Estuarine water circulation is controlled by the inflow of rivers, the tides, rainfall and evaporation, the wind, and other oceanic events such as an upwelling, an eddy, and storms. Estuarine water circulation patterns are influenced by vertical mixing and stratification, and can affect residence time and exposure time.
Residence time
The residence time of water is a key variable determining the health of an estuary, particularly from human-induced stresses. Rapid flushing ensures that there is insufficient time for sediment accumulation or dissolved oxygen depletion in the estuary; thus a well flushed estuary is intrinsically more robust than a poorly flushed estuary. Residence time also affects other parameters such as heavy metals, dissolved nutrients, suspended solids, and algal blooms that may affect the health of estuaries.
A simple way to calculate residence time is using a simple classic estuary model, which can be useful for acquiring a conceptual understanding of an estuary, but is coarse in time and space. A classic estuary has the following components: 1) freshwater inflow with a discharge and a salinity (generally ); 2) oceanic inflow with a discharge and salinity ; and 3) outflow to the ocean with a discharge of and a salinity . The inflow and outflow of water is equal because mass is conserved. Salt is also a conserved, therefore, the inflow and outflow fluxes of salinity are also equal. If groundwater inputs and evaporation are ignored, the continuity equation is:
The residence time is the volume of water within the estuary (Vol) divided by the flow rate of the river:
Exposure time
Residence time considers the time it takes for the water particles to leave the estuary, however, some water particles that leave the estuary during an ebb tide may re-enter the system during a flood tide. The amount of time a water particle spends in the estuary until it never returns is called exposure time. The exposure time can be much larger than the residence time if the water particles are leaving with the ebb tides and returning with the rising tide. The ratio between the number of water particles returning to the estuary and the number of water particles leaving is known as the return coefficient, .
In order to quantify exposure time, the water circulation outside of the estuary must be determined. However, the circulation near the mouth of the estuary is complex due to the tidal mixing processes that occur between the estuarine and ocean waters. If the coast is rugged with headlands, a mosaic of complex flow fields consisting of eddies, jets, and stagnation zones will occur, further complicating the circulation patterns outside of the estuary.
In cases involving deltas or wetlands that drain into multiple tidal creeks, such as Missionary Bay, Australia, water leaving one creek at the ebb tide may enter another estuary during the flood tide. When there is a series of estuaries involved, a large exposure time (larger than that of the individual estuaries), will occur if the tidal outflow from one estuary re-enters a different estuary during the flood tide. Along a rugged coastline with headlands, however, mixing of estuary and oceanic waters can be intense. When estuarine water leaves the estuary, it gets flushed out to coastal waters, so exposure time and residence time are close equal.
In some cases it is possible to measure the fluxes of volume, salt, and temperature across the mouth of an estuary through a tidal cycle. Using this data, can be calculated ( is the return coefficient): it is equal to the fraction of the volume of water (mean tidal prism volume) leaving the estuary during the ebb tide that is replaced with coastal waters prior to re-entering the system. When , the same water is re-entering the estuary, and if , the estuarine water that has left the estuary during the ebb tide has been replaced with coastal waters entering the estuary during the rising tide. The exposure time { { math|τ'''}} is estimated by:
{ { math|τ' Vestuary Ttide / (1-r) VTP}}
is defined as the mean estuarine volume and is the tidal period.
The total fluxes of brackish water through the river mouth during tidal events is often much higher (often by a factor of 10 to 100) than the volume flux from riverine inflow. Therefore, if measurements are not precise, the estimate of the net flux will be unreliable. Direct measurements of the return coefficient are often complicated by unsteady oceanic events such an upwelling, the passage of an eddy, or storms, so the success of a correct direct measurement of the return coefficient is rare.