Has streamflow changed in the Nordic countries? – Recent trends and comparisons to hydrological projections

SummaryA pan-Nordic dataset of 151 streamflow records was analysed to detect spatial and temporal changes in streamflow. Prior to undertaking analyses, all streamflow records with significant levels of autocorrelation were pre-whitened to remove the adverse effect of temporal autocorrelation on the test results. The Mann–Kendall trend test was applied to study changes in annual and seasonal streamflow as well as floods and droughts for three periods: 1920–2005, 1941–2005 and 1961–2000. Field significance was evaluated to determine the percentage of stations that are expected to show a trend due to the effect of cross-correlation. The period analysed and the selection of stations influenced the regional patterns found, but the overall picture was that trends of increased streamflow dominate annual values and the winter and spring seasons. Trends identified in summer flows differed between the three periods analysed, whereas no trend was found for the autumn season. In all three periods, a signal towards earlier snowmelt floods was clear, as was the tendency towards more severe summer droughts in southern and eastern Norway. These trends in streamflow result from changes in both temperature and precipitation, but the temperature induced signal is stronger than precipitation influences. This is evident because the observed trends in winter and spring, where snowmelt is the dominant process, are greater than the annual trends. A qualitative comparison of the findings with available streamflow projections for the region showed that the strongest trends found are generally consistent with future changes expected in the projection periods, for example increased winter discharge and earlier snowmelt floods. However, there are predicted changes that are not reflected in past trends, such as the expected increase in autumn discharge in Norway. Hence, the changes expected because of increased temperatures are reflected in the observed trends, whereas changes anticipated due to increases in precipitation are not.