INTERPRETATION AND APPLICABILITY OF RESIDENCE TIMES IN RESERVOIRS

The residence time scale is widely applied in engineering, for example, it has been used to optimize reactors and to predict eutrophication levels of water bodies. It has been used to define the hydraulic detention time, which is the ratio between the volume and the outlet flow of the system. Although this simple approach is widely used, the method is unable to identify dead zones within the system. Therefore, the present study aims to analyze, evaluate and propose methods to estimate transport time scales (in literature called generically residence time) that are able to identify the spatial variability of the transport of substances within a real system. In addition, this study aimed to identify potential applications for transport time scales using the Vossoroca hydropower reservoir in Brazil as a case study. The transport time scales were tested in a numerical experiment: a channel with a harbor. This analysis verified how bulk and local scales behave in a non-uniform system. The second analysis compared qualitatively scales with the transport phenomena and verified the ability to identify dead zones. Finally, the scales were applied to a real case and the links between the transport time scales and water quality parameters were investigated. All local scales tested were able to identify dead zones. Moreover, it was verified that the arrival time is related to the influence of the inflow and export time indicates where system export elements quickly. The case study indicated the existence of a relationship between transport time scales and water quality parameters, thus water quality information were extrapolated to regions without measurements.