IMPACT OF CLIMATE CHANGEON THE WATER CYCLE AND NUTRIENT LOSSES IN A FINNISH CATCHMENT F. BOURAOUI 1 , B. GRIZZETTI 1 , K. GRANLUND 2 , S. REKOLAINEN 2 and G. BIDOGLIO 1 1 Joint Research Centre of the European Commission, Institute for Environment and Sustainability, Soil & Waste Unit, TP 460, I-21020 Ispra (VA), Italy E-mail: faycal.bouraoui@jrc.it 2 Finnish Environment Institute, P.O.Box 140, FIN-00251 Helsinki, Finland Abstract. Changes in climate, either long or short-term changes, can alter significantly the hydrolog- ical behavior of catchments. A statistical analysis of a thirty-four year time series of meteorological data collected in the Vantaanjoki watershed (Southern Finland) shows an increase in temperature and precipitation. The hydrological model SWAT was applied to the Vantaanjoki watershed in order to assess the impact of the measured transient climate change on the hydro-biogeochemical behavior of the catchment. The SWAT model was calibrated and validated for a period extending from 1965 to 1998. The model performance was evaluated comparing the measured and predicted time series for flow, suspended solids, total nitrogen and total phosphorus at the watershed outlet. The model was then run for the same period with climatic data where the observed increase in temperature and precipitation was removed using non-parametric techniques. It was shown that the observed climate change was responsible for the decrease of the snow cover and increase of winter runoff. On an annual basis, small increases were noted in nutrient losses, however, with significant seasonal differences. Globally, the observed climate change was responsible for an increased contribution of diffuse nutrient losses to the total nutrient load. 1. Introduction There is a general consensus that the Earth will be subject to warming with a re- ported surface air temperature increase by 0.8 ◦ C during the 20th century with large temporal and spatial variations for Europe (Beniston and Tol, 1998). IPCC (2001) reports that the global average temperature has increased by 0.6 ◦ C(±0.2 ◦ C) over the past century, with the most warming period extending from 1976 to 2000 and the 1990s ‘likely’ being the warmest decade since 1861. Using proxy data, it is ‘likely’ that the observed increase of temperature in the Northern Hemisphere is the highest of the last 1000 years (IPCC, 2001). Tuomenvirta (2001) analyzing data from more than 100 stations in Nordic countries reports the highest increase to be more than 1 ◦ C in 100 years, with a typical range of increase similar to that given by Jones et al. (1999) and being around 0.6 ◦ C. Climate changes are expected to alter significantly the hydrological behavior of catchments (Gleick, 1999; Arnell, 1999) modifying the nutrient transformation and transport characteristics (Murdoch et al., 2000). An increase in diffuse source pollutant loads and in nutrient cycling are Climatic Change 66: 109–126, 2004. © 2004 Kluwer Academic Publishers. Printed in the Netherlands.