Claire J. Oswald

The Role of Northern Lakes in a Regional Energy Balance

There are many lakes of widely varying morphometry in northern latitudes. For this study region, in the central Mackenzie River valley of western Canada, lakes make up 37% of the landscape. The nonlake components of the landscape are divided into uplands (55%) and wetlands (8%). With such abundance, lakes are important features that can influence the regional climate. This paper examines the role of lakes in the regional surface energy and water balance and evaluates the links to the frequency–size distribution of lakes. The primary purpose is to examine how the surface energy balance may influence regional climate and weather. Lakes are characterized by both the magnitude and temporal behavior of their surface energy balances during the ice-free period. The impacts of combinations of various-size lakes and land–lake distributions on regional energy balances and evaporation cycles are presented. Net radiation is substantially greater over all water-dominated surfaces compared with uplands. The seasonal heat storage increases with lake size. Medium and large lakes are slow to warm in summer. Their large cumulative heat storage, near summer’s end, fuels large convective heat fluxes in fall and early winter. The evaporation season for upland, wetland, and small, medium, and large lakes lasts for 19, 21, 22, 24, and 30 weeks, respectively. The regional effects of combinations of surface types are derived. The region is initially treated as comprising uplands only. The influences of wetland, small, medium, and large lakes are added sequentially, to build up to the energy budget of the actual landscape. The addition of lakes increases the regional net radiation, the maximum regional subsurface heat storage, and evaporation substantially. Evaporation decreases slightly in the first half of the season but experiences a large enhancement in the second half. The sensible heat flux is reduced substantially in the first half of the season, but changes little in the second half. For energy budget modeling the representation of lake size is important. Net radiation is fairly independent of size. An equal area of medium and large lakes, compared with small lakes, yields substantially larger latent heat fluxes and lesser sensible heat fluxes. Lake size also creates large differences in regional flux magnitudes, especially in the spring and fall periods.

Thermal Characteristics and Energy Balance of Various-Size Canadian Shield Lakes in the Mackenzie River Basin

This study addresses the thermal and energy budget characteristics of four different-size Canadian Shield lakes in the Mackenzie River basin during the ice-free season of 2000. The objectives are to characterize and compare the surface temperature and thermal structures, and to quantify the magnitudes and flux rates of the energy balance components of each lake. This study highlights the variability in thermal and energy balance characteristics arising from differences in mean lake depth and surface area. The lakes exhibit similar temporal patterns for air temperature, net radiation, wind speed, and wind direction. Net radiation and wind speed are highest over the largest lake, Great Slave Lake, due to colder surface temperatures and lengthy across-lake wind fetch, respectively. During the warming phase of the summer, surface temperature is inversely related to mean depth; however, during the cooling phase this relationship reverses. The shallowest of the four lakes remains isothermal during the entire ice-free period, while the three larger and deeper lakes are all dimictic. A lag in the onset of thermal stratification in the dimictic lakes is positively correlated with mean depth and surface area. Large evaporative water losses correspond to periods of low net radiation and cold dry air over Great Slave Lake. However, over the smaller, shallower lakes periods of high evaporation occur on days with high net radiation and warm, dry air. The capacity of larger lakes to store more heat results in longer ice-free periods and higher evaporation. Maximum heat content increases and occurs later for lakes of greater depth. Maximum evaporative rates occur later and cumulative evaporation is highest for lakes of greater depth and area. The ratios of total open water evaporation for the four lakes in order of size (smallest 5 1.0) are 1.0 : 1.2 : 1.3 : 1.4. Evaporation magnitudes are discussed in the context of other temperate and high-latitude lake studies.

An Investigation of the Thermal and Energy Balance Regimes of Great Slave and Great Bear Lakes

Great Slave Lake and Great Bear Lake have large surface areas, water volumes, and high latitudinal positions; are cold and deep; and are subject to short daylight periods in winter and long ones in summer. They are dissimilar hydrologically. Great Slave Lake is part of the Mackenzie Basin flowthrough system. Great Bear Lake is hydrologically isolated in its own relatively small drainage basin and all of its inflow and outflow derive from its immediate watershed. Great Slave Lake’s outflow into the Mackenzie River is more than 8 times that from Great Bear Lake. Input from the south via the Slave River provides 82% of this outflow volume. These hydrological differences exert pronounced effects on the thermodynamics, hydrodynamics, and surface climates of each lake. The quantitative results in this study are based on limited datasets from different years that are normalized to allow comparison between the two lakes. They indicate that both lakes have regional annual air temperatures within 2°C of one another, but Great Slave Lake exhibits a much longer open-water period with higher temperatures than Great Bear Lake. During the period when the lakes are warming, each lake exerts a substantial overlake atmospheric cooling, and in the period when the lakes are cooling, each exerts a strong overlake warming. This local cooling and warming cycle is greatest over Great Bear Lake. Temperature and humidity inversions are frequent early in the lake-warming season and very strong lapse gradients occur late in the lake-cooling season. Annually, for both lakes, early ice breakup is matched with late freeze-up. Conversely, late breakup is matched with early freeze-up. The magnitudes of midlake latent heat fluxes (evaporation) and sensible heat fluxes from Great Slave Lake are substantially larger than those from Great Bear Lake during their respective open-water periods. The hypothesis that because they are both large and deep, and are located in high latitudes, Great Slave Lake and Great Bear Lake will exhibit similar surface and near-surface climates that are typical of large lakes in the high latitudes proves invalid because their different hydrological systems impose very different thermodynamic regimes on the two lakes. Additionally, such an extensive north-flowing river system as the Mackenzie is subjected to latitudinally variable meteorological regimes that will differentially influence the hydrology and energy balance of these large lakes. Great Slave Lake is very responsive to climatic variability because of the relation between lake ice and absorbed solar radiation in the high sun season and we expect that Great Bear Lake will be affected in a similar fashion.

Energy Budget Processes of a Small Northern Lake

Abstract There is a paucity of information on the energy budget of Canadas northern lakes. This research determines processes controlling the magnitude of energy fluxes between a small Canadian Shield lake and the atmosphere. Meteorological instruments were deployed on a floating platform in the middle of a 5-ha lake during the 1999 and 2000 open-water seasons. High attenuation of incoming radiation at shallow depths and the sheltered location of the lake allows a strong thermocline to develop during the summer months, which prevents deeper water from exchanging energy with the atmosphere. Only after the lake becomes isothermal in late August do deeper waters interact with the atmosphere. When the lake is warming, evaporation is controlled by net radiation, but when the lake is cooling, turbulent energy fluxes are mainly influenced by the vapor pressure deficit. An empirically derived logarithmic relationship was identified between the Bowen ratio and the vapor pressure deficit. The Canadian Global Energ...

The Influence of Lakes on the Regional Energy and Water Balance of the Central Mackenzie River Basin

The goal of this study is to define the role of lakes in the energy and water cycling of the lake-rich central Mackenzie River Basin and discuss the impacts of climate variability on the regional terrestrial water balance. This is pursued by synthesizing the results of measured data on a regional scale. Our results indicate that lake-rich regions are high energy landscapes in the thaw season. Lakes have larger net radiation and much larger water vapor fluxes and smaller sensible heat fluxes than their terrestrial surroundings. Energy exchange with the atmosphere is dominated by the annual evaporative heat flux. The presence of large lakes in a region substantially decreases the interannual variability in evaporation totals. A hypothetical region with no lakes shows a positive annual terrestrial water balance for wet and average precipitation years and only a small negative water balance for the driest years. The existing lake-rich region has a positive annual water balance only in the wettest years. Comparisons of the region with all small, all medium or all large lake scenarios indicates increased regional evaporation of 8% and 10% respectively for the latter two scenarios. Basin evaporation is a significant source for precipitation within the Mackenzie River Basin during the summer. It is hypothesized that fall and early winter evaporation from medium and large lakes enhances downwind snowfall. In response to climate warming, lake-rich high latitude basins will witness substantially increased annual evaporation.

Climate-Lake Interactions

Lakes cover an estimated 11% of the Mackenzie River Basin (MRB) and they are an integral part of the Basin’s energy and water cycling regime. For convenience they are classified into small, medium and large in terms of surface area. MRB lakes are subjected to a wide range in air temperature from south to north, and to substantial differences in precipitation. The ice-covered period is a function of lake size and location. Small lakes have a longer ice-covered period (6–9 months) than large lakes (4–7 months) and northern lakes have a longer icecovered period than their southern counterparts. Lake ice duration and thickness is strongly influenced by both air temperature and overlying snow depths. During the open water season, for small shallow lakes, the seasonality of convective fluxes is similar to the surrounding land surfaces, but for medium and large lakes there is a large time lag. Their total seasonal evaporation is significantly greater than for terrestrial surfaces and their considerable heat storage capacity accounts for the temporal lag in energy and water cycling. A large lake such as the Great Slave Lake is highly sensitive to interannual climate variability and achieves substantially greater heat storage, higher temperatures and greater evaporative and sensible heat fluxes during a warmer year than during an average year. Instead of exhibiting strong diurnal evaporation cycles, its thermal and evaporative behavior is dominated by synoptic systems that approach a three-day cycle. Mass transfer methods of calculating evaporation works well, but are specific to size of the lake and its exposure to atmospheric forcing. Slab models employed to describe the energy cycles of different size lakes have met with some success. Lakes of all sizes are strongly impacted by climate variability and change and this has large influences on the regional hydrologic regimes.

Fate and transport of ambient mercury and applied mercury isotope in terrestrial upland soils: insights from the METAALICUS watershed.

The fate of mercury (Hg) deposited on forested upland soils depends on a wide array of biogeochemical and hydrological processes occurring in the soil landscape. In this study, Hg in soil, soilwater, and streamwater were measured across a forested upland subcatchment of the METAALICUS watershed in northwestern Ontario, Canada, where a stable Hg isotope (spike Hg) was applied to distinguish newly deposited Hg from Hg already resident in the watershed (ambient Hg). In total, we were able to account for 45% of the total mass of spike Hg applied to the subcatchment during the entire loading phase of the experiment, with approximately 22% of the total mass applied now residing in the top 15 cm of the mineral soil layer. Decreasing spike Hg/ambient Hg ratios with depth in the soil and soilwater suggest that spike Hg is less mobile than ambient Hg over shorter time scales. However, the transport of spike Hg into the mineral soil layer is enhanced in depressional areas where water table fluctuation is more extreme. While we expect that this pool of Hg is now effectively sequestered in the mineral horizon, future disturbance of the soil profile could remobilize this stored Hg in runoff.

Antecedent moisture conditions control mercury and dissolved organic carbon concentration dynamics in a boreal headwater catchment

The fate and transport of mercury (Hg) deposited on forested upland soils depends on the biogeochemical and hydrological processes occurring in the soil landscape. In this study, total Hg (THg) and dissolved organic carbon (DOC) concentrations were measured in streamwater from a 7.75 ha upland subcatchment of the METAALICUS watershed in northwestern Ontario, Canada. THg and DOC concentration-discharge relationships were examined at the seasonal-scale and event-scale to assess the role of antecedent moisture conditions on the mobilization of these solutes to receiving waters. At the seasonal-scale, subcatchment discharge poorly explained THg and DOC concentration dynamics; however, the inclusion of antecedent water storage and precipitation metrics in a multiple regression model improved the prediction of THg and DOC concentrations significantly. At the event-scale, a comparison of THg and DOC concentrations for two small summer storms with similar total discharge showed that the storm following the wet snowmelt period had a significantly lower total flux of THg and DOC than the storm following warm and dry conditions in late summer due to a distinct shift in the concentration-discharge relationship. Measurements of soil water and groundwater THg and DOC concentrations, as well as a three-component mixing analysis, suggest that there was an accumulation of potentially-mobile DOC-bound THg in the well-humified organic soil layer in the catchment during the warm and dry summer period and that as the catchment became wetter in the autumn, there was an increase in soil water THg and DOC concentrations and these solutes were subsequently flushed during the autumn storm.

Modeling Lake Energy Fluxes in the Mackenzie River Basin using Bulk Aerodynamic Mass Transfer Theory

Multiple years of micrometeorological and energy flux measurements for four Canadian Shield lakes were used to develop bulk aerodynamic mass transfer coefficients (C D ) for each lake and for groups of lakes. Transfer coefficients determined from multiple years of data for the two smallest lakes were similar (1.26 x 10 -3 and 1.30 x 10 -3 ) while that for the largest lake was slightly smaller (1.10 x 10 -3 ). The coefficient for the medium-size lake was erroneously high (2.14 x 10 -3 ) likely due to generalizations in the calculation of heat storage. No strong relationships were found between the coefficient values and morphometric parameters. The linear regression comparison of measured and modeled daily fluxes using multi-year coefficients gave an average r2 of 0.78. The same coefficients performed the best at estimating cumulative latent and sensible heat loss. Absolute percent errors suggest that the multi-year coefficients give acceptable results for small and medium-size lakes only, and that these coefficients cannot be transferred from one lake to another, unless the lakes are similar in size.

Spatial distribution and extent of urban land cover control watershed-scale chloride retention

In some cold regions up to 97% of the chloride (Cl-) entering rivers and lakes is derived from road salts that are applied to impervious surfaces to maintain safe winter travel conditions. While a portion of the Cl- applied as road salt is quickly flushed into streams during melt events via overland flow and flow through storm sewer pipes, the remainder enters the subsurface. Previous studies of individual watersheds have shown that between 28 and 77% of the applied Cl- is retained on an annual basis, however a systematic evaluation of the spatial variability in Cl- retention and potential driving factors has not been carried out. Here we used a mass balance approach to estimate annual Cl- retention in 11 watersheds located in southern Ontario, Canada, which span a gradient of urbanization. We evaluated the influence of multiple landscape variables on the magnitude of Cl- retention as well as the long-term rate of change in stream Cl-concentration for the same systems. We found that mean annual Cl- retention ranged from 40 to 90% and was higher for less urbanized watersheds and for watersheds with urban areas located farther from the stream outlet. This result suggests that less urbanized watersheds and ones with longer flow pathways have more Cl- partitioned into storage and hence the potential for legacy Cl- effects on aquatic organisms. While we did measure statistically significant increasing trends in stream Cl- concentration in some watersheds, there was no consistent relationship between the long-term rate of change in stream Cl- concentrations and patterns of urbanization and the magnitude of Cl- retention. Based on our results we present a detailed conceptual model of watershed Cl- dynamics that can be used to guide future research into the mechanisms of Cl- retention and release within a watershed.