ABSTRACT: Toward modelling seasonal thaw and subsurface runoff in arctic tundra environments.

CGRG Bibliography of Canadian Geomorphology
Search Results

Author :

Date :

Title :

Publication :

Issue :

Page(s) :

Abstract

Field studies conducted at permafrost sites in arctic-tundra near Inuvik, NWT, Canada, examined the thermal regime of the active layer and subsurface runoff during snow ablation and ground thawing. The results of these studies suggest a physically based approach for estimating the rate of drainage through the saturated layer from physical, hydraulic and thermal properties of the active layer. Estimation of subsurface flow from hillslopes in the arctic tundra requires information on the elevation and thickness of the saturatedlayer because the soil permeability decreases with depth. During the early stages of snow ablation subsurface flow is confined within the surface layer of living vegetation and partially decomposed, highly permeable organic matter. In this period, flow can be calculated assuming the hydraulic conductivity of the layer is constant. Similarly, late in the season, when flow is confined within thedeeper dense, highly decomposed and less permeable organic matter, the hydraulic conductivity can be assumed constant. Between these two conditions, the saturated zone overlaps layers of widely different conductivity as the water table subsides into the organic soil. It is shown that the flow regime during soil thawing (falling water table) that the coefficient C of the relation between friction factor, f and Reynold’s Number, NR, (i.e., f = C/NR) increases linearly with the depth to the middle of the saturated zone, d. Since C = 2D2/K, where D is the geometric mean pore diameter of the material encountered by the saturated layer, and K is the soil permeability, the relation between C and d allows an approximation of the variation in permeability with depth from estimates of the variation in mean pore geometry with depth (from laboratory analysis of soil samples) and water table position. Field measurements suggest that heat conducted downwards from the soil surface governs thawing of the seasonal frost. Thus, it is reasonable to expect that temperature-index models might provide reasonable estimates of the rate of ground thawing. Strong correlations between cumulative degree-day air temperature and cumulative ground heat flux is demonstrated. However, such regressions have only limited value as a predictive tool because the association ignores the energy exchanges occurring at the soil surface and the thermal properties of the active layer. These deficiencies restrict the transposability of an empirical association for estimating the ground heat flux at a site where soil thermophysical properties may differ appreciably from those where theregressions were derived. A preferred approach uses surface temperature as an index rather than air temperature since surface temperature is directly influenced by the energy exchanges occurring at and within the active layer. This approach also offers the prospect of using remotely sensed, thermal infrared data as a method of obtaining the input variable for the model. This paper uses field measurements to demonstrate the relation between surface temperature and ground heat flux in arctic tundra following disappearance of the seasonal snowcover. The depth of thaw derived from an estimate of ground heat flux is shown to be in close agreement with the measured depth to the frost table.

Bibliography of Canadian Geomorphology