Estimating the variability of active-layer thaw depth in two physiographic regions of northern Alaska

The active layer is the zone above permafrost that experiences seasonal freeze and thaw. Active-layer thickness varies annually In response to air and surface temperature, and generally decreases poleward. Substantially less is known about thaw variability across small lateral distances in response to topography, parent material, vegetation, and subsurface hydrology. A graduated steel rod was used to measure the 1998 end-of-season thaw depth across several transects. A balanced hierarchical sampling design was used to estimate the contribution to total variance in active-layer depth at separating distances of 1, 3, 9, 27, and 100 meters. A second sampling scheme was used to examine variation at shorter distances of 0.3 and 0.1 meter. This seven-stage sample design l;ns applied to two sites in the Arctic Foothills physiographic province, and four sites on the Arctic Coastal Plain province In northern Alaska. The spatial variability for each site was determined using, ANOVA and variogram methods to compare intersite anti inter-province variation. Spatial variation in thaw depth was different in the Foothills and Coastal Plain sites. A greater percentage of the total variance occurs at short lag distances (0-3 meters) at the Foothills sites, presumably reflecting tile influence of frost boils and tussock vegetation on ground heat flow. In contrast, thaw variation at the Coastal Plain sites occurs at distances exceeding 10 meters, and is attributed to tile influence of well developed networks of ice-wedge polygons and the presence of drained thaw-lake basins. This information was used to determine fin ongoing sampling scheme for each site and to assess the suitability of each method of analysis.