Spatial and temporal patterns of active layer thickness at Circumpolar Active Layer Monitoring (CALM) sites in northern Alaska, 1995-2000

Maximum annual development of the active layer above permafrost has been monitored at seven 1-km(2) Circumpolar Active Layer Monitoring ( CALM) sites in northern Alaska since 1995. Grid nodes are spaced at 100-m intervals, yielding a regular array of 121 (11 x 11) data collection points. Three sites are located in the Arctic Foothills physiographic province, and four are on the Arctic Coastal Plain. Air and soil temperature measurements are made at each site, and soil moisture is monitored at most. Six years of record permit several general conclusions: ( 1) At the landscape scale, end-of-season thaw depth is strongly correlated with local air temperature on an interannual basis. All sites experienced maximum average thaw depth in 1998 and a minimum in 2000, consistent with the warmest and coolest summers during the period of record. The active layer, however, may exhibit Markovian behavior over multidecadal periods. ( 2) There is significant intrasite variation in thaw depth and near-surface soil moisture content within each 1-km(2) grid, reflecting the local influence of vegetation, substrate properties, snow cover dynamics, and terrain. ( 3) On the Coastal Plain, thaw depth is significantly greater in drained thaw-lake basins, resulting in a bimodal distribution of thaw depth related to primary landscape elements. ( 4) Foothills sites demonstrate large spatial and interannual variability resulting from microtopography and temporal variations of soil moisture content, making predictive mapping of thaw depth problematic at the scale and resolution of the grids. The spatial pattern of thaw depth across sites on the Coastal Plain is relatively consistent, although lake margins exhibit more complex patterns attributable to fluctuating water levels.