Characterisation of Drought and Wet Events in the Paraíba do Sul River Basin—Brazil

ABSTRACT In Brazil, the Paraíba do Sul River Basin, spanning São Paulo, Minas Gerais and Rio de Janeiro, has experienced extreme precipitation events that, whether through excesses or deficits, have generated significant socioeconomic impacts in this key industrial and demographic region. Thus, this study aimed to analyse and characterise the occurrence and spatiotemporal patterns of drought and wet events in the Paraíba do Sul River Basin, using the Standardised Precipitation Index (SPI) at both 3‐month and 12‐month time scales. Based on historical rainfall data from 87 pluviometric stations spanning the period from 1963 to 2024, the SPI enabled the identification of short‐ and long‐term hydrological patterns. To detect statistically significant trends, the non‐parametric Mann‐Kendall statistical test was used. The magnitude of the trends was obtained using the Sen's slope estimator. SPI‐12 identified severe droughts in 1963 and 2014, with annual deficits above 500 mm, and the most intense wet event in 1983, with surpluses over 650 mm. SPI‐3 confirmed recurrent seasonal droughts in 2014 and showed that wet events are not limited to the rainy season. Trend analysis revealed statistically significant changes in accumulated precipitation at the 5% significance level, with marked decreases in municipalities of São Paulo and Rio de Janeiro, such as Cunha‐SP (−278 mm/year/decade), Nova Friburgo‐RJ (−154.7 mm/year/decade), and Caçapava‐SP (−113.5 mm/year/decade). In contrast, notable increases were observed in Minas Gerais, including Carangola‐MG (+104.4 mm/year/decade), Patrocínio do Muriaé‐MG (+105.4 mm/year/decade), and Miradouro‐MG (+124.6 mm/year/decade). These results underscore a spatially heterogeneous pattern, reinforcing the need for adaptive and region‐specific water resource management strategies.