Scientific publications

We are experiencing very rapid climate change. Humans have warmed the Earth’s climate, primarily through emissions of greenhouse gases. In Sweden, both temperature and precipitation have increased, while the number of days with snow cover has decreased. This in turn affects runoff and water flows. The changes vary between different times of the year and between different parts of Sweden.

This report presents results from calculations of the water balance for the normal periods 1991–2020 and 1961–1990 on a general scale. The concept of water balance used here implies that all precipitation falling over an area either evaporates or flows into a watercourse. In equation form: precipitation = evaporation + runoff. If the water balance is calculated over a longer period, storage in snow, lakes, soil, or groundwater can be assumed to be negligible.

The modeled mean annual precipitation is highest in the western parts of the country, while the lowest precipitation is found in the southeastern and northernmost parts. Evaporation is greatest in the western parts of Götaland and Svealand and lowest in the northern mountain regions. Modeled runoff, which is the difference between precipitation and evaporation, varies considerably across the country. Annual runoff is highest in the mountain regions and lowest in the eastern parts of Götaland and Svealand.

From the normal period 1961–1990 to 1991–2020, both precipitation and evaporation have increased in large areas of the country. Annual mean runoff has also increased in most parts of the country, however - decreased runoff is observed in eastern Svealand and parts of eastern Götaland.

The report also provides an overview of how and why Sweden’s hydrology has changed and is expected to change in the future.

Flood events recorded by the Swedish Transport Administration and the Swedish CivilContingencies Agency in recent decades have been analysed to create a betterunderstanding of flood problems in Sweden and to guide the development of the warningservice. The results show that floods are most common in the summer and in denselypopulated areas, and usually occur far from major rivers and lakes. An overwhelmingmajority of the events also occurred without any warning criteria being exceeded, either inmeteorology or hydrology, and only a minority of the events were therefore preceded byany warning. To improve the warning service for flood events, it is recommended to (1)develop routines for weighing risk factors from both meteorology and hydrology in futureinterdisciplinary flood warnings, (2) place greater focus on runoff calculations as a riskfactor, especially with short time steps and in densely populated areas, and (3) supplementthe current methodology for evaluating warnings to utilize reported flood events from theSwedish Transport Administration, the Swedish Civil Contingencies Agency or othersources.