Tests on Vindel river (Vindelälven) in northern Sweden have shown that the new method is capable of producing more accurate forecasts of the volume of the spring flood than the methods currently employed.
“Predicting the weather and water flows several months in advance is extremely difficult in our part of the world. However, the latest development in meteorological long-term forecasts also provides the conditions for improved spring flow forecasts,” says Jonas Olsson, researcher in hydrology at SMHI.
Time of the spring flood and forecast reliability
Where the new method works, it also gives better estimates of the probability that the volume will exceed different levels. In the latter part of spring it can also provide a reasonably reliable signal whether the spring flood will start earlier or later than normal.
“In areas with shifting temperatures around 0 degrees the snow starts to melt earlier in the season and it is then difficult to predict when the spring flood will arrive and the volumes involved”, says Jonas Olsson.
Seasonal forecast the basis of the new method
The new method is based on seasonal forecasts from the European Centre for Medium Range Weather Forecasts, ECMWF, and has been developed in a research project funded by Energiforsk, a research and knowledge organisation for the energy sector in Sweden.
The project studied the possibilities to perform a spring flood forecast that is better than the climate-based method used up until now. The project examined the conditions for two Swedish rivers, Vindel river and Ljusnan river.
Best forecast for a well-defined melting period
The conclusion is that for the Vindel river, which is unregulated and located in the far north of the country in an area with comparatively a great deal of snow in the winter and cold until the spring flow begins, the new method is better than the previous one. It gives a lower systematic error and a more reliable assessment of the probabilities for different spring flood volumes. There is a possibility, about one month in advance, to determine whether the spring flood will be early, normal or late.
Other prerequisites apply for the Ljusnan river. It is a regulated water flow further in the south of the country, where the melting away of snow starts earlier in the season, which makes forecasting more difficult. For the Ljusnan river the new forecast method is not a clear improvement over today’s method.
Additional research is necessary in order to transfer the results to more Swedish rivers.