Magnitude and frequency of flooding in the Tagus basin (Central Spain) over the last millennium

A relational database using Access, and an interface based on a Geographical Information System (GIS) with ArcView program, were created to allow spatial-temporal analysis of documentary flood data collected for the Tagus basin (Central Spain). High flood frequencies were registered in the periods: AD 1160-1210 (3%), 1540-1640 (11%; peak at 1590-1610), 1730-1760 (5%), 1780-1810 (4%), 1870-1900 (19%), 1930-1950 (17%) and 1960-1980 (12%). Flood magnitudes of those documentary events were estimated at four sites (Aranjuez, Toledo, Talavera and Alcantara) along the Tagus River using HEC-RAS Computer program, matching the calculated water surface profiles with historical references of flood stage. The largest floods took place during the periods AD 1168-1211 (Toledo and Talavera), AD 1658-1706 (Talavera), AD 1870-1900, and AD 1930 1950 (Aranjuez, Toledo, Talavera and Alcantara). The temporal distribution of flood magnitude and frequency is discussed within the context of climatic variability experienced by the Iberian Peninsula. Although flood producing mechanisms of Atlantic and Mediterranean basins of the Iberian Peninsula are related to different, independent atmospheric patterns, there is a clear coincidence between most periods showing high flood frequencies. These periods of high flooding seem to correspond to the initial and final decades of periods with climate deterioration described at the continental scale. This suggests that climatic variability over the last millennium has induced a response in hydrological extremes (positive or negative), irrespective of the flood-producing mechanism.