Daily variability of estuary-shelf exchange at the Lagoa dos Patos’s mouth

Abstract

The Lagoa dos Patos is one of the largest coastal lagoons in the world, covering an area of 10,000 km2, with a drainage basin of 200,000 km2, with an average river flow of around 5000 m3/s. The lagoon is of the choked type, and exchanges between the lagoon and the ocean occur through a narrow section with nealy 500 m wide. The hydrodynamics during periods of low river flow are mainly controlled by wind patterns, which can either favor the inflow or outflow of water from the lagoon. A field experiment was conducted to investigate the daily variability of lagoon-ocean exchanges during a period of low flow, fortuitously capturing an extreme condition with a 1 % probability. Flow and salinity data were monitored daily near the lagoon's mouth for one month (from February 23rd to March 23rd, 2020). Additionally, a hydrological balance was calculated using data on river flow from the lagoon's main tributaries (Guaíba river system, Rio Camaquã, and São Gonçalo Channel) and lagoon-atmosphere exchanges through precipitation and evaporation. The average results obtained from direct measurements (∼ 1000 m3/s) agree reasonably well with the data obtained from the hydrological balance (∼850 m3/s), especially considering the limitations of both approaches. It was identified that direct precipitation events over the lagoon can produce rapid responses in the freshwater outflow from the lagoon, often exceeding the values of river flow contribution.