P. Marsh and J. Pomeroy National Hydrology Research Institute 11 Innovation Blvd., Saskatoon, Sask. S7N 3H5

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1 WATER AND ENERGY FLUXES DURING THE SNOWMELT PERIOD AT AN ARCTIC TREELINE SITE P. Marsh and J. Pomeroy National Hydrology Research Institute 11 Innovation Blvd., Saskatoon, Sask. S7N 3H5 INTRODUCTION Snow plays an important role in the water and energy fluxes of Arctic regions. For example, its high albedo has a dramatic impact on the surface energy balance, and as a result snowpack removal in the spring results in the rapid and dramatic change in surface energy fluxes. In addition, since the snow stored on the ground may represent 6 to 10 months of precipitation, the brief snow melt period represents a sudden release of water to the stream channels. As a result, the spring runoff event often accounts for over half of the total runoff. This runoff has important implications to northern ecosystems, and it also plays a role in controlling circulation patterns in the Arctic Ocean. As a result, it is critical for hydrological, weather and climate predictions to properly estimate both the timing and volume of meltwater release. In Arctic regions, the snow and ground at the end of winter typically have temperatures many degrees below freezing. As a result, much of the initial melt water freezes within the snowcover, with the freezing determined by the cold content of the snowpack and the soil heat flux. Because of cold soil temperatures and the existence of permafrost, the soil heat flux is always negative (ie from snow to the soil) during the melt period, with reported magnitudes as large as 80 W/m2 (Marsh and Woo, 1987). In temperate areas these processes are not as important, and as a result are often ignored in snowmelt runoff models. In addition, wetting fronts do not move uniformly through a snow cover. Instead, flow fingers develop at the leading edge of the wetting fronts, allowing melt water to move very quickly through the snow. This process plays an important role in energy fluxes within the snowpack and in initiating runoff much earlier than would otherwise be expected. The objectives of this paper are: (1) to utilize physically based models of snowmelt and water flux through cold snow covers, (2) demonstrate the application of this model for predicting the initiation of meltwater runoff in Arctic areas, and (3) demonstrate how the spatial variation of the initiation of runoff at the basin scale, can be estimated by applying model results in conjunction with modelled snow cover accumulation as described by Pomeroy et al. (1995). METHODS Field studies of water flux through cold snow have been camed out at Trail Valley Creek (68 kn? in area, 68 "45N, 133 "301W), located approximately 40 km north of Inuvik, N.W.T. The methodology for determining snow cover distribution is provided by Pomeroy et al. (1993). The snow distribution is highly variable, with thin (3050 cm) snow covers on the wind blown tundra, cm snow cover in the high shrubs, cm in the forests, and cm in drift areas. Snow properties, including grain size, snow temperature, and density for example, were

2 determined at snowpits located in each of the landscape classes. Meteorological data was obtained from a remote weather station located within the TVC basin. Water flux through cold snow was measured using a variety of melt water collection lysimeters, varying in size up to 1 m2. A multicompartment lysimeter was used to determine the variability of flow over small areas. MODELLING Surface enerq balance The surface energy balance during the melt period was modelled using an hourly bulk aerodynamic method (Dunne et al., 1976) during the period when the surface is completely snow covered. It was found, however, that once the snowcover became patchy, advectionfrom the bare patches to the snow covered areas limited the accuracy of this method for predicting basin inowmelt. In order to overcome this difficulty, a simple parameterization scheme using radiation and sensible heat fluxes distributed by snowcovered area, was developed. This scheme estimates the advection of sensible heat fr6m the bare patches to the ~nowcover~d areas. Meltwater percolation Percolation through the snowpack was estimated from a variable flow path, melt metamorphism model, with the melt flux applied to modelled snowcover (Pomeroy et al., 1995) in various landscape types to determine the spatial variability in the timing and magnitude of meltwater released for runoff. The percolation model is described in Marsh and Woo (1984) and Marsh (199 1). This model parameterizes the percolation processes as follows: (1) the wetting front is idealized as a two component wetting front. The background fiont, above which all snow is wet and isothermal at O C, and a finger fiont which describes the deepest penetration of meltwater into the snow. (2) the size and flow volume in the finger front is determined fiom measurements in the Canadian Arctic (Marsh and Woo, 1984) and in the Sierra Nevada Mountains of California (McGurk and Marsh, 1995). These studies have demonstrated that flow fingers vary in size only over a very small range of sizes, and that on average, they cover 22% of a horizontal surface, while carrying 48% of the total flow. (3) ice layers are allowed to grow at premelt strata boundaries. With the rate of growth controlled by the temperature gradient above and below the strata boundaries. (4) the snow and soil temperature are linked, and therefore the large negative soil heat fluxes are accounted for. RESULTS Snow survey, mapping, and modelling (Pomeroy et al., 1995) results have been used to map snowcover within the TVC study area. Mean snowcover depths varied from 45 cm for tundra, 100 cm in high bush areas, to 180 cm for drifts. Likewise, densities were 150,230 and 250 Mm3 respectively. Spring melt began in May, with the first day of melt occurring on May 12, 1993 (Julian Day 132). Low rates of melt occurred over the period May 15 to May 19 (JD 135 to 139), but then melt ceased for 4 day period when air temperatures were below 0 C between May 20 and 23 (ID 140 to 143). Starting on May 24 (JD 144), melt began and continued until June 29 (ID 180). During this period, melt rates gradually increased, reaching a maximum value of over 60 mmlday (Figure 1).

3 Figure 1. Surface snowmelt and basin snowcovered area. Also shown are the predicted and finger (FF) wetting fronts for tundra, high bush, and drift sites. The arrival of these fronts at the snowpack base indicate the dates when each snowcover was not contributing water (NC) and hlly contributing meltwater to runoff PC). For the period between these, the snowpack was partially contributing (PC) meltwater to runoff. Tundra (45 cm) FC High bush (100 cm) Julian Day (1993) The meltwater percolation model was then used to estimate the initiation of the timing of runoff for each landscape class within TVC. Figure 1 illustrates the background and finger wetting front advances, and the availability of meltwater at the base of the snowpack for each landscape class. For the period before the finger front reaches the base of the snowcover, none of the surface melt reaches the base of the snowcover and the snowpack is defined as not contributing. Once the finger front reaches the base of the snow, then it is partially contributing meltwater to

4 Figure 2. Spatial v a r i n t i o n in r?.c!t l.vater rc.!,z;~szfrom the ~ ~ o i v p for a ~ I'rnil k l'alley Creek, indicating those portiofij of tke basin u. h i s h sr? r,ot cont:ibu:ing to flow and partially contributing flow for bray Yatz that none of t h e basin is fully contributing flow.

5 runoff. The volume of runoff is dependent on the surface melt rate, and the flow within the flow fingers, which in this case is set at 22% of the surface melt. Finally, once the background front reaches the base of the snowcover, the snowpack is filly contributing meltwater, and therefore all of the surface melt is available for runoff Note that runoff in this paper only implies runoff from the snowpack to the soil or as overland flow, or as storage at the base of the pack. Ongoing work is addressing the issue of contribution to the stream channel (Quinton and Marsh, 1995). During the spring of 1993, meltwater first reached the base of the snowpack, and therefore was available to infiltrate the frozen soil or to runoff, at the tundra on May 16 (JD 136), approximately four days after the start of melt. The background front however, did not reach the base of the pack until May 24 (JD 144). As a result, all melt water was not available to runoff until this time. The drift areas were at the other extreme, with the finger front not reaching the base of the snowpack until May 20 (JD 140), 8 days after the start of melt. At these sites, the background front reached the base of the snowpack 7 days later. The high bush areas were intermediate between these extremes (Figure 1). Combining model results with a map of landscape types for TVC allows the contributing areas of meltwater runoff to be mapped on a daily basis. Figure 2 shows an example for May 17, 1993 when 84% of the basin was partially contributing meltwater to runoff, while the remaining 16% of the basin with deeper snowcovers was not contributing any meltwater. Future work will use such model output, in conjunction with ongoing work of flowpaths (Quinton and Marsh, 1995) to model basin streamflow. References Dume, T., A.G. Price, and S.C. Colbeck The generation of runofffrom subarctic snowpacks. Water Resources Research, 12, Marsh, P Water flux in melting snow covers. In Advances in Porous Media, Vol. 1. M.Y. Corapcioglu (ed.). Elsevier, Amsterdam, Marsh, P. and Mk. Woo Wetting front advance and fieezing of meltwater within a snowcover 2: A simulation model. Water Resources Research, 20, Marsh, P. and Mk Woo Soil Heat Flux, Wetting Front Advance and Ice Layer Growth in Cold, Dry Snow Covers. Snow Property Measurements Workshop, Ottawa: National Research Council Canada, Marsh, P., J. Pomeroy and W.L. Quinton Application of snow and evaporation models for predicting water fluxes at the arctic treeline in northwestern Canada. International GEWEX Workshop on ColdSeasoflegion Hydrometeorology, this volume. McGurk, B. and P. Marsh Flowfinger continuity in serial thicksections in a melting Sierran snowpack. IAHS. Biogeochemistry of seasonally snowcovered catchments. in press. Pomeroy, J.W., P. Marsh, and L. Lesack Relocation of major ions in snow along the tundrataiga ecotone. Nordic Hydrology, 24, Pomeroy, J.W., P. Marsh, and D.M. Gray Application of a blowing snow model in the Arctic. International GEWEX Workshop on ColdSeasoflegion Hydrometeorology, this volume. Quinton, W.L. and P. Marsh Subsurface runoff from tundra hillslopes in the continuous permafrost zone. International GEWEX Workshop on ColdSeasodRegion Hydrometeorology, this volume.