Freshwater runoff

	Author: Bertil Håkansson, SMHI

Key message

During the 1950 - 2002 year period total runoff to the Baltic Sea area show no long term trends. On the other hand the period is characterised by dry and wet periods lasting for a couple of years to a decade by and large following the NAO index. From a regional point of view the runoff enters the Gulf of Bothnia (6000 m³/s), the Gulf of Finland and the Baltic Proper (3500 m³/s) and to a lesser extent in Gulf of Riga and in Kattegat (1000 m³/s).

Results and Assessment

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Figure 1. Runoff from years 1950 - 2002 to the Baltic sub basins based on annual mean values.

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.

The indicator shows the annual runoff from drainage areas but integrated over the Baltic sub-regions. 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 contributes to a lesser extent to the total runoff.

Assessment

There is no obvious trend in the annual runoff during the last 50 years, 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 2. Total runoff from years 1950 - 2002 to the Baltic Sea except Kattegat based on annual mean values and the NAO index.

References

Bergström, S. And B. Carlsson 1994. River runoff to the Baltic Sea 1950 - 1990. AMBIO Vol. 23, No. 4-5, 280 - 287.
Graham, Phil 1999. Modelling runoff to the Baltic Sea. AMBIO Vol. 28, No. 4, 328-334.
http://www.jisao.washington.edu/data_sets/nao/nao.ascii

Data

Observations are collected at the BALTEX Hydrological Data Centre (http://www.smhi.se/sgn0102/bhdc/bhdc.htm), whereas modelled data is done at SMHI using the HBV-model (Graham-99). Bay of Bothnia, the Sounds and the Kattegat are based on observations. Gulf of Riga runoff is based observations up through 2001,while simulations are used for 2002. Gulf of Finland runoff is based on observations up through 1997, while simulations are used for 1998-2002. Baltic Proper runoff is based on observations up through 1996, while simulations are used for 1997-2002.

Additional information

	More information on NOA index can be found from The Finnish Institute of Marine Research  NOAA

BALTEX Hydrological Data Centre: http://www.smhi.se/sgn0102/bhdc/bhdc.htm

Last update: 20 November 2003