Riverine nitrogen : Analysis and modelling under Nordic conditions
Sammanfattning: The Baltic Sea in northern Europe is a brackish water body where eutrophication is considered a serious problem, partly as a result of high nitrogen load from rivers. This thesis includes statistical analysis of nitrogen behaviour in 55 small catchments. The results were used for further development of a dynamic model (HBV-N). This model was then applied in case studies of southern Sweden, including dynamic modelling in 3725 subbasins (totalling 145,000 km2) using time-series from 722 sites. The riverine nitrogen transport to the Swedish coast was simulated. Nitrogen reduction measures within the agricultural sector undertaken during 1985-1994 were found to decrease the normalised load by 7%. Constructed wetlands were found to reduce the load by 16% if 1% of a drainage basin were converted into wetlands.The spatial and temporal variations in terrestrial nitrogen leakage were to a great extent explained by catchment characteristics and hydrometeorological conditions. More detailed studies are needed, however, to distinguish between the influences of the various underlying processes (e.g., flow composition and washout processes).Large-scale studies of nitrogen transport and measure-allocation strategies must consider transformation processes in the aquatic system. The net load was considerably lower than the gross load, and it was temporally and spatially highly variable depending on climate, landscape mosaic, flow path, and watercourses.The HBV-N model is shown to have good potential as a decision-support tool for the improvement of water-quality management and environmental planning under Nordic conditions. For more sophisticated retention modeling, empirical studies are needed to quantify the impact of the various retention processes in the aquatic system.
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