Hydrology and Earth System Sciences www.hydrol-earth-syst-sci.net 1027-5606 1607-7938 12 2 2008 10.5194/hess-12-393-2008 http://www.hydrol-earth-syst-sci.net/12/393/2008/ http://www.hydrol-earth-syst-sci.net/12/393/2008/hess-12-393-2008.html http://www.hydrol-earth-syst-sci.net/12/393/2008/hess-12-393-2008.pdf 393 403 2008-03-05 Winter climate affects long-term trends in stream water nitrate in acid-sensitive catchments in southern Norway H. A. de Wit heleen.de.wit@niva.no A. Hindar L. Hole Norwegian Institute for Water Research (NIVA), Gaustadalléen 21, 0349 Oslo, Norway Norwegian Institute for Air Research (NILU), P.O. Box 100, 2027 Kjeller, Norway Controls of stream water NO₃ in mountainous and forested catchments are not thoroughly understood. Long-term trends in stream water NO₃ are positive, neutral and negative, often apparently independent of trends in N deposition. Here, time series of NO₃ in four small acid-sensitive catchments in southern Norway were analysed in order to identify likely drivers of long-term changes in NO₃. In two sites, stream water NO₃ export declined ca 50% over a period of 25 years while in the other sites NO₃ export increased with roughly 20%. Discharge and N deposition alone were poor predictors of these trends. The most distinct trends in NO₃ were found in winter and spring. Empirical models explained between 45% and 61% of the variation in weekly concentrations of NO₃, and described both upward and downward seasonal trends tolerably well. Key explaining variables were snow depth, discharge, temperature and N deposition. All catchments showed reductions in snow depth and increases in winter discharge. In two inland catchments, located in moderate N deposition areas, these climatic changes appeared to drive the distinct decreases in winter and spring concentrations and fluxes of NO₃. 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