Calculations of yearly averages regarding nitrogen leaching, reduction in rivers and lakes, net transport to the sea and source apportionment for the south of Sweden are presented for the period 1985-1994. The calculation are based on daily catchment modelling in 3 725 subbasins with the conceptual HBV-N model that has been calibrated in 722 sites against measured time-series. Two input data sets have been used: one with the land use and leaching concentrations related to year 1985-87 and another with the values for 1994. Some conclusions from the study:
- The nitrogen reduction from the source to the sea is about 61 000 ton a year ( 45 % of the gross load) (1985-94), which leaves about 75 000 ton to the annual mean net transport to the sea.
- The annual reduction varies spatially from 0-100 % in the area studied and it varied in time up to 30 % between different years in single basins. Most of the the reduction occurred in lakes. Consequently, the highest reduction was found for areas upstreams large lake systems and thelowest reduction near the coast where there are few lakes.
- The source apportionment showed that the contribution from arable land dominated ( 45 % ), but contribution from sewage treatment plants are important (21 % ) as well as the forest leakage (8-14 %) and the atmospheric deposition on lake surfaces (11 %).
- The simulation of the total net nitrogen transport to the sea showed a reduction of 7 % between1985 and 1994 due to changes in the agricultural sector.