Modeling combined effects of forestry, agriculture and deposition on nitrogen export in a northern river basin in Finland.

The development of the spatial N export and retention model N_EXRET for large river basins is presented, utilizing remote-sensing-based land use and forest classification. Export coefficients describing the contribution from agriculture, forestry and peat harvesting were estimated based on empirical studies. Representativeness of forest treatment coefficients have been evaluated by use of data from a small, well-documented test catchment. Simulation results from the application of the model to the Oulujoki river basin (22,840 km2) are discussed. Model estimated N fluxes were compared with measured N fluxes in separate points of the river basin. Based on source apportionment, agriculture contributes 17% of the total export, varying between 8% in the uppermost subbasin and 38% in the lowermost subbasin close to the sea. Forestry contributes almost as much, 16%, with less pronounced variation (11-24%) between the different subbasins. In the separate subbasins, 7-37% of the incoming gross N export was retained. Based on mass balances and sensitivity analyses, retention was estimated to be in the range of 5-10 kg ha-1 yr-1 in lakes and 0-1 kg ha-1 yr-1 in peatlands. The model results were validated by testing to data from the 3 closely situated river basins. However, further testing is needed in river basins where land use and deposition patterns differ clearly from these northern basins.