Residence time

Residence time is the average time a substance or parcel of water remains within a defined system. Its value depends on the system boundaries and how residence time is measured.

Mean residence time: The expected duration that material spends inside a system before leaving. This is the statistical quantity most commonly meant by residence time.
Water residence time: The average time water remains within a specified reservoir or hydrologic system. This is the hydrological application of the general concept.
Chemical reaction kinetics: The study of how fast chemical reactions occur and what controls their rates. Residence time determines how long reactions can alter material before it exits.
Transit time: The time a particular parcel takes to travel between two specified locations. A parcel-specific journey differs from the system-wide average residence time.
Residence-time distribution: A probability distribution describing how long portions of material spend inside a system. It reveals variation hidden by a single average residence time.
Residence time of groundwater: The time groundwater spends in an aquifer before discharge or extraction. Long groundwater residence times can indicate slow circulation and prolonged contact with rock.
Damköhler number: A dimensionless ratio comparing the timescales of reaction and transport. It indicates whether reaction or passage through a system dominates outcomes.
Age of water: The time elapsed since a water parcel entered a specified system or reservoir. Age is measured at a point or for a parcel, unlike a system's mean residence time.
Mass balance: An accounting of material entering, leaving, and accumulating within a defined system. At steady state, inventory divided by throughput can estimate mean residence time.
Ocean residence time: The average time a dissolved substance remains in the ocean before removal. It describes how rapidly ocean processes remove or redistribute dissolved materials.

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