ABSTRACT: The effect of frost on the water balance of two subarctic slopes

CGRG Bibliography of Canadian Geomorphology
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Abstract

The water balance of slopes in a discontinuous permafrost environment was studied in the Wolf Creek Basin, Yukon. The sites include a South-facing slope with a dense aspen forest underlain by silt soils with seasonal frost and a North-facing slope with open stands of spruce and organic layer that rests atop mineral soils with permafrost. Precipitation inputs were similar on both slopes, though the North slope had over 30% more snow at the end of winter. During snowmelt, the seasonally frozen soils of the South slope did not hinder infiltration and the soil profile was recharged but no surface runoff was generated. On the North slope, ground ice restricted infiltration of meltwater and about 80% of the snow storage was discharged as surface runoff. As ground thaw progressed, surface runoff declined. After the thawing front advanced beneath the organic layer, two modes of flow occurred: 1. quick flow at the base of the organic mat and 2. slowerDarcian flow in the mineral soil. On the South slope, soil moisture was mostly depleted by evaporation, the magnitude of which about equalled precipitation. During snowmelt on the North slope, large surface detention of meltwater supported high evaporation rates. As the frost table descended, near-surface soil moisture declined, accompanied by a reduction in evaporation rates. As the frost table descended, near-surface soil moisture declined, accompanied by a reduction in evaporation rates. Seasonally, evaporation was slightly larger than total precipitation. For the period of August 1996 to August 1997, the moisture storage changes were about +5% for both slopes. These field results indicate that seasonal frost not accompanied by large percentages of ice content does not hinder downward movement of water into the mineral soils. This promotes vertical exchange of water at the expense of lateral runoff. Conversely, permafrost with ice rich layers inhibitsinfiltration and percolation of water, confining the soil water within the surface thawed zone and promoting surface runoff and lateral subsurface drainage. Knowledge of permafrost disposition within subarctic basins is therefore critical in determining the contributing source areas for streamflow and basin water balances as a whole.

Bibliography of Canadian Geomorphology