Results and assessment
Relevance of the indicator for describing developments in the environment
Runoff is a quantitative background indicator on the freshwater discharge, carrying the nutrients from the drainage areas to the coast.
Runoff is an important parameter on the change of pressure on nutrient supply due to varying climate and climate change. Also change in land-use can influence runoff. To evaluate the change of pressure on nutrient supply to the Baltic region it is necessary to know the variability of runoff and normalise for this natural variability.
Assessment
The indicator shows the annual runoff from drainage areas but integrated over the Baltic sub-regions (Figure 1).
Runoff is governed by the precipitation - evaporation on land areas and is also influenced by air temperature. It is the sum of direct river and diffusive runoff. In all sub-regions a strong seasonal, annual and decadal variability can be distinguished. Especially wet and dry periods are characterising the runoff. The 1970s were a fairly dry period compared with the 1980s and the later part of the 1990s. Geographically, the runoff is of about the same size in the Gulf of Bothnia, Gulf of Finland and the Baltic Proper, whereas the Gulf of Riga and the Kattegat is contributes to a lesser extent to the total runoff.
There is no obvious trend in the annual runoff during the last 50 years (Figure 2), neither in the total nor from sub-regions. The dry period found during the 1970s could have masked the marine eutrophication since the runoff was lower than average and hence also the total load of nutrients. During the wet periods the total nutrient load (pressure) increased making marine eutrophication (effects) even worse.
Figure 1: Runoff to the Baltic subregions based on annual mean from 1950 to 2001.
Figure 2: Total runoff to the Baltic Sea based on annual mean from 1950 to 2001 (m3/s).
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Data
Data from 1950 to 1998 is based on observations from national focal points, whereas the data from 1999 to 2002 is based on modelling by SMHI using the HBV-model for the Baltic Sea drainage area.
References
Bergström, S. And B. Carlsson 1994. River runoff to the Baltic Sea 1950 - 1990. Ambio Vol. 23, No. 4-5, 280 - 287.
Summary (<20 words) There is no obvious trend in the annual runoff during the last 50 years; neither in the total nor from sub-regions.
Helsinki Commission