Hydrologic and thermal properties of the active layer in the Alaskan Arctic

Abstract

Almost all biological activity in far north regions takes place within a shallow zone above the permafrost called the active layer. The active layer is the surficial layer of the soil system which thaws every summer. In Imnavait watershed, a small headwater watershed north of the Brooks Range on the North Slope of Alaska, the active layer is an extremely variable multilayered system consisting of a mat of mosses and sedges on about 10 cm of organic soil over silt. The layer of organic soil tends to mollify thermal and hydrologic fluctuations below it. The thermal conductivity of the surface organic layer at average moisture contents is about one-third that of the silt and thus functions as a layer of insulation for the permafrost. Before spring melt or after a period of low precipitation, the organic mat is desiccated and will absorb 1.5 cm of water before downslope runoff occurs. The hydraulic conductivity of the organic soils is 10 to 1000 times greater than the silt, thus during a large rainfall event, downslope movement will primarily occur in the organic layer. The subsurface mineral soil tends to remain saturated with little annual variation and shows little response to precipitation events. In comparison, the moisture content of surficial organic soil fluctuates between 10 and 90% by volume. To adequately model physical processes, we need a detailed understanding of the thermal and hydrologic properties because these properties vary so dramatically over short vertical distances.