Predicting water age distribution in the Pearl River Estuary using a three-dimensional model

The age of water (AW) concept is applied to investigate the water exchange process in a large and density stratified estuary, namely the Pearl River Estuary. A three-dimensional AW model has been built based on an existing hydrodynamic and solute transport model. The model is used to determine the AW distributions inside the Pearl River Estuary under various hydrodynamic conditions. The predicted mean AW values during the dry and wet seasons are approximately 25 and 10 days, respectively. In general, lower AW values are observed in the upper layers near the water surface, while higher values are observed in the lower layers near the bed. The difference becomes more pronounced during the wet season when the river discharge is very large. The tidal influence is relatively small. The variation of tidal forcing impacts mainly on the AW fluctuation range, but it has limited impact on the tidally averaged AW value. Moreover, the AW vertical difference is enhanced during neap tides. A comparison between the predictions using barotropic and baroclinic models indicates that the density-induced circulation has a significant impact on the water exchange rate.