| Estuaries are complex ecosystems that provide a habitat for many living organisms. Unfortunately, these ecosystems can sometimes be fragile and rely on delicate balances to stay self-sustaining. Residence time is one of a larger set of parameters used to characterize the environmental state of estuaries and other water bodies. Though the science of evaluating large ecosystems is still under development, time scale calculations can greatly help in the understanding of the various processes within an estuary. These metrics help to define an estuary from an ecological and engineering standpoint. As first defined by J.T.F. Zimmerman (1976), residence time is the time it takes for a particle in an estuary, channel or river to travel from one preselected point to another preselected point in the estuary, channel or river. Since residence time is an intimate function of the estuary (i.e., it is a function of freshwater discharge, tides, estuary geometry, etc.) it can be used to make general comparisons of habitats within an estuary and between estuaries. This knowledge can contribute to restoration of estuaries already in decline as well as future projects that aim to construct sustainable habitats. After a brief introduction, we present relevant time scales and provide each with a precise definition. Then we discuss various models for residence time; namely the tidal prism model, a transient storage model, a variable residence time model, a forward Eulerian method and a probabilistic Lagrangian method. Also discussed is how tracers can define residence time and the implications of choosing among different tracer types. Reviews of studies that used residence time in an effort to make systems quantifiable are then presented. Lastly, the paper qualitatively reviews how time scales can be used in various problems dealing with characterization of an estuary. |
