MODELING NITROGEN TRANSPORT AND RETENTION IN THE CATCHMENTS OF SOUTHERN SWEDEN

The Baltic Sea is suffering from eutrophication and attempts are being made to reduce nutrient loads. This article focuses on nitrogen trans port from southern Sweden (145 000 km(2)), and presents a model approa ch (HBV-N) that has been used in the national decision-making process for best management practices. Calculations of nitrogen leaching, rete ntion in the freshwater system, net transport to the sea, and source a pportionment are presented for the period 1985-1994. Input data were h andled in GIS, including results from SOIL-N and MATCH. Daily simulati ons were made in 3725 subbasins with calibration against measured time series at 722 sites. Diffuse source pollution was normally retained b y 10-25% before entering the river network. Lakes normally reduced nit rogen transport by 30-40 kg ha(-1) yr(-1) of lake area. On average, 45 % of the annual gross load was reduced during transport, but temporal and spatial variations were great. 75 000 tonnes N yr(-1) reached the sea.