Climate impact on floods – changes of high-flows in Sweden for the past and future (1911–2100)

There is an on-going discussion whether floods are more frequent nowadays than in the past and whether they will increase in a future climate. To explore this for Sweden we merged observed time-series from 69 sites across the country (450 000 km2) for the past century with high-resolution dynamic scenario modeling of the up-coming century. The results show that the changes of daily annual high flows in Sweden oscillate between decades, but there is no significant trend for the past 100 years. A small tendency for high flows to decrease by 0.3–0.4% per decade in magnitude and 10-year flood frequency was noted, but not statistically significant. Temperature was found to be the strongest climate driver for river high-flows, as these are mainly related to snow melt in Sweden. Also in the future there will be oscillations between decades, but these were difficult to estimate as climate projections were not in phase with observations. However, in the long term, the daily annual high-flows may decrease by on average 1% per decade, mainly due to lower peaks from snow melt in the spring (–2% per decade) caused by higher temperatures and shorter snow season. On the contrary, autumn flows may increase by 3% per decade due to more intensive rainfall. This indicates a shift in flood generating processes in the future, with more influence of rain generated floods. This should be considered in reference data for design variables when adapting to climate change. Uncertainties related to the study are discussed in the paper, both for observed data and for the complex model chain of climate impact assessments in hydrology.