Analysis of Inorganic Nitrogen Leaching in a Boreal River Basin in Northern Finland

In this study the dynamic， semi-distributed INCA-N model was applied to the boreal Simojoki river basin in northern Finland to outline inorganic nitrogen (N) leaching patterns and N processes in catchment scale. Special emphasis was paid to the quality assurance of the modelling work. The dominant human impacts in the area are forestry， agriculture， scattered settlement and atmospheric deposition. In order to assess the effectiveness of current environmental policies and to implement river basin management plans， it is essential to know the relative significance of the different sources of pollution. INCA-N explained main features of the hydrological pattern and seasonality of inorganic N concentrations in river water when N processes in soil in sub-zero temperatures were included. Over-winter N mineralization processes in soil accounted for 38% of annual N mineralization. The lowest concentrations during the growing season were not reproduced， which indicates that there are some retention processes missing from the model. As summer is typically a low flow period the simulation results are reliable as long as the interpretation is based on daily or annual loads. Loading from the river basin was mostly dependent on annual hydrology and it was concentrated to peaks during the snow melting period. In the upper parts of the river inorganic N load originated mainly from commercial forests. At the outlet of the river anthropogenic sources accounted for more than half of the overall inorganic N load， with agriculture， forestry and scattered settlements making almost equal contributions. Expected changes in atmospheric N deposition would not have any significant effect but the combination of water protection measures both in agricultural and forestry areas and in scattered settlement areas would decrease inorganic N load by 18% of the total load. The expected increase in forest felling of 20% would not have an influence， but increase in agricultural land due to the EU's Common Agricultural Policy might lead to increased N load to the sea. (ISSN 1456-2596; <isbn 951-22-8064-7; <isbn 951-22-8076-0(pdf); Helsinki University of Technology Water Resources Publications TKK-VTR-13; 65p.)