Sub-daily extreme events distribution and changes in Northeastern Brazil in the 20th century

Abstract. The regional analysis of extreme hydrological events is connected with the availability of a dense network of rainfall data that is absent or inaccessible in Brazil, especially for sub-daily information. In engineering, extreme events rainfall information is represented by intensity–duration–frequency (IDF) relationships which are the most commonly used tools in water resources engineering for planning and design. Even if the sub-daily information that is included in the relationships is not available, the extreme rainfall information rest in the fundamentals of the IDF. This paper analyzes spatial distribution and track changes in sub-daily precipitation over Northeastern (NE) Brazil. Precipitation was estimated from IDF relationships information in Brazil based in rainfall measured from 1920's to 1950's (but still used in engineering projects) and information from the last half of the 20th century obtained from several IDFs gathered from municipalities' manuals, local symposia and books in many cases not easily obtainable. Results showed an intensification of extreme events in recent years, especially in shorter duration rainfall (less than 12 h). Hourly rainfall is bigger in almost all the Brazilian region, but especially in littoral and Northern portion, however, 12 and 24 h rainfall exhibit increases in the North, but, lower values in the Southern half of the region in concordance with flood changes reported by Milly et al. (2005). Analyzing the ratio between 1 and 24 h rainfall is possible to confirm its increase in all the region, with up to 35% in some areas. These results were able to show insight of sub-daily extreme events changes during 20th century in NE Brazil were previous reports were not found. The results also alerts for the necessity of engineering projects review, as outdated information is still being used for design purposes.