A modelling study of residence time and exposure time in the Pearl River Estuary, China

• The residence and exposure times show significant seasonal variations in the PRE.
• In the PRE the exposure time is about two times longer than the residence time.
• The two time scales decrease exponentially with an increasing discharge.
• The monsoon wind forcing shortens the exposure time in the dry season.

This paper presents an investigation into two transport timescales, i.e. the residence time and exposure time, of a conservative matter in the Pearl River Estuary (PRE) using a depth integrated hydrodynamic-dispersion model. The model has been verified against field measured tidal and salinity data in three typical seasons, including the wet, dry and average rainfall seasons. The model predicted distributions of tidal wave amplitude and salinity level agree generally well with the field measurements. The model is then enhanced by including capabilities for calculating the two timescales. The numerical model predictions show that both the freshwater discharge and tidal water elevation affect significantly the values of residence time and exposure time. The return coefficient is found to be about 0.5. Using a regression analysis, an exponential function has beenderived to correlate the timescales to the freshwater discharge. In the dry season the average residence time and exposure time are up to about 6 days and 12 days, while in the wet season these values are reduced to 3 days and 5 days, respectively. Generally, in all three types of seasons, the exposure time is about two times greater than the residence time, which demonstrates that there is a high possibility for water to re-enter the PRE after leaving the estuary. Both the residence time and exposure time decrease as the initial water elevation increases, which indicates that a contaminant will stay in the PRE for a longer time if it is released at a low tide. The effects of monthly averaged wind forcing on the resident time and exposure time are also investigated.