A CASE STUDY OF 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 . 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 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. Differences in a values among mineral soils FIELD OBSERVATIONS are attributed to differences in soil properties, thaw Ground thaw (Figure 1) began between June 3 (O4) development, and the degree of saturation. Non- and June 28 (PD3), depending upon the time of saturated polar desert soils and O4 had larger a at snowmelt termination above each plot. Deepest thaw depth which also varied more than the saturated soils among the organic soils was O1 (0.47 m), followed by (O1, O2, O3). O3 (0.37 m) and O2 (0.33 m). The non-saturated plots O4 and O5 had an active layer thickness of about 0.2 m ENERGY BALANCE only. Although ground thaw at the polar desert soil Total vertical ground heat flux (Qg) into the soil were plots began later than at the organic soil plots, thaw calculated at all sites using surface temperature gra- depths exceeding 0.5 m were attained at PD1 and PD2 dients and rainfall inputs (Table 1). Heat advected later- and thaw was suppressed slightly at PD3. The water ally by groundwater flow at O1 and O2 was not deter- table (Figure 1) at O1, O2 and O3 remained close to the mined. Values of Qg were on average greater in the ground surface throughout the summer. Sites O4 and polar desert soils. The effect of convection from artifi- O5 had no water table at the onset of thaw, yet O4 cially increased rainfall (PD2) on ground heat flux was developed a thin saturated zone in early August due to less than 4 % of Qg, and a lower value of Qg at PD3 was Table 1. Ground Energy Balances and maximum thaw depth due to its prolonged snow cover. Values of Ql were si- for the period June 1 to August 6, all fluxes in MJ/m2 (see milar among the polar desert sites, as were their thaw text for explanation of symbols) depths. The variability of Qg and Ql among the organic soils was much greater. Ground heat flux was higher at the saturated O2 site, and lowest at the desiccated O5 site. Variations in Ql were accompanied by differences in the thaw depths among the sites: O1 and O3 had the largest Ql while sites with a shallow active layer (O4, O5) had lower values. Sensible heating (Qs) of the active layer accounted for less than 10 percent of Qg.

Sean K. Carey, Ming-ko Woo 129 Heat flux into the permafrost (Qp), determined as a GROUND ICE residual of the energy balance equation, accounted for Heat flux into permafrost (Qp) is facilitated by the 17 to 69 per cent of Qg. Organic soils underlain by large high k for frozen soils with abundant ice, notably at O2, volumes of ground ice (O2, O4) and polar desert soils O4 and the polar desert soils near their permafrost with an ice-enriched zone at the base of the active layer tables (Table 1). In contrast, sites O1 and O3 that do not have large concentrations of ice in their lower active had larger values of Qp than the other soils. layers have lower Qp and high Ql. The increase in ice content at a particular depth causes a sudden retarda- Discussion tion in the descent of the thawing front. This is due to the prolonging of the zero curtain effect at the zone Unlike most field-based comparisons of ground ener- where the large ice content is encountered. Examples gy balance and thaw which involved dissimilar site include the lower part of the active layer at O2 where conditions, the present case study examined ground the ice content reached >0.80, and near the permafrost thaw and their related ground energy terms among tables of PD1, PD2 and PD3 where the ice content sites with similar environmental (climate, geothermal, increases to almost 0.50. On the whole, the organic soils soil, topographic) settings. Our findings elucidate the with larger ice content than the polar desert soils, relative roles of ground heat flux, soil thermal proper- developed shallower active layers. These observations ties and ground ice content on active layer thaw. from the field provide direct evidence that the ground ice content is inversely proportional to the rate of thaw, GROUND HEAT FLUX as given in equation 2. Although larger Qg is expected to cause deeper thaw depths, only site O5 conformed to this expected rela- Conclusions tionship. Among the organic soils, there was no clear relation between Qg and thaw depths. Sites O2 and O4 All the variables presented in Equation 3 affect had the two largest seasonal values of Qg but deve- ground thaw but their relative significance cannot be loped shallower active layers than O1 and O3. Among differentiated without field comparisons. By maintain- the polar desert sites, despite the reduction in Qg from a ing similar environmental conditions among experi- prolonged snowcover at PD3, thaw depths were only mental plots located in close proximity to one another, slightly suppressed. The effect of artificially increasing differences in their thaw depths were compared directly and the relative significance of the governing factors rainfall by 130 mm was negligible on Qg and thaw, con- was discriminated. By contrasting active layer thickness firming NixonÕs (1975) numerical conclusion that con- in two major soil types found in the Canadian Arctic vection is not a major factor in active layer develop- Islands, this study shows that: ment. In general, results from this study indicate that factors other than ground heat flux play a more impor- (1) Thaw depths do not correlate directly with heat tant role in determining active layer thaw. flux into the active layer.

THERMAL PROPERTIES (2) Artificial modifications of surface conditions While the thermal diffusivity is affected by the chang- such as increased snow-cover or augmented rainfall ing moisture conditions of the soil, most thermal mo- have little influence on the maximum thaw depths. dels use fixed values for computing ground thaw for the entire season. This case study shows that a is highly (3) Thermal diffusivity (k/c) affects the rate of heat variable in time and among the sites. However, a does transfer in soils and is important in determining thaw not always explain the differences in active layer thick- depths. ness. At O5, low a throughout the humified organic profile may explain the shallow thaw depths. For O1, (4) Large fractional ice contents increase the ther- O2 and O3, a was similar for both organic and mineral mal conductivity and facilitate heat transfer into the layers (Figure 2), yet thaw depths were approximately permafrost. 0.15 m greater at O1 than O2 (which also had greater Q ). Furthermore, O1 had significantly lower a values g (5) Large ground ice contents limit thaw by pro- than the other polar desert soils, yet its thaw depth was longing the zero-curtain effect. only slightly less. Despite the general tendency that thaw depth decreases with a, several sites with similar a and Qg values have significantly different thaw depths.

130 The 7th International Permafrost Conference Acknowledgments

This work was supported by a grant from the Natural Sciences and Engineering Research Council of Canada and a Northern Training grant from the Department of Indian and Northern Affairs. We are grateful to the Polar Continental Shelf Project for logistical support. Financial assistance towards presentation of this paper at the Seventh International Conference on Permafrost was provided by the Royal Canadian Geographical Society.

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