Abstract
The most significant changes in climate, due to the well-known enhanced greenhouse effect, are generally expected to occur at northern high latitudes. Sub-arctic environments, that are dominated by the presence of a seasonal snow cover, may therefore be particularly sensitive to global warming. The impact of human-induced climate changes on
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the hydrological system of sub-arctic environments was analysed in a series of studies, undertaken in the Tana River Basin in northernmost Finland and Norway. Central to the approach was a large-scale hydrological model of the study area, that has been coupled to a regional climate model. The hydrological model was based on a conceptual water balance model originally developed for the River Rhine. In this study, special attention was paid to the spatial distribution of snow coverage, snowmelt and evapotranspiration, in addition to river discharge. In order to be able to make proper estimates of the consequences of climate change, the empirical methods of estimating snowmelt and evapotranspiration in the water balance model were replaced by two physically-based models. In developing the hydrological model, field measurements were used that have been collected in a small catchment in Northern Finland. The model performance was evaluated by comparing the results with satellite observations of snow cover depletion in the Tana Basin, as well as observations on snow depth and river discharge. The improved, physically-based model version appeared to simulate the hydrological behaviour of the Tana Basin realistically. The impact of changes in climate was analysed by driving the hydrological model with climate change scenarios. For the Tana Basin, these scenarios imply that the mean annual temperature in the coming century may rise by more than 5 degrees Celsius, and precipitation may increase by 25 %. As a consequence, the snow-free season is extended by more than 30 days at higher elevations, to 70 days or more days close to the Barents Sea. Consequently, the amount of solar radiation that is received during the snow-free season, increases by about 16 %. This increase in the annual radiation sum may be of importance for the tundra vegetation, that consists mainly of low shrubs and heath. Due to the shorter snow season, the annual amount of sublimation decreases by 30 %, while the amount of evapotranspiration in summer increases by about 15 %. River discharge may increase as well, by almost 40 % on an annual basis. Changes in climate, as predicted for the coming century, will therefore have a significant impact on snow coverage, evapotranspiration and freshwater runoff in the Tana Basin. These changes will influence the ecological processes in the sub-arctic landscape. Due to changes in albedo and the amount of freshwater runoff towards the Arctic Ocean, changes in the hydrological cycle of sub-arctic areas may also provide a feedback to the global climate system. Understanding the hydrological processes operating in sub-arctic environments, and evaluating their sensitivity to climate change, is therefore an essential part of global change research.
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