Modelling impacts of climate and deposition changes on nitrogen fluxes in northern catchments of Norway and Finland

Abstract. The Integrated Nitrogen model for Catchments (INCA) was applied to three upland catchments in Norway and Finland to assess the possible impacts of climate change and nitrogen (N) deposition on concentrations and fluxes of N in streamwater in cold regions of Europe. The study sites cover gradients in climate and N deposition from the southern boreal Øygard Brook (2.6 km²) in SW Norway, via the southern/middle boreal Simojoki River (3610 km²) in northern Finland to the sub-arctic Dalelva Brook (3.2 km²) in northern Norway. The INCA scenario simulations included future N deposition scenarios (current legislation and maximum feasible reduction) and climate scenarios for 2050 (ECHAM4/OPYC3; HadCM3) treated separately and in combination. As a result of climate change, the INCA model predicted markedly reduced duration and amounts of snow cover in all catchments. The occurrence of winter rainfall and melting periods was predicted to become more frequent so that more frequent floods in winter will to a large extent replace the regular snowmelt flood in spring. At the northernmost catchment, Dalelva, the predicted temperature increase might result in a doubling of the net mineralisation rate, thereby greatly increasing the amount of available inorganic N. At all catchments, the increased N supply was predicted to be largely balanced by a corresponding increase in N retention, and relatively small increases in NO₃^- leaching rates were predicted. This dynamic relationship is, however, strongly dependent on the temperature responses of the key N transformation processes modelled. A future reduction in N emissions and deposition, as agreed under current legislation, would have pronounced effects on concentrations of NO₃^- in streamwater at the southernmost catchment, Øygard, even following a climate change around 2050. At the more remote Dalelva and Simojoki catchments, the N emission reductions will be small compared to the internal N recycling processes, and climate change will to a large extent offset the effects of reduced N deposition.

Keywords: catchments, surface water, scenarios, climate, hydrology, nitrogen deposition, nitrate leaching