Will the flooding of rivers in Germany become more frequent?

Various factors may cause an increase or decrease of flooding of rivers. Firstly, there is human intervention in the flow rate of the river or its tributaries, for instance river straightening, the building of dams or irrigation systems or changes in land use in the area around the river (e.g. deforestation). These factors vary from river to river and are difficult to predict for the future.

Secondly, the frequency and severity of floods are fundamentally affected by climate change. Changes in the frequency of extreme rainfall events are particularly important in this respect. In order to estimate possible changes in precipitation, and thus climate change, from anthropogenic (human) effects, climate models are used. These calculate the changes in, amongst other things, precipitation distributions given different assumptions of human behaviour for the future. Based on these assumptions, such calculations have factors like increasing concentrations of greenhouse gases set as input parameters. The results of climate simulations conducted by various international groups have been published, for example in the report of the Intergovernmental Panel on Climate Change (IPCC).

Given an increase in greenhouse gas concentrations, all the models predict an average warming and with it an intensification of the hydrologic cycle. This means that, averaged globally, there will be more precipitation, but also more severe droughts for dry regions. We can illustrate what this might mean for individual areas by means of a pair of simulations by ECHAM4, a model of the global atmosphere. First, climate for the period 1970-1999 was simulated, using observed concentrations of greenhouse gases. Climate for 2060-2089 was then simulated using estimates of future greenhouse gas emissions projected using IPCC scenario IS92a..

According to these calculations, precipitation levels for southern Europe will be reduced whilst levels for northern Europe will increase (Figure 1a). Along with these changes in average rainfall, some regions show local increases in heavy rain storms. In north and central Europe, the number of days with more than 20mm/day of rain increases significantly (Figure 1b). Heavy rain storms only become less frequent in some Mediterranean regions. The average amount of rain in these areas is also reduced. A comparison with the simulation of current climate (Figure 1c) shows that some areas (e.g. off the coast of Norway) see a doubling in the frequency of heavy rain storms.

To investigate the influence of this changed rainfall on the river system, these results were fed into a model of the horizontal runoff from the land surface. Human impacts (e.g. dam building, river straightening) were not included in this model.

In all the regions where both the average amount of rain and the frequency of extreme rainfall events increased, severe floods also became more frequent. This increase is particularly marked in northern Europe, as well as parts of central Europe. In contrast, the probability of flooding for rivers in southern Europe decreases. The German rivers, (e.g. the Elbe, Oder, Rhein and Donau) lie in a region for which only small changes in precipitation are predicted.

These predictions from model calculations can only give us clues as to what the future developments will be. For example, the impact of human intervention in the course of rivers and their regions of influence is still unknown.

What can observations from past decades tell us?
An increase has been noted in the number of floods on many rivers in southern Germany, Austria, the Benelux countries and west Germany since the 1970s. One researcher, Caspary, ascribes the changes for various rivers in south-west Germany to a rise in the number of low pressure systems, brought in from the Atlantic, in the general meteorological picture for winter. Caspary advances the hypothesis that when the frequency of these trains of low pressure systems increases, there is a stronger risk of flooding for south-west Germany.

Literatur
[1] Intergovernmental Panel on Climate Change (IPCC) (2001)
Cambridge University Press , ISBN 0521 56436 0
[2] Roeckner et al. (1996)
The atmospheric general circulation model ECHAM-4: model description and simulation of present-day climate. MPI Report No. 218, Max-Planck-Institute for Meteorology, Hamburg
[3] Hagemann, S. and L. Dümenil (1998)
A parameterization of the lateral waterflow for the global scale Clim. Dyn. 14 (1), 17-31
[4] Caspary, H.-J. (1998), Hochwasserabflussverschärfung infolge von Klimaänderung in :
"Warnsignal Klima ", Herausgeber : Lozán, J. L., Graßl H. und Hupfer P., ISBN 3-00-002925-7