The regional water and energy balance is governed by the atmospheric circulations that transport the water and energy into, through and out of the region, as well as their complex interactions with the physical features and processes of the region. To better appreciate the water and budgets evaluated for the MRB, a description of the basin physiographic characteristics and the large-scale atmospheric circulations that affect the region will be given in this section.
The Mackenzie River in northwestern Canada is the longest river in N. America, and the 10th longest in the world. The Mackenzie River Basin (MRB, Figure 2.1) includes all of the land area (~ 1.8x10^6 km2) that contributes runoff to the Mackenzie River, and discharge from the river is the largest N. American source of fresh water into the Arctic Ocean. The basin stretches from Jasper, Alberta at about 52°N to the Beaufort coast near 70°N. It is made up of six main sub-basins, three great lakes (Great Slave Lake, Great Bear Lake and Lake Athabasca ), and three major deltas. It has four major physiographic zones: an Arctic coastal plain, a mountainous Cordilleran region in the west, flat interior plains with myriads of lakes and wetlands, and to the east, the rolling to rugged Canadian Shield with thin soil mantles and a patchwork of lakes, wetlands and uplands. It is separated from the Pacific Ocean by the mountainous coastal regions of British Columbia and Yukon. The mountains to the west of the basin rise to 2-3 km (Figure 2.3) and act as an effective barrier to the moisture that is transported into the continent from the Pacific.
Due to its northern location, about three-quarters of the MRB experiences no absorbed solar radiation for nearly three months of the year; much of the basin is snow-covered for 6-8 months of the year and about 75% of the basin lies within the continuous and discontinuous permafrost zones. Pingos and patterned ground features associated with continuous permafrost are found in the north, while agriculture and forestry are important economic activities in the southern parts of the basin. Because of the diverse physiographic features climatic conditions that characterize the basin, eight of Canada’s 15 major terrestrial ecozones can be found within the basin (Figure 2.2). Other features of the Mackenzie basin are summarized in Table 2.1.
|Area:||1.8 millon km2|
|Number of sub-basins:||6 (Figure 2.5)|
|Number of lakes:||3 great lakes and 32,370 lakes of smaller sizes (Figure 2.6)|
|Mean annual precipitation:||410mm|
|Mean annual evapotranspiration:||237mm|
|Average monthly temperature:||15°C in the summer and -25°C in the winter|
|Ecozones:||Taiga Cordillera, Boreal Cordillera, Montane Cordillera, Southern Arctic, Taiga Plain, Boreal Plain, Taiga Shield, and Boreal Shield (Figure 2.2)|
Outline vectors are available for the Mackenzie Basin and are as shown in Figure 2.4. The data in the following .zip files are provided as a courtesy of Research Scientists at the National Hydrological Research Institute of Saskatoon. The files are in SPANS GIS compatible format .vec (vector data) and .veh (header) files. However, SPANS is not necessary to read these files since they contain ASCII text and are easily displayed and understood using any word processor. The vector coordinates are found in the .vec files. The .veh files are SPANS header information. The .vec file values are easily recognizable as decimal longitudes and latitudes, one pair per line. Gary Weiss (ECB Regina) has converted the basin outlines from the SPANS raster format into ESRI's Arcview file format. To view the files, use Geomantica.
|Zip filename||Contents of unzipped files|
|basinvec.zip||Outline of the whole Mackenzie Basin (.dbf, .sbn, .sbx, .shp, & .shx formats)|
|Mackenzie_basins.zip||Outline of the Mackenzie river basin/sub-basins in Arcview format|
Outline vectors are available for all sub-basins of the Mackenzie Basin. They are each labelled with a different colour in Figure 2.5. The data in the following .zip files are provided as a courtesy of Research Scientists at the National Hydrological Research Institute of Saskatoon. The files are in SPANS GIS compatible format .vec (vector data) and .veh (header) files. However, SPANS is not necessary to read these files since they contain ASCII text and are easily displayed and understood using any word processor. The vector coordinates are found in the .vec files. The .veh files are SPANS header information. The .vec file values are easily recognizable as decimal longitudes and latitudes, one pair per line. Gary Weiss (ECB Regina) has converted the basin outlines from the SPANS raster format into ESRI's Arcview file format. To view the files, use Geomantica.
|Zip filename||Contents of unzipped files (.vec & .veh)|
|athbasca.zip||Athabasca River basin outline|
|bear.zip||Great Bear Lake basin outline|
|liard.zip||Liard River basin outline|
|peace.zip||Peace River basin outline|
|peel.zip||Peel River basin outline|
|slave.zip||Great Slave Lake basin outline|
In the central MRB there are about 32,370 lakes (Figure 2.6) and for the whole MRB lakes of various sizes dominate about 11% of the surface area. These lakes have distinctive energy cycles and surface processes as summarized in the following excerpt of lake features in the MRB contributed by W. Rouse to the (MAGS) website:
For the small and shallow lakes, lake-ice melt can proceed rapidly with a combination of above-freezing temperatures and strong absorption of solar radiation. This allows vigorous evaporation to proceed early in the thaw season. The heating and cooling of shallow lakes is then governed by the temperature of the overlying air masses. Typically, a small subarctic lake will average a complete heating-cooling cycle in 5 days during the ice-free period. Subfreezing temperatures in early winter coincide with short daylight periods and freeze-up occurs quickly. The initial formation of ice puts an effective lid on outgoing turbulent heat fluxes and the evaporation cycle ends. The period of evaporative heat loss from shallow subarctic lakes is in the order of 4 months and this becomes less in northern tundra regions.
In large deep lakes there is a strong asymmetry between heating and cooling rates due to large heat storage in spring and summer and increased evaporation during the fall ice-free period as the warmer water surface exchanges both heat and mass with the cold overlying air. Evaporation operates over a period ranging from 5 to 6 months.
Lakes of all sizes have the highest evaporation rates of any high latitude surface. Higher temperature and greater solar radiation can increase annual evaporation from a shallow lake by more than 50% and from very large lakes by more than 30%. Shallow lakes warm quickly in spring and have very high evaporation rates until they freeze in the fall. Large deep lakes take substantial periods to warm, but stay thawed into early winter, and their total evaporation amounts are significantly greater. Intermediate-sized lakes display characteristics that lie between these extremes. The frequency and size of lakes greatly influence the magnitude and timing of landscape-scale evaporative and sensible heat inputs to the atmosphere and are important to regional climatic and meteorological processes.
While the soil type varies considerably over this vast basin, its most noticeably soil characteristic is the abundance of organic soil (mostly peat lands) in the region (Figs. 2.7 and 2.8). The 1 km2 resolution soil texture data presented in Figs 2.7 and 2,8 are derived from the Soil Landscape of Canada (SLC) version 2.1 soil data and are vertically mapped to the soil layers of the CLASS land surface model. These soil data were used in the specifications of the CRCM runs described later.
The Mackenzie river basin comprises many different types of vegetation as Figure 2.9 shows. Vegetation cover varies from the farm and cold temperate grassland in the south, through boreal forests and subarctic woodlands to the arctic tundra in the north (Fig. 2.9). In terms of areal coverage, coniferous and transitional forests cover almost 50% of the basin. Various shrub-lands cover roughly 30% of the region and tundra covers about 4 % of the basin’s surface area that is located mainly in the northeastern-most part of the region. The remaining area of the basin is covered largely by various water bodies and wetlands.
The types of vegetation present in the Mackenzie river basin are outlined in Table 2.2. The Canada Center for Remote Sensing (CCRS) CCRS-2 classification scheme is used, as it was found to be the most accurate by the MAGS community to represent the Mackenzie river basin.
|Type of vegetation||Percentage|
|Mixed Wood Forest||0.5%|
|Ice and snow||0.3%|
The large-scale atmospheric conditions that control the transport of water and energy into the MRB during the winter and summer as derived from the NCEP reanalysis dataset are presented in Fig. 2.10. It is well known that the net poleward transport of water and energy in the atmosphere is accomplished by the Hadley circulation in the tropics and by transient eddies and stationary eddies that are associated with cyclones and meandering planetary waves in the mid- and high latitudes. As a vast high-latitude continental region, the MRB thus plays the role as one of the major heat sink regions in the global climate system, and the atmospheric transport of water and energy into and through the MRB is determined by both extra-tropical cyclone activities and planetary wave motions that affect the region.
The propagating synoptic disturbances that occur over the N. Pacific stormtrack dominate the weather of the North Pacific throughout the cool season from September through May, and the importance of these transient storm systems in the transport of water and energy into the MRB has been highlighted in several studies (e.g., Lackmann and Gyakum 1996; Smirnov and Moore 1999). In addition to the eddy transport by the transient systems, the southwesterly mean flow from the N. Pacific also plays a significant role in transporting the warm and moist air from the subtropical Pacific into the region.
The summer (June-August) mean flow over the N. Pacific is dominated by a quasi-stationary wave pattern forced by differential diabatic heating that occurs over oceanic and continental regions. The main climatological feature of interest is the low-level Pacific subtropical anticyclone that develops in response to the monsoonal heating over Asia and N. America. The mid-to-upper level large-scale wave structure is much weaker when compared to its cool-season counterpart. Consequently, the role of the upper level airflow in transporting heat and moisture into the region diminishes during the summer and the stationary eddy transport of moisture into the region from the Pacific is largely accomplished by the mean low-level southwesterly flow in the northern flank of the subtropical anticyclone. Although the mean westerly still dominate the heat and moisture transports in the region during the summer, meridional transports also play a non-negligible role in affecting the warm-season water and energy budgets of the basin.