Abstract

Coastal lagoons, channels, and wetlands are sensitive ecosystems with high productivity and biodiversity. These systems often provide society with the valuable freshwater resources and many ecosystem services. In the subtropics, the hydrological systems inherit the typically high climatic and weather variability characteristics of these zones. Therefore, knowing the natural variability of these characteristics is crucial for the sustainable use of the water resources of these systems. In the present paper, we investigated the hydrodynamics of the Mirim-Sao Goncalo system, which comprises a large, shallow coastal lake, a 78 km-long channel and their adjacent floodplains and wetlands, with a focus on water level oscillations. We used a combination of numerical simulations, gauge station data and Synthetic Aperture RADAR imaging to evaluate the influence of the incident winds and the discharges of the main tributaries on the system's water levels and the establishment of barotropic gradients. We analyzed the water level variability of the Mirim lagoon and the Sao Goncalo Channel and their overflows to the adjacent floodplains. The simulations are five years long and cover the period starting in January 2000 to December 2004. The analysis indicates that the discharge of the tributaries mainly governs the system's temporal patterns. Wind action has two distinct influences on the systems. First, the wind dams the water at the southern portion of the Mirim Lagoon, which creates a persistent barotropic gradient in the surface. This gradient is often destroyed and dislocated northwards in a rotational fashion when the wind blows from the southern quadrant. Second, the wind perturbs the temporal variability patterns, resulting in high-frequency oscillations of the water level temporal signal. Flooding is frequent in the lands adjacent to the lagoon and the channel and results from the combination of the geomorphological and climatic settings of the region. The system is surrounded by ancient lagoon deposits with an extremely flat and low-lying topography, which favors the flooding of large extents. The large catchment area and the high precipitation rate over this region result in significant water level oscillations, causing the system to overflow and leading to subsequent floods.