Subsídios hidrológicos para a análise geossistêmica: estudo da bacia hidrográfica do rio Cachoeira – Ilhéus/BA.

Abstract

The ongoing climate changes on our planet, whether natural or anthropogenic, directly impact the water supply for populations, especially the most disadvantaged, who face problems arising from scarcity and hydrological flooding. Therefore, it is increasingly necessary to undertake integrated studies on the processes of the hydrological cycle, as they guide the forces responsible for the spatialization of water on the planet and thus influence the dynamics of society's appropriation of space. Thus, the research aimed to analyze how hydrological elements can contribute to geosystemic studies in river basins. To this end, a case study was conducted, focusing on the Cachoeira River Basin (BHRC), located in the South and Southwest of Bahia, in the Eastern Basin and in the Eastern RPGA, under No. VII, encompassing 13 municipalities and a population of 542,692 inhabitants. In addition to using the geosystemic approach, the characterization of the study area was carried out using the prisms of the Geoenvironmental Complex and Advanced Experimental Hydrology (HAE), in the latter case, with the use of historical series of rainfall, elevation and flow. The geosystems were identified through field observations, a study of the longitudinal profile of the main river, analysis of cartographic material and CBERS-4A images, and correlations with other information gathered during the research. Thus, the geosystems of Alto Cachoeira (GEO1), Médio Cachoeira (GEO2), and Baixo Cachoeira (GEO3) were identified, as well as their constituent geofacies. The data revealed two historical models of spatial appropriation in the basin: extensive livestock farming predominated in the west, where social interventions were more prominent, and cocoa cultivation in the east, where cultivation under the forest contributed to the relative preservation of the forest. Significant landscape changes were observed in the basin, affecting its systemic dynamics. From this perspective, deforestation, sewage discharge into rivers, and irregular riverbank occupation, among other factors, have contributed to the recurring flood and drought events in the Middle and Lower Cachoeira Rivers, causing serious problems for the local population. Floods occurred in 48% of the years in the sampled historical series, while droughts occurred in 80% of them. Based on Bahian regulations, the data showed that the BHRC supplies only 11% of the water demand of the population covered by the basin. Lithological insubordination was also observed, as the drainage density (Dd = 0.63) and hydrographic density (Dh = 0.39) showed low values, failing to reflect the characteristics of the basement. In Alto Cachoeira, Dd and Dh presented lower values, when compared to Médio Cachoeira and Baixo Cachoeira, demonstrating that in Alto Cachoeira there is a greater contribution of vegetation to water infiltration, when rainfall occurs. In some geofacies of the three delimited geosystems, a certain balance between ecological potential and biological exploitation was observed, indicating fewer interventions resulting from the spatial appropriation model established in the basin. Furthermore, it was found that the hydrological elements that presented the greatest potential for research in river basins, from a geosystemic perspective, were drainage density and hydrographic density, as they demonstrated more qualified responses to the demands of the identified geosystems.