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How is water residence time calculated for a reservoir in the water cycle?

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Diagram illustrating Earth's environmental spheres, with labeled sections showing the geosphere (land and crust), biosphere (living things), atmosphere (air), and hydrosphere (water), each extending vertically from the Earth's surface. — hydrosphere Earth's environment includes the atmosphere, the hydrosphere, the lithosphere, and the biosphere.

In the study of the water cycle, the water residence time for a reservoir is calculated by dividing the total volume of water contained within that reservoir by the rate at which water is added to it or the rate at which it is lost from it.12

This calculation helps determine how long a mass of water persists in a specific medium or location. Because different reservoirs—such as oceans, lakes, rivers, ice, and groundwater—vary significantly in volume and flow rates, their residence times differ greatly. For example, water in the atmosphere has a short residence time of approximately 10 days, while water in the oceans may remain for 3,000 to 3,230 years.314