Efeito da variabilidade climática sobre os baixos níveis de reservatórios na bacia hidrográfica do Rio São Francisco

Abstract

The number of occurrences of extreme precipitation events has increased over the last few years, in some cases, with greater intensity and duration, thus favoring natural disasters such as severe droughts. This research aims to analyze the level of the main reservoirs of the four sub-basins that make up the São Francisco River Basin, identifying the pluviometric events, particularly those of drought, and their possible climatic causes. The study was carried out with precipitation and quota data obtained from the National Water Agency (ANA) for the period from 1968 to 2020. Extreme drought events were classified and characterized by the standardized precipitation index (SPI). The Mann-Kendall test was used to detect climatic trends in the rainfall series of the four subregions of the BHSF and were statistically analyzed by the T-Student test. Of the four sub-regions, three of them show a downward trend in precipitation and elevation, while only one shows an upward trend, highlighting the possible changes in the levels of the sub-regions' reservoirs. Wavelet analysis was used with the intention of verifying cycles of extreme rainfall and their causes through the temporal scales observed in the precipitation series of the BHSF sub-regions. The results show that all sub-regions have higher occurrences of droughts than rains, however, extreme drought events are infrequent, whereas mild drought events were more recurrent. The years characterized as almost normal were the ones with the highest number in the four subregions, however, in all of them, the episodes of mild drought had expressive percentages. In the Wavelet analysis, it was found that the seasonal and interannual scales dominated, associated with the persistent El Niño signal Southern Oscillation (7 years). In the cluster analysis, regions with similar behavior were identified. In relation to the annual averages, two large climatic groups were formed that are indirectly similar to each other, and in relation to the seasons of the year, for the summer, four groups were formed, for the autumn and winter, five groups and for the spring, three groups emerged. Knowledge of homogeneous groups in BHSF can make an essential contribution to the management of water resources.