| Summary: | The characteristics of climate and relief on the Iberian Peninsula favour the generation of floods. In Spain, these floods have historically had serious socio-economic impacts, with over 1,525 fatalities in the last five decades. Floods are the consequence of abnormal weather at a limited spatial-temporal scale and, so far, cannot be simulated with the physical models that predict the different scenarios of future climate change. Possible scenarios of the impact of climate change on flood regimes can be diagnosed through the use of millennial scale relationships of flood response to changes in climate, these obtained from geological and documentary data. In Atlantic basins, the generation, duration and magnitude of floods are very much associated with changes in winter rainfall. Palaeoflood and documentary flood records (e.g. Benito et al., 2003, 2008) show greater frequency of ordinary and extraordinary events during the initial and final phases of cold periods such as the Little Ice Age (1550-1850). In the instrumental period (1910 to the present), Atlantic rivers underwent a decline in the frequency of extraordinary floods, although the magnitude of the most catastrophic floods has remained the same, despite the flood control effect of reservoirs. This upward trend of hydrological variability is expected to continue in the forthcoming decades (medium level uncertainty) if we take into account the intensification of the positive phase of the North Atlantic Oscillation (NAO). In the case of rivers Duero and Ebro, peak discharges might be affected by the sudden snowmelt resulting from sudden variations in winter and spring temperatures. In Mediterranean basins, past flood series indicate that extreme floods occur during periods of high irregularity of both seasonal and annual rainfall (Barriendos and Rodrigo, 2006), In recent periods (the seventies and eighties) an increase has been observed in intense rainfall episodes, some of which have caused extraordinary floods. These recent floods reached maximum ...
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