Ninth International Conference on Permafrost ... - IARC Research

Seasonal and Interannual Variability of Active Layer Development in PermafrostWetland SystemsC.M. ChiuDepartment of Agronomy, Purdue University, West Lafayette, IN, USAL.C. BowlingDepartment of Agronomy, Purdue University, West Lafayette, IN, USAIntroductionOver the last decades, the arctic region has been warmingat an accelerated rate with sometimes puzzling effectson lakes and wetlands. Wetlands are common landscapecharacteristics in the northern high latitudes. Permafrost andseasonal frost can be a powerful factor affecting wetlandhydrology. The interaction of soil thermal and moistureregimes controls the structure of the seasonally thawedactive layer and hydrological response in permafrost regions.Soil thermal and moisture properties regulate the transfer ofheat though the active layer, thus affecting the annual thawand frost depth. The maximum active layer thickness canvary substantially with annual soil temperature change,given similar moisture conditions (Brown & Hinkel 2000).Spatially, the active layer thickness can vary over shortlateral distances due to differences in heat transfer underdifferent soil moisture conditions subject to freezing andthawing (Outcalt et al. 1990). For example, Woo and Xia(1996) found that a wetland site had shallower maximumthaw depth than a drier site due to the buffering effect ofthe large ice content. The spatial variation may also reflectthe interaction of a large number of highly localized factors,including vegetation type, snow cover, organic layerthickness, soil thermal properties, microtopography, and theoperation of nonconductive heat transfer processes (Outcaltet al. 1990). This study will examine the spatial and temporalvariation of soil temperature and soil moisture content in acontinuous permafrost environment and how they relate tothe duration and thickness of seasonal active layer in uplandand wetland systems. In particular, the ability of the VariableInfiltration Capacity (VIC) land surface model to reproducethe observed relationship between moisture condition andmaximum annual thaw depth will be evaluatedStudy AreaThe study area is the 471 km 2 Putuligayuk River watershedlocated on the coastal plain south of Prudhoe Bay on theAlaskan North Slope. Observations of soil temperaturesand weather are taken from the Betty Pingo weather station(70°16′46.3ʺN, 148°53′44.5ʺW) operated by the Universityof Alaska Fairbanks. The watershed is dominated by drainedlakes and numerous permafrost features such as high- andlow-centered polygons, pingos, hummocky terrain, frostboils, and strangemoor ridges (Kane et al. 2000). The poordrainage results in extensive wetlands, ponds, and lakes. Themaximum thickness of the permafrost along the arctic coastis about 600 m, whereas the maximum depth of thaw of theactive layer was about 53 cm between 1993 to 2002 (Brown& Hinkel 2000). The vegetation in this area is dominated bysedge tundra, with shrubs in wetter areas and tussock tundrain higher and drier areas. Observed meteorological and soiltemperature data for the period 1994–2004 was obtainedthrough the National Snow and Ice Data Center (Kane &Hinzman 1997).Model DescriptionThe Variable Infiltration Capacity (VIC) model is amacroscale hydrologic model which has been applied tomany environmental studies associated with global climateand land use change prediction (Liang et al., 1994). VICmodel features relevant to this application include (1) a finitedifference soil thermal solution (Cherkauer & Lettenmaier1999, Cherkauer & Lettenmaier 2003) and (2) representationof the surface storage and energy dynamic of sub-grid lakesand wetlands (Bowling 2003, Cherkauer & Lettenmaier2003). In this study, we will examine the ability of the VICmodel to simulate upland and wetland soil temperature, soilmoisture, and active layer depth. The VIC model is run as asingle point centered on the two sites. A constant temperaturebottom boundary condition is used with a fixed temperatureof -10°C at 4 m damping depth. Two scenarios are run withcontrasting initial moisture conditions, bulk density andbase flow parameters to simulate the contrasting moistureconditions of the site.Preliminary ResultAdjacent upland and wetland sites were instrumented byUAF beginning in 1994. The upland site was instrumentedwith 12 temperature probes vertically arranged at depths of0, 1, 2, 3, 4, 5, 6, 7, 8, 15, 35, and 60 cm. The adjacentwetland site was also instrumented with 12 temperatureprobes vertically arranged at depths of 0, 0.5, 1, 2, 3, 4, 5, 6,15, 30, 50, and 75 cm. Mean annual soil temperatures for theupland and wetland sites derived from hourly observationsbetween 1996–2001 are shown in Figure 1. The patterns atboth sites are very similar, although observed annual averagetemperatures are 0.27 to 0.74°C warmer for the wetter soils.The simulated temperatures are 0.35 to 0.6°C warmer forthe wetter soil. In the wetter soil, water or ice displaces airin soil pores, increasing the bulk conductivity as well as thesoil heat capacity. Thus, more energy may be required tochange the temperature within the same depth resulting in ashallower active layer depth and shorter thawed period in thewetter soil (Figs. 2, 3).43