SOURCE APPORTIONMENT OF RIVERINE NITROGEN TRANSPORT BASED ON CATCHMENT MODELING

Source apportionment of river substance transport, i.e. estimation of how much each source in each subbasin contributes to the river-mouth t ransport is a vital step in achieving the most efficient management pr actices to reduce pollutant loads to the sea. In this study, the spati ally lumped (at sub-catchment level), semiempirical PULSE hydrological model, with a nitrogen routine coupled to if was used to perform sour ce apportionment of nitrogen transport in the Soderkopingsan river bas in (882 km(2)) in south-eastern Sweden, for the period 1991-93. The ri ver basin was divided into 28 subbasins and the following sources were considered: land leakage from the categories forest arable and ley/pa sture; point sources, and; atmospheric deposition on lake surfaces. Th e calibrated model yielded an explained variance of 60%, based on comp arison of measured and modelled river nitrogen (Total N) concentration s. Eight subbasins, with net contributions to the river-mouth transpor t exceeding 3 kg ha(-1) yr(-1), were identified as the most promising candidates for cost efficient nitrogen management. The other 20 subbas ins all had net contributions below 3 kg ha(-1) yr(-1). Arable land co ntributed 63% of the nitrogen transport at the river mouth and would t hus be in focus for management measures. However, point sources (18% c ontribution to net transport) should also be considered due to their r elatively high accessibility for removal measures (high concentrations ). E.g., the most downstream subbasin, with the largest wastewater tre atment plant in the whole river basin, had a net contribution of 16 kg ha(-1) yr(-1). This method for source apportionment may provide autho rities with quantitative information about where in a river basin, and at which sources, they should focus their attention. However, once th is is done, an analysis with higher resolution has to be performed in each of the interesting subbasins, before decisions on actual manageme nt measures can be taken. Copyright (C) 1996 IAWQ.