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Factors affecting the distribution of permafrost, Mackenzie Delta, N.W.T. Smith, Michael William

Abstract

Variations in ground temperature regime and permafrost distribution were studied in a small area about 50 km northwest of Inuvik, N.W.T., in the east-central part of the Mackenzie Delta. The ground thermal regime is influenced by surface conditions, the nature of which varies spatially and temporally. Relationships obtained for the present set of environmental conditions are applied to the analysis of permafrost dynamics where surface boundary conditions have changed. The Delta is an area of active sedimentation and erosion and about 50 percent of the study area is covered by water bodies. A major distributary in the study area is undergoing lateral migration and the local configuration of permafrost is closely related to the history of river migration. Further, vegetation shows a successional sequence related to river migration, and there is thus a complex interaction between vegetation, topography and microclimate. The major objectives of the study were: 1) To describe the permafrost and ground temperature variations in the study area. 2) To understand how local environmental factors influence the ground temperature field. 3) To analyse the development of the present ground temperature field in terms of its position in a long-term geomorphological sequence. Temperature boreholes were drilled to various depths up to 30 m. Temperatures were measured with thermistors, and these measurements were augmented with seismic and resistivity surveys. Lake and river temperatures were also recorded, and ground materials were sampled from boreholes. In summer, active layer temperatures, net radiation and ground heat flux were recorded at five sites. Measurements of groundtemperatures, snow depths and ice cover were made in winter. Permafrost is generally perforated in nature, being absent beneath the river channel and larger lakes. Ground temperatures are warmer close to water bodies, and permafrost plunges steeply beneath cut banks. Observations indicate permafrost thicknesses of between 50 to 65 m in stable, spruce-covered areas. Calculations show that the maximum thickness in the area is about 100 m, at sites most distant from water bodies. Beneath slip-off slopes ground temperatures are warmer than beneath cut banks, and permafrost is between only 2.5 to 9 m thick. Here, permafrost thickens away from the river, and wedges out towards it; permafrost is absent in some places, where the winter snow drifts are deep. Significant differences in thermal regime exist under the various types of vegetation. In summer, higher ground temperatures are associated with high values of net radiation and ground heat flux. Mean daily 25-cm temperatures varied from 0.7° to 11.2°C between sites. In winter, snow cover is the decisive factor affecting ground temperatures. Significant variations in snow accumulation occurred between sites, and winter 1-meter temperatures varied from -6.4° to -0.6°C within 12 m. Through the framework of simple heat conduction theory, a consistent explanation of permafrost distribution in terms of local environmental factors is developed. The heat conduction models are suitable for ground temperature prediction, with agreement typically within ±0.5° of observed values for most sites. Calculations of the thermal disturbance due to channel shifting are in general agreement with observations, although omission of the latent heat term leads to some errors.

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