Dependence of the water residence time in Ria of Pontevedra (NW Spain) on the seawater inflow and the river discharge

The dependence of the residence time on the seawater inflow and the river discharge was analyzed by means of a stationary box model in the Ria of Pontevedra. The Ria, which is a partially stratified estuary, is divided into three boxes containing 11 CTD measuring points. The mean residence time calculated during the period October 1997 to October 1998 varies from around 3 days at the inner box to around 8 days at the external one. The residence time is shown to depend simultaneously on the river discharge and on the seawater inflow, which is characterized in terms of the upwelling index. This dependence on the upwelling index is observed to decrease upstream.