Runoff from hummock-covered Arctic tundra hillslopes in the continuous permafrost zone
Quinton, William Leo
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Runoff processes at three hummock-covered hillslopes within a 95 ha catchment (Siksik Creek) in the tundra region of the Canadian western Arctic were studied for three years (1992-1994). Measurements over the study included all hydrological inputs, monitoring of active layer and water table depths, chemical tracer-based measurements of subsurface flow velocities, continuous and spot measurements of surface and subsurface discharges, detailed mapping of the hummock distribution and soil types, and continuous gauging of the main stream channel. Tundra is a heterogeneous medium in both horizontal (due to the presence of periglacial features such as mineral earth hummocks) and vertical (due to abrupt changes in hydraulic conductivity with depth) directions. Hillslope runoff occurs through the unfrozen saturated layer of the interhummock zone (IHZ), owing to its relatively high permeability and connected nature. The subsurface runoff rate is controlled by the saturated layer position and flowpath type. Subsurface flow follows downslope oriented flowpaths (primary and macrochannnel flowpaths), and flowpaths obstructed from conducting directly downslope (secondary flowpaths). The water velocity, discharge and residence time of the subsurface flowpaths are highly variable spatially and temporally as a result of variations in the thickness and elevation of the unfrozen saturated layer; and the composition and arrangement of flowpaths on hillslopes. Rapid subsurface flow (i.e. subsurface flow occurring at surface runoff-like velocities) occurs through the highly conductive near-surface peat as well as through macrochannels. Although surface runoff is rare, the task of transmitting runoff rapidly to the stream is accomplished by rapid subsurface flow. All flowpath types conduct runoff at similarly high rates when the saturated layer is within the highly conductive near surface peats. When the saturated layer is below this near-surface zone, but still within the depth range where macrochannels occur, the macrochannel flowpaths (i.e. flowpaths containing active macrochannels) continue discharging at a relatively high rate, while primary and secondary flowpaths conduct flow at a lower level. Aside from their impact on the composition and arrangement of flowpaths in the IHZ, the presence of mineral earth hummocks on hillslopes increases the tortuosity of flowpaths in the IHZ, and as a result the residence time of runoff on hillslope is increased. Mineral earth hummocks also reduce the volume of runoff reaching the streambank during a runoff event, as a small amount of runoff in the IBZ flows into the hummocks. The presence of hummocks also enhances runoff from hillslopes by displacing the saturated layer of the interhummock zone upward where the hydraulic conductivity can be orders of magnitude higher. The near-stream area has a large impact on hillslope runoff and streamflow because of its location in the basin (surrounding the stream channel), and its deeper peat accumulation (and therefore greater water storage capacity). The rate at which the saturated layer falls away from the surface in the near-stream area varies among plots, and as a result the duration of the spring runoff period (high flow rates), and the nature of its adjustment to the summer regime (low flow rates) varies among the hillslopes. The major hillslope water fluxes and related processes are presented in a conceptual model, which is used as an explanatory framework for the level of flow in the stream channel.