How Long Does Water Stay in Each Reservoir?

The popular claim that over a 100-year period a water molecule spends 98 years in the ocean, 20 months as ice, two weeks in lakes and rivers, and less than a week in the atmosphere is inaccurate. Residence time depends on the reservoir and on how it is estimated. Approximate figures put ocean water at thousands of years, atmospheric water at about nine days, and river water at roughly 12–20 days. Lakes are often estimated at about ten years; glacier water may remain for centuries to millennia. These are reservoir averages, not a schedule followed by every individual molecule.