At an average depth of just 53 metres, the Baltic Sea is much shallower than most of the world’s seas. It contains 21,547 km³ of water, and every year rivers bring about 2% of this volume of water into the sea. The Baltic Sea’s catchment area is almost four times larger than the sea itself.
The amount of freshwater runoff entering the Baltic Sea from its catchment area determines the quantities of nutrients entering the sea, and also affects salinity levels and the stratification of the sea water into layers of different densities. The rain and snow that fall directly into the sea only play a minor role in the water balance of the Baltic Sea, but they account for most of the atmospheric nitrogen load entering surface waters. Climatic factors such as precipitation and evaporation determine levels of runoff.
During the last 50 years, annual runoff into the Baltic Sea has remained approximately stable. While long-term cyclical fluctuations with alternating wet and dry periods are observable, year-to year variations are dominant along with the annual runoff cycle. This assessment period was relatively wet compared to previous years.
Figure 2. Total runoff from years 1950 - 2002 to the Baltic Sea except Kattegat based on annual mean values and the NAO index.
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