The Effect of Organic Terrain and Surface Modification on Active Layer Development

Abstract

<p>During the summer of 1995, ground energy balance was calculated for six organic soils at different stages of development and nine mineral soils subject to forced modification near Resolute, N.W.T. Ground temperature, frost and water tables, and soil moisture were monitored at each site on 9 m² plots, allowing inter-comparisons in an attempt to assess how active layer development was related to changes in soil structure, hydrology and boundary conditions.</p> <p>In organic soils, both the degree of humification and the position of the water table were the principal controls on active layer thaw. Developing organic soils with water tables near the surface had deeper thaw and warmer soil profiles than decomposing organics. The presence of ground ice also limits thaw depth because of large latent heat requirements for ice melt. In organic soils, both conduction and convection of heat are important processes. A feedback mechanism may exist whereby wetland organics are maintained by keeping the active layer shallow through a reduction in heat transfer when dry.</p> <p>The responses of mineral soils to changing boundary conditions were inconclusive. Modifications to surface albedo, rainfall, snowfall and soil structure all affected soil parameters. Decreased albedo and increased snow cover had the greatest influence on active layer thaw when compared with unmodified conditions of the previous year. Placing a gravel pad over surfaces modified thaw depths in both mineral and organic soils.</p> <p>The results of this study show the importance of hydrologic conditions, properties of surface organic layer, soil heterogeneity and ground ice on active layer thermodynamics. In all soils, heat flowing into the permafrost was the greatest component of the ground energy expenditure, followed by heat consumed in phase change and lastly by sensible heat used in warming the active layer. Current numerical models of active layer development are simplified, and this study shows that complexities not accounted for in models may hinder their ability to predict active layer responses to changing environments.</p>