Variabilidades pluvial e fluvial na região fisiográfica do Baixo São Francisco

Abstract

Recent studies have pointed out that ongoing climate changes could result in a reduction in rainfall and an increase in droughts in the Brazilian Northeast, thus aggravating the situation of water vulnerability, especially in areas that already present a certain degree of susceptibility, such as the region of Lower São Francisco (RBS). Aiming to contribute to tackling these issues, the main objective of this work was to characterize rainfall and river variability and trends in the physiographic region of Baixo São Francisco. To analyze rainfall variability and trends, a series of monthly precipitation data was used from 1980 to 2013, whose data were extracted from 39 points of the RBS, from the database of Xavier et al. (2016). To analyze the variability and fluvial trends of the lower course of the São Francisco River, monthly data obtained from the ANA (National Water Agency) database for the period from 1982 to 2020 were used, extracted at three points (Piranhas/AL, Pão de Açúcar/AL, Propriá/SE). Monthly flow data obtained from reports from CHESF (São Francisco Hydroelectric Company) were used to evaluate river variability in Sobradinho/BA in the period from 2013 to 2018. Descriptive statistics were calculated and the nonparametric Mann-Kendall test was applied to identify whether there are trends in rainfall and flows. The statistical significance adopted was 10%, 5% and 1%. The results of the analysis of rainfall variability showed that the region has higher amounts of precipitation from April to July, and lower amounts from September to December. For annual precipitation, accumulated amounts ranging from 522 mm to 1241 mm were identified to the west and southeast of the RBS, respectively. The analysis of standard years showed precipitation amounts ranging from 230 mm to 1100 mm in the dry year (1993), 230 mm to 1390 mm in the usual year (1984) and from 520 mm to 1982 mm in the rainy year (1989). In turn, the application of the Mann-Kendall test showed significant trends of increased precipitation on the annual (from 5,4 to 11,2 mm) and seasonal (3,2 to 5,2 mm) scales. The analysis of river variability showed higher flows in the period from January to April, with a variation between 2040 and 2470 m³/s, and lower flows from June to August, with a variation between 1520 and 1727 m³/s. Average monthly flows showed significant negative trends, with higher reduction amounts between January and April. Average annual flows also showed decreasing trends, with amounts varying between -35,84 m³/s and -42,53 m3 /s. The analysis of the interannual variability of rainfall and flows reveals that there is a low participation of the rains that occur in the RBS in the formation of flows in the lower course of the São Francisco River, which indicates that the greatest contributions to the flows of this stretch of the river are from the regions amount. Finally, it was verified that the flow regime has been strongly affected by the regulation operated at the Sobradinho dam, which has culminated in the reduction of periods of floods and droughts and has also led to more severe reductions, as was observed between 2013 and 2018, when the flows flowing into Sobradinho were continuously lowered, leading to a series of socio-environmental impacts in the Baixo São Francisco region.