Subsídios para gestão integrada dos recursos hídricos na Bacia Hidrográfica do Rio Cachoeira – Bahia

Abstract

The present work carried out in the Cachoeira River Basin (CRB), located in the southern portion of the State of Bahia, aimed to contribute with scientific foundations that make it possible to guide the integrated management of water resources in that area. Two scientific articles published in the journals were developed: Anuário do Instituto de Geociências and Boletim do Museu Paraense Emílio Goeldi. Ciências Naturais. In the first article, entitled “Intervening factors in the groundwater salinization process, Cachoeira River Basin, Northeastern Brazil”, hydrogeochemical diagrams and multivariate statistics were applied in order to identify the factors that potentially influence the salinity of the waters in the crystalline aquifer of the study region. The data used came from the hydrogeological database of the Companhia de Engenharia Hídrica e Saneamento da Bahia (CERB). Groundwater was predominantly classified as calcium-bicarbonate with salinity ranging from sweet to brackish. The cation that prevails in the region is Ca2+, while carbonate and bicarbonate correspond to the dominant anions. With the factor analysis it was possible to recognize two factors that explain approximately 93% of the total variance of the data. Hierarchical cluster analysis resulted in 5 distinct clusters based on dissolved ion concentrations. This study indicates that the natural mechanisms that control the chemical composition of groundwater are mainly related to the weathering of rocks associated with evaporation contributions, and the most significant parameters in the variability of water salinity are Ca2+, Mg2+, K+, Fe(total), Cl-. The second article was entitled "Precipitation in the Cachoeira River Basin: Variability and Trends (1970 - 2020)", this study aimed to identify rainfall trends and variability. The data used comprise historical series of total monthly rainfall from six rainfall stations for the period from 1970 to 2020 (51 years) obtained from the Hydrological Information System (HidroWeb) of the Agência Nacional de Águas e Saneamento Básico (ANA). The Mann-Kendall, Spearman's Rho and Sen's slope tests were satisfactorily applied, allowing the identification of significant trends and precipitation magnitude. The analysis showed that statistically nonsignificant trends prevail in the area. However, a significant negative trend was identified in the centralwest region in two rainfall stations that characterize a reduction of around 300 mm of rain in the studied period. Periods with extreme drought/humidity were recognized and compared with ENSO phases where it was identified that 62.5% of drought events and 42.9% of rainy events can be related to El Niño/La Niña phenomena, respectively. With the coefficient of variation, low rainfall variability was recognized in the eastern portion and high variability in the western portion of the area. The results obtained can be used as a reference for the management of water resources in the river basin.