Dinâmica de parâmetros limnológicos em reservatório e índice de vegetação em bacia hidrográfica no semiárido brasileiro

Abstract

The disorderly exploration and occupation of watersheds lead to the reduction of forested areas, resulting in surface runoff and sedimentation into water bodies, impairing water quality. In this scenario, the use of facilitating tools such as Remote Sensing is crucial to obtain data assertively and inexpensively. Therefore, the objective was to analyze how preserving vegetation in the watershed influences water quality in its reservoir. For the research, on-site data was collected at Pereira de Miranda reservoir, Pentecoste, Ceará, at five different points. Concentrations of total suspended solids (SST), fixed suspended solids (SSF), volatile suspended solids (SSV), and water transparency using the Secchi disk were evaluated. To analyze the vegetation in the Pereira de Miranda reservoir watershed (BHPM), Landsat 8 satellite images obtained from the United States Geological Survey (USGS) were used, employing bands 4 and 5 to calculate the Normalized Difference Vegetation Index (NDVI) for each collection period: second semester of 2021 and 2022, and first semester of 2022 and 2023. Data on reservoir water volume and regional rainfall measurements during the study were also collected from the Fundação Cearense de Meteorologia e Recursos Hídricos (FUNCEME) and the Companhia de Gestão dos Recursos Hídricos do Estado do Ceará (COGERH). It was observed that the highest SST concentration and lowest water transparency were recorded in the first semester of 2022 at point P05: 9,33 mg.L-1 and 0,53 m, respectively. NDVI indicated larger areas of exposed soil in the watershed in the first semester of 2023, occupying 10,65% of the total area, and marked that vegetation with low photosynthetic activity peaked in the second semester of 2021 (96,83%) and second semester of 2022 (88,57%), respectively. In the Pearson correlation analysis (r), a negative and moderate association (-0,43 and -0,54) was observed between regions with high photosynthetic activity vegetation and limnological parameters (SST and SSF). Additionally, the Pearson correlation (r) showed a very strong positive relationship (0,85) between areas with high photosynthetic activity vegetation and water transparency. These results suggest that the presence of vegetation in a watershed is inversely related to the amount of suspended sediments, as its removal promotes increased surface runoff, which in turn intensifies erosive processes, resulting in more suspended sediments in the reservoir water.