Rivers with memory

Leipzig meteorologist calculates risks for floods and droughts

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The "long-term memory" of rivers can lead to the occurrence of extreme flood situations or long-lasting dryness. This was proven by a Leipzig meteorologist in a study of six rivers in Europe, the US and Africa. The new findings have now been published in the journal "Water Resources Research" of the American Geophysical Union.

The trigger for the memory effect is the ability of bodies of water to create evasion possibilities or reservoirs as flow rates increase - a "short-term memory". The further downstream and thus below further inflows you are, the stronger becomes of the short-term "long-term memory" that ensures the best possible water flow.

Long-term measurements on the Elbe

Dr. Manfred Mudelsee from the University of Leipzig was working on his research with long-term measurements of the amount of runoff in the rivers. "The best runoff time series is for the Elbe, there is nothing better around the world, " explains the scientist. Thus, for example, in Dresden since 1852, the water flow rates have been registered daily, not even war events interrupted the series of measurements. However, the data was collected over longer periods of time in other measuring stations above and below the Saxon metropolis.

From the measurement series and with the help of the mathematical method of probability calculation, the meteorologist can now determine how probable it is that certain sections of the river are subject to flooding. "Although I can not predict the flood event as such, it is possible to calculate how high the risk of such an event is."

Flood prevention as an economic factor

What may sound rather unspectacular at first may be crucial for the economy, but also for politics. If the flood risk can be presented for medium-term periods, investment and resettlement decisions can be made dependent on it. How important this can be, becomes clear at the Elbe flood of 2002: In the floods in the Czech Republic and Germany numerous people were killed, the property damage went into the billions. display

Since he has dealt extensively with the long series of measurements on the Elbe, Mudelsee can give the all-clear in the climate risk analysis for this river for winter floods. As a result of the climate change, the river does not iced up so much that thawing ice floes will accumulate to a greater extent and meltwater can flow away almost unhindered. However, the scientist warns against simply transferring these insights: "Every river has to be considered individually and separately in order to carry out a qualified risk analysis."

(University of Leipzig, 26.06.2007 - AHE)