Impacts of warmer winters on water discharge
and water level
1. Impacts on water discharge
Warmer winters seem to cause a higher water discharge during winter, especially in January (see figure), probably because river and soil water is no longer deeply frozen. In addition, snowfall becomes rare and more and more precipitation falls in form of rain that can immediately flow into lakes. The decreased snowfall has drastic effects on the spring flood. Spring floods that used to appear in April in central Sweden are purely developed nowadays due to the lack of snow, causing that the water discharge in April tends to become less and less with increasing winter air temperatures (see figure).
2. Impacts on the water level
As soon as the water discharge is affected by a warmer winter climate also the water level experiences changes. Most extreme during the past 40 years were the water levels in Lake Vänern and Lake Mälaren in the beginning of 20011,2,3. The water level in Lake Vänern reached 45.7 m above sea-level which is 0.4 m higher than the highest so far registered water level in this lake (available observations since the middle of 1930 when water level regulations started in Lake Vänern). The exceptionally high water levels in both Lake Vänern and Lake Mälaren were the result of extreme floods at the end of 2000. According to climate experts the probability of an increase of extreme flood events in Sweden in the near future is high although there are large regional differences4,5,6.
1 Weyhenmeyer, G. and Sonesten, L. 2002. Klimat och vattenstånd under 2001. Chapter 1, pp. 12-14. In: A. Christensen (ed.), Vänern Årsskrift 2002 (in Swedish). Vänerns vattenvårdsförbund. Report 22. 83 pp.
2 Andersson, B. and Weyhenmeyer, G. 2002. Miljöövervakning i Mälaren 2001 (in Swedish). SLU-Report, Dept. of Environmental Assessment, Uppsala, Sweden. Report 2002:10, 41 pp.
3 Weyhenmeyer, G. A., WillÚn, E. and Sonesten, L. 2004. Effects of an extreme precipitation event on water chemistry and phytoplankton in the Swedish Lake Mńlaren. Boreal Environment Research 9: 409-420.
4 Bergström S., Carlsson B., Gardelin M., Lindström G., Pettersson A. and Rummukainen M. 2001. Climate change impacts on runoff in Sweden assessments by global climate models, dynamical downscaling and hydrological modelling. Climate Research 16: 101-112.
5 Christensen J.H., Räisänen J., Iversen T., Bjorge D., Christensen O.B. and Rummukainen M. 2001. A synthesis of regional climate change simulations a Scandinavian perspective. Geophysical Research Letters 28: 1003-1006.
6 Rummukainen M., Räisänen J., Bringfelt B., Ullerstig A., Omstedt A., Willén U., Hansson U. and Jones C. 2001. A regional climate model for northeastern Europe: model description and results from the downscaling of two GCM control simulations. Climate Dynamics 17: 339-359.
Easterling D.R., Meehl G.A., Parmesan C., Changnon S.A., Karl T.R. and Mearns L.O. 2000. Climate extremes: observations, modeling, and impacts. Science 289: 2068-2074.
IPCC. 2001. Climate change 2001: Impacts, adaptation, and vulnerability. Cambridge University Press, New York, USA.
Meehl G.A., Karl T., Easterling D.R., Changnon S., Pielke R., Changnon D., Evans J., Groisman P.Y., Knutson T.R., Kunkel K.E., Mearns L.O., Parmesan C., Pulwarty R., Root T., Sylves R.T., Whetton P. and Zwiers F. 2000. An introduction to trends in extreme weather and climate events: Observations, socioeconomic impacts, terrestrial ecological impacts, and model projections. Bulletin of the American Meteorological Society 81: 413-416.
Prudhomme C., Reynard N. and Crooks S. 2002. Downscaling of global climate models for flood frequency analysis: where are we now? Hydrological Processes 16: 1137-1150.