Climate change and flood sensitivity in Spain

The temporal and spatial distribution of more than 2500 records of historical floods in Spain, gathered within a database, were analysed. Our results suggest that the last millennium can be divided into periods with similar flood frequency, magnitude and distribution. This flood variability seems to correspond to changes of the prevailing atmospheric circulation patterns affecting the Iberian Peninsula. The present climatic flood-generating conditions were used in order to identify those changes for each period. In the Atlantic river basins, large floods between AD 1400-1500 and AD 1850-1910 resulted from intense, widespread rainfalls associated with Atlantic west and northwest frontal systems transported by the westerlies. In fact, both periods seem to experience a high hydrological variance, corresponding to a transition between different climatic conditions. Between AD 1150 and 1400 two hydroclimatic periods were distinguished: the first (AD 1150-1290) was characterized by a high flood frequency, especially over the Atlantic basins, some of these floods affecting more than one drainage basins. The flood distribution pattern for the second phase between AD 1290 and 1400, was characterized by a decrease in the number of floods in the Atlantic basins and a relative increase of flooding in the Ebro and Jucar basins. Finally, the AD 1500-1850 interval was characterized by a higher flood frequency, especially in the south and southeastern basins, due to cold pool conditions, as well as by a high irregularity of large floods associated with a high temperature contrast between winter and summer.