Análises de eventos extremos de precipitação diária no estado de Alagoas, Nordeste do Brasil

Abstract

Extreme Precipitation Events (EPEs) are rare and intense phenomena capable of causing major disasters. In Brazil, EPEs are among the leading causes of natural disasters, especially in densely populated coastal areas. Northeastern Brazil, in particular, is highly vulnerable due to the large variability in precipitation. In this context, the main objective of this study was to statistically analyze daily EPEs in the state of Alagoas, Northeastern Brazil. The specific objectives were: (I) to define and characterize the climatology of EPEs at temporal (daily, monthly, and annual) and spatial (102 municipalities) scales; (II) to regionalize areas with homogeneous characteristics of extreme rainfall; (III) to estimate the return period of daily EPEs by month and year; and (IV) to verify the existence of significant trends in event frequency and intensity. For this purpose, a daily EPE database provided by Xavier et al. (2022) was used for the period 1961–2023 (63 years), with a spatial resolution of 0.1° × 0.1°. After data acquisition, daily EPEs were defined using the percentile technique at monthly and annual scales for each of the 102 municipalities. The events were then regionalized using the multivariate statistical technique of cluster analysis. After identifying EPE sub-regions in Alagoas, characterization was carried out using descriptive statistical analyses, as well as comparative analyses among sub-regions and within each sub-region (by monthly, seasonal, annual, decadal periods, and climatological normals). The probabilities of occurrence and return periods of daily EPEs were analyzed. Non-parametric statistics were applied to assess the direction (Mann–Kendall), magnitude (Sen), and change point (Pettitt) of trends, adopting a 5% significance level. The results indicate high temporal and spatial variability of EPEs in Alagoas. Three sub-regions were identified (S1, S2, and S3). The most intense daily EPEs were concentrated in S1, although they exhibit seasonal and spatial variations. The concentration of daily EPEs occurs mainly in S1 and S2 (between April and July); in contrast, in S3, events predominate between December and March. The return period showed an increasing trend over time, especially in the eastern portion of the state, where precipitation events exceeding 100 mm in a single day are now expected at intervals of 25 to 30 years. Trends in daily EPEs were identified in Alagoas, with distinct and sometimes opposite behaviors depending on location. In terms of daily EPE frequency, S1 showed a trend in only one municipality, characterized by an increase. S2 concentrated the largest number of municipalities with trends, with increases in frequency prevailing, although some areas experienced decreases. S3 stood out for the predominance of decreasing frequency trends, especially in its western portion, although some municipalities showed increases. These results provide relevant scientific support for territorial planning, risk management, and the mitigation of impacts associated with EPEs.