Bormann, H., Pinter, N. and Elfert, S. 2011. Hydrological signatures of flood trends on German rivers: Flood frequencies, flood heights and specific stages. Journal of Hydrology 404: 50-66.
In the introduction to their empirical study of the hydrology of German rivers, the authors write that "following several severe floods in Germany during the past two decades, [the] mass media as well as scientists have debated the relative contributions of climate and/or anthropogenic processes to those floods."
What was done
In the Bormann et al. study, which was driven by a desire to help resolve this climate-change impact debate, long time-series of stage and discharge data obtained from 78 river gauges in Germany were analyzed for trends in flood frequency, peak discharge, peak stage and stage-discharge relationships, where all variables investigated had to have a temporal history on the order of at least half a century.
What was learned
The three researchers first established the nature of Germany's temperature history, noting that Schonwiese (1999) identified a homogenous positive trend of 0.5-1.0°C over the course of the 20th century, which was subsequently confirmed by Gerstengarbe and Werner (2008) and Bormann (2010). Then, in terms of land use change between 1951 and 1989, they report that "agricultural area in Germany decreased from 57.8% to 53.7%, while forested areas remained almost constant." During this same time period, they report that "impervious areas increased sharply from 7.4% to 12.3%," and they say that "this trend has continued since 1989," with impervious areas further increasing from 11.2% to 13.1%, forest areas increasing from 29.3% to 30.1%, and agricultural area decreasing from 54.7% to 52.5%. And as a consequence of the net increase in impervious surfaces, they say that "runoff generation can be expected to increase and infiltration and groundwater recharge decrease," which would be expected to lead to increases in river flow and a potential for more frequent and extreme floods. However, they report that "most stations analyzed on the German rivers did not show statistically significant trends in any of the metrics analyzed [italics added]."
What it means
In light of these several observations -- plus the fact that "most decadal-scale climate-change impacts on flooding (Petrow and Merz, 2009) are small compared to historic peaks in flood occurrence (Mudelsee et al., 2006)" -- Bormann et al. conclude their report by stating that these significant facts "should be emphasized in the recent discussion on the effect of climate change on flooding." And if this is done, there is no other conclusion to be drawn but that the warming experienced in Germany over the past century has not led to unprecedented flooding throughout the country. In fact, it has not led to any increase in flooding.
Bormann, H. 2010. Changing runoff regimes of German rivers due to climate change. Erdkunde 64: 257-279.
Gerstengarbe, F.-W. and Werner, P.C. 2008. Climate development in the last century -- global and regional. International Journal of Medical Microbiology 298: 5-11.
Mudelsee, M., Deutsch, M., Borngen, M. and Tetzlaff, G. 2006. Trends in flood risk of the river Werra (Germany) over the past 500 years. Hydrological Sciences Journal 51: 818-833.
Petrow, T. and Merz, B. 2009. Trends in flood magnitude, frequency and seasonality in Germany in the period 1951-2002. Journal of Hydrology 371: 129-141.
Schonwiese, C.-D. 1999. Das Klima der jungeren Vergangenheit. Physik in unserer Zeit 30: 94-101.Reviewed 20 July 2011