Residence time of water in the Mondego estuary (Portugal)

The residence time of water is widely used as an indicator of how long a substance will remain in an estuary, a harbour, or a lagoon, and it is used to enable comparisons among different water bodies. In this work, the residence time in the Mondego Estuary, Portugal, is calculated by using two methodologies: the first one is based on field data and a freshwater fraction model, and the second one is based on a Lagrangian transport model. The Lagrangian model is coupled to a two-dimensional hydrodynamic numerical model that solves the depth-averaged advection and diffusion equations. Boundary conditions are provided by the Portuguese Coastal Operational Model, downscaled by using four nested domains with increasing resolution from the large to the local scale. The spatial variation of the residence time is characterized by subdividing the Mondego Estuary into boxes. The observed average salinity for each box is applied to the freshwater fraction model. With the Lagrangian model, boxes are filled with tracers and the path of the particles passing through them is quantified. The overall results of the two methodologies are similar, with a value of the residence time varying over the year between 1 and 12 days computed with the Lagrangian transport model and 2 and 9 days with the freshwater fraction model. Several scenarios were built by applying the Lagrangian transport model to investigate the history of water renewal and the influence of freshwater inflows and geomorphologic factors on the residence time. The overall results indicate that freshwater inflow is the main factor influencing the residence time. The analysis of the history of the water renewal was carried out by calculating the water exchange among boxes inside the estuary, pointing to the river flow as the main factor contributing to the water renewal of boxes.