<Tr> <Td> Atmosphere </Td> <Td> 9 days </Td> </Tr> <P> The residence time of a reservoir within the hydrologic cycle is the average time a water molecule will spend in that reservoir (see adjacent table). It is a measure of the average age of the water in that reservoir . </P> <P> Groundwater can spend over 10,000 years beneath Earth's surface before leaving . Particularly old groundwater is called fossil water . Water stored in the soil remains there very briefly, because it is spread thinly across the Earth, and is readily lost by evaporation, transpiration, stream flow, or groundwater recharge . After evaporating, the residence time in the atmosphere is about 9 days before condensing and falling to the Earth as precipitation . </P> <P> The major ice sheets - Antarctica and Greenland - store ice for very long periods . Ice from Antarctica has been reliably dated to 800,000 years before present, though the average residence time is shorter . </P>

Processes of the hydrological cycle that return water to the oceans