Floods Are Becoming Less Severe, Not More

By Paul Homewood

 

We are often told that global warming will lead to more extreme floods, for instance here.

To test this out, a team of scientists analysed daily discharge data from many sites across the world, and found that there was no evidence at all that floods were becoming worse. Indeed, if anything the reverse was true.

 

 

 

Long-term properties of annual maximum daily river discharge worldwide

Dimitris Bouziotas, Georgios Deskos, Nikos Mastrantonas, Dimosthenis Tsaknias, Grigorios Vangelidis, Simon -Michael Papalexiou, and Demetris Koutsoyiannis

Department of Water Resources and Environmental Engineering, National Technical University of Athens, Greece

ABSTRACT

We use a database of annual maximum daily discharge time series (World Catalogue of Maximum Observed Floods, IAHS Press, 2003) and extract those with length greater than 50 years. We analyse extreme floods at several stations worldwide focusing on the long-term properties of the time series including trends and persistence (else known as Hurst-Kolmogorov dynamics), which characterizes the temporal streamflow variability across several time scales. The analysis allows drawing conclusions, which have some importance, given the ongoing and intensifying discussions about worsening of climate and amplification of extreme phenomena.

CONCLUSION

The World Catalogue of Maximum Observed Floods (Herschy, 2003) and its supplementary documentation, provide a good basis for assessing flood trends worldwide.

Analysis of trends and of aggregated time series on climatic (30-year) scale does not indicate consistent trends worldwide. Despite common perception, in general, the detected trends are more negative (less intense floods in most recent years) than positive. Similarly, Svensson et al. (2005) and Di Baldassarre et al. (2010) did not find systematical change neither in flood increasing or decreasing numbers nor change in flood magnitudes in their analysis.

Future work will try to investigate the reasons for the decreasing trends at each of the basins, with particular emphasis on whether the river basins upstream the gauging location are intact or perhaps affected by human activities