[1] Regional variations in evaporation losses and water budget are interpreted from systematic isotopic patterns in surface waters across a 275,000 km 2 region of northern Canada. Differential heavy isotope enrichment in a set of >255 nonheadwater lakes sampled by floatplane during 1993 and 1994 is strongly correlated to varying hydroclimatic conditions across the region. Calculated catchment-weighted evaporation losses typically range from $10-15% in tundra areas draining into the Arctic Ocean to as high as 60% in forested subarctic areas draining to the Mackenzie River via Great Bear or Great Slave Lakes. Because of the diversity in drainage order and the ratio of catchment to surface area, lakes in the region may inherit as little as 30% to as much as 99% of their isotopic enrichment signal from upstream water bodies. Open-water evaporation generally decreases with increasing latitude and accounts for 5-50% of total evapotranspiration. Coupling of meteorological and isotopic data permits a novel assessment of regional evaporation-transpiration flux partitioning in the three major ecoclimatic zones (high-boreal forest, subarctic forest-tundra, and low-arctic shrub tundra), while the differing frequency distributions of lake water balance in these zones provides a new index of landscape-scale hydroclimatology that may have significant potential for investigating ongoing (or past) changes in response to high-latitude climate change.
Abstract:The oxygen-isotope composition of local precipitation (d 18 OP) is reconstructed from carbonate lakesediment components in a sediment core covering the last 10000 calendar years from Lake Tibetanus, a small, hydrologically open, groundwater-fed lake in the Abisko area, northern Sweden. Comparison of the d 18O P history with a pollen-based palaeotemperature record from the same core clearly reveals pronounced deviations from the normally expected temporal d 18 O P -temperature relation (so-called 'Dansgaard relation') that may be a function of changing oceanicity. The transition from relatively moist, maritime conditions in the early Holocene to a much drier climate after 6500 cal. BP is re ected by major changes in forest extent and composition as recorded by pollen and plant macrofossil data. At the time of maximum in uence of westerly air-mass circulation (high zonal index) c. 9500 cal. BP, brought about by high summer insolation and enhanced meridional pressure gradients, d 18 OP at Lake Tibetanus was about 2‰ higher than would be predicted by the modern isotope-temperature relation. The occurrence of long-term changes in d 18O P -temperature relations, which are more sensitive measures of palaeoclimate than either d 18O P or temperature alone, needs to be taken into account when extracting palaeoclimatic information from continental oxygen-isotope records.
Abstract:An overview of current research in isotope hydrology, focusing on recent Canadian contributions, is discussed under the headings: precipitation networks, hydrograph separation and groundwater studies, river basin hydrology, lake and catchment water balance, and isotope palaeohydrology from lake sediment records. Tracer-based techniques, relying primarily on the naturally occurring environmental isotopes, have been integrated into a range of hydrological and biogeochemical research programmes, as they effectively complement physical and chemical techniques. A significant geographic focus of Canadian isotope hydrology research has been on the Mackenzie River basin, forming contributions to programmes such as the Global Energy and Water Cycle Experiment. Canadian research has also directly supported international efforts such as the International Atomic Energy Agency's (IAEA) Global Network for Isotopes in Precipitation and IAEAs Coordinated Research Project on Large River Basins. One significant trend in Canadian research is toward sustained long-term monitoring of precipitation and river discharge to enable better characterization of spatial and temporal variability in isotope signatures and their underlying causes. One fundamental conclusion drawn from previous studies in Canada is that combined use of υ 18 O and υ 2 H enables the distinction of precipitation variability from evaporation effects, which offers significant advantages over use of the individual tracers alone. The study of hydrological controls on water chemistry is one emerging research trend that stems from the unique ability to integrate isotope sampling within both water quality and water quantity surveys.
[1] Global trends in the d 2 H-d 18O enrichment slope of continental lakes and shallow soil water undergoing natural evaporation are predicted on the basis of a steady state isotope balance model using basic monthly climate data (i.e., temperature and humidity), isotopes in precipitation data, and a simple equilibrium liquid-vapor model to estimate isotopes in atmospheric moisture. The approach, which demonstrates the extension of well-known conceptual models in stable isotope hydrology to the global scale, is intended to serve as a baseline reference for evaluating field-based isotope measurements of vapor, surface water, and soil water and as a diagnostic tool for more complex ecosystem models, including isotope-equipped climate models. Our simulations reproduce the observed local evaporation line slopes (4-5 range for lakes and 2-3 range for soil water) for South America, Africa, Australia, and Europe. A systematic increase in slopes (5-8 range for lakes) toward the high latitudes is also predicted for lakes and soil water in northern North America, Asia, and Antarctica illustrating a latitudinal (mainly seasonality-related) control on the evaporation signals that has not been widely reported. The over-riding control on the poleward steepening of the local evaporation lines is found to be the isotopic separation between evaporation-flux-weighted atmospheric moisture and annual precipitation, and to lesser extents temperature and humidity, all of which are influenced by enhanced seasonality in cold regions.
The stable isotope-mass balance method can provide useful water balance information in ungauged catchments. The method has been used to evaluate evaporation and water balance at two contrasting sites in northern Canada. Areally weighted evaporative discharge from an 850 km2 tundra catchment in south-central District of Keewatin is estimated to be about 7 % of total water discharge (≈ 16 mm/yr), compared to about 19 % (≈ 65 mm/yr) from a 300 km2 forested watershed in the Upper Mackenzie Valley. Lakes in both watersheds exhibit broad ranges of evaporation/inflow ratios related to local water balance. The potential errors in the estimates are evaluated through consideration of possible variations in basin storage, humidity, and the isotopic composition of atmospheric vapour.
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