Source apportionment of river substance transport, i.e. estimation of how much each source in each subbasin contributes to the river-mouth transport, is a vital step in achieving the most efficient management practices to reduce pollutant loads to the sea. In this study, the spatially lumped (at sub-catchment level), semi-empirical PULSE hydrological model, with a nitrogen routine coupled to it, was used to perform source apportionment of nitrogen transport in the Söderköpingsån river basin (882 km2) in south-eastern Sweden, for the period 1991–93. The river basin was divided into 28 subbasins and the following sources were considered: land leakage from the categories forest, arable and ley/pasture; point sources, and; atmospheric deposition on lake surfaces. The calibrated model yielded an explained variance of 60%, based on comparison of measured and modelled river nitrogen (Total N) concentrations. Eight subbasins, with net contributions to the river-mouth transport exceeding 3 kg ha−1 yr−1, were identified as the most promising candidates for cost efficient nitrogen management. The other 20 subbasins all had net contributions below 3 kg ha−1 yr−1. Arable land contributed 63% of the nitrogen transport at the river mouth and would thus be in focus for management measures. However, point sources (18% contribution to net transport) should also be considered due to their relatively high accessibility for removal measures (high concentrations). E.g., the most downstream subbasin, with the largest wastewater treatment plant in the whole river basin, had a net contribution of 16 kg ha−1 yr−1. This method for source apportionment may provide authorities with quantitative information about where in a river basin, and at which sources, they should focus their attention. However, once this is done, an analysis with higher resolution has to be performed in each of the interesting subbasins, before decisions on actual management measures can be taken.

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