Hydrology and Earth System Sciences www.hydrol-earth-syst-sci.net 1027-5606 1607-7938 9 6 2005 10.5194/hess-9-657-2005 http://www.hydrol-earth-syst-sci.net/9/657/2005/ http://www.hydrol-earth-syst-sci.net/9/657/2005/hess-9-657-2005.html http://www.hydrol-earth-syst-sci.net/9/657/2005/hess-9-657-2005.pdf 657 674 2005-12-31 Water and nitrogen processes along a typical water flowpath and streamwater exports from a forested catchment and changes after clear-cutting: a modelling study A. Laurén L. Finér H. Koivusalo T. Kokkonen T. Karvonen S. Kellomäki H. Mannerkoski M. Ahtiainen Finnish Forest Research Institute, Joensuu Research Centre, Yliopistokatu 6, P.O. Box 68 FIN-80101 Joensuu, Finnland Laboratory of Water Resources, Helsinki University of Technology, P.O. Box 5300 FIN-02015 TKK, Finnland University of Joensuu, Faculty of Forestry, P.O. Box 111, FIN-80101 Joensuu, Finnland North Karelia Regional Environment Centre, P.O. Box 69 FIN-80101 Joensuu, Finnland A two dimensional model, FEMMA, to describe water and nitrogen (N) fluxes within and from a forested first-order catchment (Kangasvaara in Eastern Finland) was constructed by linking the most significant processes affecting the fluxes of water, ammonium, nitrate and dissolved organic nitrogen along a hillslope from the water divide to the stream. The hillslope represents the average flowpath of water in the catchment and the model was used to estimate the N fluxes for a catchment in eastern Finland before and after clear-cutting. The simulated results were in reasonable agreement with the nitrate, dissolved organic N and dissolved total N measurements from the study catchment and with other results in the literature. According to the simulations, the major sinks of N after clear-cutting were immobilisation by soil microbes, uptake by ground vegetation and sorption to soil. These sinks increased downslope from the clear-cut area, indicating the importance of an uncut buffer zone between the stream and the clear-cut area in reducing N exports. The buffer zone retained 76% of the N flux coming from the clear-cut area. Nitrification was a key process in controlling the N export after clear-cutting and N increases were mainly as nitrate. Most of the annual N export took place during the spring flood, when uptake of N by plants was minimal.