The ebb and flood of tides is very important in the renewal of water within an estuary. In southern New England estuaries that do not receive large inputs from rivers, this tidal exchange is the most important factor influencing the turnover of water in the estuary, although wind-driven circulation and the input of freshwater from streams and groundwater seepage can be important. If the volume of seawater that enters and exits the estuary on each tidal cycle is known, along with the freshwater input rate, then these can be used with the total volume of the estuary to estimate how long it takes (on average) for the water in the estuary to be replaced. This is the residence time (or turnover time) of water in the estuary. A simplified calculation of residence time, assuming complete mixing, is:
