Spatial-temporal analysis of flood patterns in semiarid river catchments of SE Iberian Peninsula since CE 1850

The province of Almería, southeastern Iberian Peninsula. The main objective of this study was to analyse the evolution of the spatial distribution and the frequency of floods in the four main basins of the Province of Almería since 1850. A database of the main historical floods reported in written historical documents in the Almanzora, Andarax, Antas and Aguas river basins was compiled, and information on the damage caused by the floods was retrieved from 15 historical archives, allowing an understanding of how floods have behaved over the past 170 years and how they are likely to behave in the near future. The river basins of the province of Almería are drained by ephemeral flows mostly related to episodes of heavy rainfall or to long-lasting rainfall attributed to low pressure systems that can persist for several days. The region's climatic, topographical, and lithological characteristics make this region prone to floods that lead to economic damage in the floodplains. In total, more than 106 flood events were recorded between 1850 and 2020 throughout the study area. Since 1850, the frequency of floods has increased and they have been heterogeneously spatially distributed. Three flood-rich periods have occurred, from 1870 to 1900 (43 floods), from 1966 to 1982 (18 floods), and from 2000 to 2020 (19 floods), each with different characteristics in terms of the distribution of flood events. Specifically, the period 1870–1900 shows a higher frequency of ordinary, extraordinary, and catastrophic floods compared to the other flood-rich periods, with a very high trend of events observed across all the studied basins. Three factors explain the changes in the frequency and the spatial distribution of the flood events: a) an increase in the frequency of ordinary flood events; b) new construction in highly vulnerable areas and in coastal areas close to river mouths; and c) changes in land use. Therefore, studies on low-frequency floods should be taken into account in regional planning, and the organization of flood-prone basins should be rethought as they are developed. This reorganization is needed particularly in regions sensitive to the consequences of climate change, such as the Mediterranean and arid and semi-arid areas. • Three main flood periods: 1870–1900, 1966–1982, 2000–2020. • Shift of flood damages to coastal tourist areas post-2000. • Flood frequency correlates with population growth in river basins. • The spatial distribution of damages is different in each period. • Urban expansion into floodplains increased vulnerability to floods.