Using a three-dimensional particle-tracking model to estimate the residence time and age of water in a tidal estuary

A three-dimensional hydrodynamic model that includes a Lagrangian particle-tracking simulation was applied to the Danshuei River estuarine system in northern Taiwan. The model’s accuracy was validated with data from1999; the results from themodel agreedwellwith empirical observations ofwater surface elevation, tidal currents, and salinity. The validatedmodelwas then used to investigate the residence time and water age in response to different levels of freshwater discharge. A regression analysis of the model results revealed that an exponential equation best explained the correlation between residence time and freshwater input. We found that the residence times during the low and high freshwater discharge episodes were 4.4 and 2.5 days, respectively. Thewater age during the low-flow periods was greater than that during the high-flow periods. The modelled residence time and water age values without densityinduced circulation were higher than those with density-induced circulation, which indicates that density-induced estuarine circulation may play a significant role in the estuary. & 2010 Elsevier Ltd. All rights reserved.