Extreme heat and runoff extremes in the Swiss Alps

The hydrological response of Swiss river basins to the 2003 European summer heatwave was evaluated by a combined analysis of historical discharge records and specific applications of distributed hydrological modeling. In the summer of 2003, the discharge from headwater streams of the Swiss Central Plateau was only 40%-60% of the long-term average. For alpine basins runoff was about 60%-80% of the average. Glacierized basins showed the opposite behavior. According to the degree of glacierization, the average summer runoff was close or even above average. The hydrological model PREVAH was applied for the period 1982-2005. Even if the model was not calibrated for such extreme meteorological conditions, it was well able to simulate the hydrological responses of three basins. The aridity index phi describes feedbacks between hydrological and meteorological anomalies, and was adopted as an indicator of hydrological drought. The anomalies of phi and temperature in the summer of 2003 exceeded the 1982-2005 mean by more than 2 standard deviations. Catchments without glaciers showed negative correlations between phi and discharge R. In basins with about 15% glacierization, phi and R were not correlated. River basins with higher glacier percentages showed a positive correlation between phi and R. Icemelt was positively correlated with phi and reduced the variability of discharge with larger amounts of meltwater. Runoff generation from the non-glaciated sub-areas was limited by high evapotranspiration and reduced precipitation. The 2003 summer heatwave could be a precursor to similar events in the near future. Hydrological models and further data analysis will allow the identification of the most sensitive regions where heatwaves may become a recurrent natural hazard with large environmental, social and economical impacts.