subarctic, subalpine catchments.