Which term describes the length of time water remains in a reservoir before leaving?

• A. Hydrologic cycle
• B. Groundwater
• C. Residence time
• D. Fluxes

Answer: (C)

The length of time water remains in a reservoir before leaving is described by residence time. It’s the average duration a water molecule spends in that system, determined by how much water is stored (the reservoir’s volume) and how fast water leaves (the outflow rate). In steady conditions, you can think of it roughly as residence time ≈ volume divided by outflow rate. For example, if a reservoir holds 500 million cubic meters and water exits at 250 million cubic meters per year, the residence time is about 2 years, meaning typical water molecules stay in the reservoir for around two years before exiting.

The hydrologic cycle refers to the entire global set of water movements, not the time a specific reservoir holds water. Groundwater describes water stored underground, not the duration in a surface reservoir. Fluxes are the rates of movement (how much water flows per unit time), not the time water spends in a reservoir.