Summary This paper summarizes and analyses available data on the surface energy balance of Arctic tundra and boreal forest. The complex interactions between ecosystems and their surface energy balance are also examined, including climatically induced shifts in ecosystem type that might amplify or reduce the effects of potential climatic change. High latitudes are characterized by large annual changes in solar input. Albedo decreases strongly from winter, when the surface is snow‐covered, to summer, especially in nonforested regions such as Arctic tundra and boreal wetlands. Evapotranspiration (QE) of high‐latitude ecosystems is less than from a freely evaporating surface and decreases late in the season, when soil moisture declines, indicating stomatal control over QE, particularly in evergreen forests. Evergreen conifer forests have a canopy conductance half that of deciduous forests and consequently lower QE and higher sensible heat flux (QH). There is a broad overlap in energy partitioning between Arctic and boreal ecosystems, although Arctic ecosystems and light taiga generally have higher ground heat flux because there is less leaf and stem area to shade the ground surface, and the thermal gradient from the surface to permafrost is steeper. Permafrost creates a strong heat sink in summer that reduces surface temperature and therefore heat flux to the atmosphere. Loss of permafrost would therefore amplify climatic warming. If warming caused an increase in productivity and leaf area, or fire caused a shift from evergreen to deciduous forest, this would increase QE and reduce QH. Potential future shifts in vegetation would have varying climate feedbacks, with largest effects caused by shifts from boreal conifer to shrubland or deciduous forest (or vice versa) and from Arctic coastal to wet tundra. An increase of logging activity in the boreal forests appears to reduce QE by roughly 50% with little change in QH, while the ground heat flux is strongly enhanced.

Region 2 comprises arctic and subarctic North America and is underlain by continuous or discontinuous permafrost. Its freshwater systems are dominated by a low energy environment and cold region processes. Central northern areas are almost totally in¯uenced by arctic air masses while Paci®c air becomes more prominent in the west, Atlantic air in the east and southern air masses at the lower latitudes. Air mass changes will play an important role in precipitation changes associated with climate warming. The snow season in the region is prolonged resulting in long-term storage of water so that the spring¯ood is often the major hydrological event of the year, even though, annual rainfall usually exceeds annual snowfall. The unique character of ponds and lakes is a result of the long frozen period, which aects nutrient status and gas exchange during the cold season and during thaw. GCM models are in close agreement for this region and predict temperature increases as large as 48C in summer and 98C in winter for a 2 Â CO 2 scenario. Palaeoclimate indicators support the probability that substantial temperature increases have occurred previously during the Holocene. The historical record indicates a temperature increase of 418C in parts of the region during the last century. GCM predictions of precipitation change indicate an increase, but there is little agreement amongst the various models on regional disposition or magnitude. Precipitation change is as important as temperature change in determining the water balance. The water balance is critical to every aspect of hydrology and limnology in the far north. Permafrost close to the surface plays a major role in freshwater systems because it often maintains lakes and wetlands above an impermeable frost table, which limits the water storage capabilities of the subsurface. Thawing associated with climate change would, particularly in areas of massive ice, stimulate landscape changes, which can aect every aspect of the environment. The normal spring¯ooding of ice-jammed north-¯owing rivers, such as the Mackenzie, is a major event, which renews the water supply of lakes in delta regions and which determines the availability of habitat for aquatic organisms. Climate warming or river damming and diversion would probably lead to the complete drying of many delta lakes. Climate warming would also change the characteristics of ponds that presently freeze to the bottom and result in fundamental changes in their limnological characteristics. At present, the food chain is rather simple usually culminating in lake trout or arctic char. A lengthening of the growing season and warmer water temperature would aect the chemical, mineral and nutrient status of lakes and most likely have deleterious eects on the food chain. Peatlands are extensive in region 2. They would move northwards at their southern boundaries, and, with sustained drying, many would change form or become inactive. Extensive wetlands and peatlands are an important component of the global carbon budget, and warm...

Abstract:A one-dimensional thermodynamic model for simulating lake-ice phenology is presented and evaluated. The model can be driven with observed daily or hourly atmospheric forcing of air temperature, relative humidity, wind speed, cloud amount and snowfall. In addition to computing the energy balance components, key model output includes the temperature profile at an arbitrary number of levels within the ice/snow (or the water temperature if there is no ice) and ice thickness (clear ice and snow-ice) on a daily basis, as well as freeze-up and break-up dates. The lake-ice model is used to simulate ice-growth processes on shallow lakes in arctic, sub-arctic, and high-boreal forest environments. Model output is compared with field and remote sensing observations gathered over several ice seasons. Simulated ice thickness, including snow-ice formation, compares favourably with field measurements. Ice-on and ice-off dates are also well simulated when compared with field and satellite observations, with a mean absolute difference of 2 days. Model simulations and observations illustrate the key role that snow cover plays on the seasonal evolution of ice thickness and the timing of spring break-up. It is also shown that lake morphometry, depth in particular, is a determinant of ice-off dates for shallow lakes at high latitudes.

Abstract. The first direct measurements of evaporation from a large high-latitude lake, Great Slave Lake, Northwest Territories, Canada, were made using eddy covariance between July 24 and September 10, 1997, and June 22 and September 26, 1998. The main body of the lake was ice-free between June 20 and December 13, 1997, and June 1, 1998, and January 8, 1999, with the extended ice-free season in 1997-1998 coinciding with 4øC above normal air temperatures and an abnormally strong E1 Nifio. Measurements extending roughly 5.0 to 8.5 km across the lake were made from a small rock outcrop located near the main body of the lake. The lake was thermally stratified between midJuly and September, with the thermocline extending down to approximately 15 m. High winds were effective in mixing warm surface waters downward and, when accompanied by cold fronts, resulted in large, episodic evaporation events typically lasting 45 hours. The daily total evaporation was best described as a function of the product of the horizontal wind speed and vapor pressure difference between the water surface and atmosphere. Seasonally, the latent heat flux was initially negative (directed toward the surface) followed by a steady increase to positive values (directed away from the surface) shortly after ice breakup. The latent heat flux then remained positive for the remainder of the ice-free period, decreasing midsummer and then steadily increasing until freeze-up. The sensible heat flux was small and often negative most of the spring and summer yet switched to positive and began to increase in the early fall. Extrapolation of evaporation measurements for the entire ice-free periods gave totals of 386 and 485 mm in 1997 and 1998-1999, respectively.

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.

The Mackenzie River is the largest North American source of freshwater for the Arctic Ocean. This basin is subjected to wide fluctuations in its climate and it is currently experiencing a pronounced warming trend. As a major Canadian contribution to the Global Energy and Water Cycle Experiment (GEWEX), the Mackenzie GEWEX Study (MAGS) is focusing on understanding and modeling the fluxes and reservoirs governing the flow of water and energy into and through the climate system of the Mackenzie River Basin. MAGS necessarily involves research into many atmospheric, land surface, and hydrological issues associated with cold climate systems. The overall objectives and scope of MAGS will be presented in this article.

This paper presents an empirical model of net ecosystem CO2 exchange (NEE) developed for a subarctic fen near Churchill, Manitoba. The model with observed data helps explain the interannual variability in growing season NEE. Five years of tower‐flux data are used to test and examine the seasonal behaviour of the model simulations. Processes controlling the observed interannual variability of CO2 exchange at the fen are examined by exploring the sensitivity of the model to changes in air temperature, precipitation and leaf area index. Results indicate that the sensitivity of NEE to changing environmental controls is complex and varies interannually depending on the initial conditions of the wetland. Changes in air temperature and the timing of precipitation events have a strong influence on NEE, which is largely manifest in gross ecosystem photosynthesis (GEP). Climate change scenarios indicate that warmer air temperatures will increase carbon acquisition during wet years but may act to reduce wetland carbon storage in years that experience a large water deficit early in the growing season. Model simulations for this subarctic sedge fen indicate that carbon acquisition is greatest during wet and warm conditions. This suggests therefore that carbon accumulation was greatest at this subarctic fen during its early developmental stages when hydroclimatic conditions were relatively wet and warm at approximately 2500 years before present.

Summary This paper examines, with examples, controls on the energy and water balance of northern wetlands. Most wetlands have organic soils and are thus peatlands. High‐latitude wetlands are underlain by ice‐rich permafrost, which helps maintain wetland systems and also imparts special characteristics to their energy and water balances. In North America, components of the radiation balance decrease linearly poleward, whereas the poleward rate of decrease of temperature and precipitation lessens. During the four‐month summer of a high subarctic wetland, net radiation is large and the latent heat flux dominates the energy cycle. The ground heat flux is substantial, especially in early summer, when the ice‐rich ground is rapidly thawing. Winter begins in October and heat loss from the ground approximately balances negative net radiation. The summer energy and water balance differs among terrain units. Large shallow lakes exhibit larger net radiation and potential evaporation rates than surrounding wetland surfaces which, in turn, exhibit substantially larger magnitudes than dryland terrain. There is a variable withdrawal rate of soil moisture depending on soil types and plant rooting characteristics, which influences the actual evaporation from the surface. Synoptic weather systems play a major role in day‐to‐day energy and water responses to climate forcing. Long‐term modelling of the water balance of a wetland shows year‐to‐year persistence in climatic patterns. Although net radiation, temperature and precipitation all influence the magnitudes of water deficit, the precipitation inputs are of paramount importance. Our ability to fully understand, model and extrapolate, in space and time, the major controls on the surface climate of wetlands, is evaluated. Spatial extrapolation is seen to be more readily achieved than temporal extrapolation.