Seasonal Variations on Characteristic Hydraulic Times in Guanabara Bay, Rio de Janeiro, Brazil – Part 2: Water Age vs. Residence Time

Understanding the processes that influence hydrodynamics and water quality parameters in estuarine systems is essential to assist environmental management policies. Guanabara Bay, located in Rio de Janeiro, is one of the most important estuarine systems of Brazil. Most of its tributary rivers are polluted by large quantities of domestic sewage, which compromises the water quality. Therefore, it is important to understand how the water renewal capacity of the system behaves for different scenarios.

Under such perspective, this work aims to study the influence of environmental seasonal changes on water renewal at Guanabara Bay through the analyses of three characteristic hydraulic times widely used in the literature: Residence Time, Time of Renewal Rates and Water Age. Although these methods are space and time varying functions, the Water Age concept was adopted as reference for comparison with the others due to its versatility.

This Part 2 focuses in analyzing similarities and differences between the Water Age (W_A) and the Residence Time (R_T). We discuss the methodologies to calculate these functions through a well-tested environmental hydrodynamic modeling system and present respective analyses concerning Guanabara Bay. The simulations considered two scenarios, a rainy summer, and a dry winter. Both include effects of astronomical tides, storm surges, coastal currents, local winds, salinity gradients, and variable discharges from tributary rivers. The influences on water exchanges of each external effect are analyzed.

Comparing the two analyses, there are noticeable differences between the results that may lead to mistaken interpretations, hence, it is crucial to properly understand the concept and computation methodology of each one. The R_T in the central area of the bay varies from 50-90 days, while the W_A varies from 60-105 days in the same region. In the northern area of the Bay, the R_T spans from 70-90 days, and the W_A from 10-80 days. The greater difference between the analyses was in the northeast and northwest areas, where the incoming river has a greater influence on the water renewal in the Bay. Comparing the seasons, we see that the north of the Bay has higher R_T in both scenarios, which is due to that area not be under direct influence of any tributary river.

This work contributes to the understanding of water mixtures in Guanabara Bay. It demonstrates how characteristic hydraulic times in general, and the concept of Water Age in particular, together with computational modeling, can assist the environmental management of Guanabara Bay.