A CASE STUDY OF ACTIVE LAYER THAWAND ITS CONTROLLING FACTORS Sean K. Carey, Ming-ko Woo School of Geography and Geology, McMaster University, Hamilton, Ontario, Canada L8S 4K1 e-mail: [email protected]; [email protected] Abstract The roles of energy input and thermal properties of the soil on active layer thaw have been considered by modelling or studied piecemeal in the field. This paper reports a case study, undertaken near Resolute, Northwest Territories, Canada, that makes direct comparisons of the relative importance of ground heat flux, thermal properties and ice content effects on ground thaw. Results from the sites did not indicate a simple rela- tionship between ground heat flux and active layer thaw. Soil thermal properties are related to thaw depth except for sites where there is abundant ground ice. Energy balance considerations revealed that large ice con- tents lead to a prolonged zero-curtain effect and facilitate downward heat conduction to the permafrost. The consequence is shallow thaws for the active layers with ice rich soils. Introduction where the first bracketed terms (Qg) represent the heat conducted and convected into the active layer, The depth of active layer thaw is important to hydrol- with (dT/dz)|s being the temperature gradient at the ogy because most water fluxes and storages are con- ground surface, ks the thermal conductivity of the sur- fined above the frozen zone, to geomorphologic and face layer, cw the volumetric heat capacity of water, DT geotechnical investigations because of the abrupt the temperature difference between the soil and the change in soil properties at the frost table, and to ecolo- water which infiltrates the soil at a rate of dF/dt. In the gy because the permafrost table marks the limit of roo- second bracketed term (Qs), c is the volumetric heat ting. The ground energy balance offers a framework capacity of the active layer, dT/dt is the daily tempera- that relates thaw depth to its causative factors. During ture change over the active layer of thickness Z. Heat the thaw period following snowmelt and before freeze flux into the permafrost (third bracket) involves the back, the one-dimensional energy balance for the active thermal conductivity, kb, and the temperature gradient layer is (dT/dz)|b at the base of the active layer. QQQQ=++ [1] gpsl The theoretical bases of the above equations are Here, Qg is the heat flux into the ground, which is dis- well understood. Geothermal modelling of ground thermal regimes and thaw in permafrost terrain under sipated as heat flux into the permafrost (Qp), as sensible both present and potential climate change scenarios has heat that warms the active layer (Q ), and to thaw the s traditionally been carried out using thermal conduction ground ice (Ql). Rate of thaw (dzh/dt) is related to the models that can accommodate phase change (Nakano latent heat term and Brown, 1972, Smith, 1977, Smith and Riseborough, 1983, Riseborough 1990, Kane et al., 1991, Hinzman et dz Q [2] al., 1992). Field studies on the energy balance of the hl= dt rl ò active layer have demonstrated the role of the organic i mat in limiting active layer thaw (e.g. Brown and PŽwŽ, where r is the density of ice, l is the latent heat of 1973, Riseborough and Burn 1988) the effect of fusion, ¦i is the volumetric fractional content of ice. increased winter snowcover on insulating the ground Expanding the components of Equation 1 and substitu- from coldness (Nicholson, 1978, Goodrich, 1982, Rouse, ting into equation 2, thaw-rate can be re-written as 1984) the role of ground heat flux (Rouse et al., 1992) and ice content (Rouse 1984, Woo and Xia, 1996) on æ dT dFö æ dT ö æ dT ö thaw depths. çks + cTwbD ÷ - c Zk+ ç ÷ dz è dz dt ø è dt ø è dz ø h = s b However, the various terms in Equation 3 do not dt rl òi operate at the same level of significance across all soils, [3] and even among soils of similar type. There remains a Sean K. Carey, Ming-ko Woo 127 lack of field studies that seek to identify the various tric ice contents for all soils were determined by sam- components of the ground energy balance, and assess pling prior to ground thaw at approximately 0.05 m their comparative importance on active layer thaw. It is depth intervals at sites adjacent to each plot (Figure 1). therefore the purpose of this study to provide a case study in permafrost soils that examines the relative Snow conditions at all plots were measured at the importance of ground heat flux, latent heat and soil beginning of the experimentation period and summer properties and ice content on ground thaw. rainfall was recorded using a Texas Electronics tipping- bucket rain gauge. A snow fence was set up before win- Study site and methods ter to increase snow accumulation at PD3; rainfall was simulated at PD2 using a simulator consisting of a This case study examines the magnitudes of all com- reservoir from which 380 nozzles showered over an ponents of Equation 3 on a comparative basis from data area of 1.5 m2 to yield an intensity of 10 mm h-1. Water collected at experimental plots set up near Resolute, Northwest Territories, Canada (74¡43ÕN, 94¡59ÕW). Two Figure 2. Daily rainfall and air temperature (30 year normal for Resolute also shown), 1995, and thermal diffusivity for the top (organic layer and types of surficial materials were considered, including 2-10cm layer at the polar desert) and bottom layers (0.1-0.25 m at the organ- polar desert soils (Cruickshank, 1971) made up of lime- ic sites and 0.1-0.5 m at the polar desert sites) of the soils. stone and dolomite pebbles in a matrix of sand and silt, and bog (or fen) soils which consist of an organic layer made up of peat, mosses, lichens and vascular plants, overlying sandy loam. The latter will be called organic soils. Five organic soil plots and three polar desert soil plots were used, each measuring 3 m x 3 m. The organic soils were divided into those with a saturated zone frequen- tly reaching the surface (plots O1, O2 and O3), and those which dry out partially in the summer (plots O4 and O5). Of the three polar desert soils, plot PD1 served as a control for two other plots that were subjected to artificially increased rainfall input (PD2) or enhanced snow accumulation (PD3). Soil properties and volume- Figure 1. Frost table and water table variation at the study plots, 1995, with 0 depth indicating the ground surface. The columns show snow accumula- tion and soil properties: SWE is Snow Water Equivalence in mm measured on June 1, f is porosity and (fi) is volumetric ice content. 128 The 7th International Permafrost Conference used for the simulations was equilibrated with air tem- the melting of ground ice at depth and rainfall. The perature before each application. polar desert sites were saturated only immediately after snowmelt. Ground temperatures were measured using thermo- couples installed the previous summer at depths of THERMAL DIFFUSIVITY 0.02, 0.1, 0.25 m and at the organic-mineral interface of Both thermal conductivity and heat capacity change the organic soils. Polar desert soils had a junction at 0.5 as ground ice is replaced by water and when water is m depth instead of 0.25 m. Plot PD1 had additional replaced by air through evaporation or drainage losses. thermocouples at 0.15, 0.2 and 0.3 m. Depth of frost Thermal diffusivity (a) of the active layer, defined as table was probed daily by driving a thin steel rod into k/c, was calculated based on values of k and c adjusted all the plots until frozen ground was reached. The preci- for daily changes in the fractional contents of ice, water sion when compared to thermocouple temperatures at and air (Figure 2) (Woo and Xia, 1996). For surface la- both organic and polar desert sites was 0.02 m. At each yers of the polar desert sites, a declined throughout the plot, water table was measured in small wells with season as water and air replaced ice. Differences in a walls reinforced by perforated PVC pipes. Soil moisture among polar desert soils were caused by variations in was obtained gravimetrically using small samples soil composition and surface wetness. For the top layer taken at two-day intervals. of the organic soils, a dropped rapidly as ice melted but afterwards, a stabilized and did not respond readily to The experiments spanned between July 1 and August changes in soil moisture. Sites O1, O2 and O3, which 6, 1995, a period that was warmer and drier than the cli- were saturated for most of the season, exhibited similar matic mean for Resolute (Figure 2). Mean snow water trends in a. Site O5, which was not saturated, also had equivalences on top of each plot are given in Figure 1. similar a values due to the high bulk density of the There was significant variation in snow depth among organic material. The palsa site (O4) had low a values the plots as some organic soils had experienced melt as would be expected of a dry organic mat, and were prior to the study. Rainfall in the study period totaled similar to those obtained by Nelson et al. (1985) (their 28 mm except that plot PD2 received an additional 130 Figure 6). mm simulated during 13 events (Figure 2). The thermal diffusivity of the lower mineral layer Results (Figure 2) exhibited trends similar to those of the sur- face layer.