<Dd> Water enters the mantle via subduction of oceanic crust . Water returns to the surface via volcanism . </Dd> <P> Water cycle thus involves many of the intermediate processes . </P> <Table> Average reservoir residence times <Tr> <Th> Reservoir </Th> <Th> Average residence time </Th> </Tr> <Tr> <Td> Antarctica </Td> <Td> 20,000 years </Td> </Tr> <Tr> <Td> Oceans </Td> <Td> 3,200 years </Td> </Tr> <Tr> <Td> Glaciers </Td> <Td> 20 to 100 years </Td> </Tr> <Tr> <Td> Seasonal snow cover </Td> <Td> 2 to 6 months </Td> </Tr> <Tr> <Td> Soil moisture </Td> <Td> 1 to 2 months </Td> </Tr> <Tr> <Td> Groundwater: shallow </Td> <Td> 100 to 200 years </Td> </Tr> <Tr> <Td> Groundwater: deep </Td> <Td> 10,000 years </Td> </Tr> <Tr> <Td> Lakes (see lake retention time) </Td> <Td> 50 to 100 years </Td> </Tr> <Tr> <Td> Rivers </Td> <Td> 2 to 6 months </Td> </Tr> <Tr> <Td> Atmosphere </Td> <Td> 9 days </Td> </Tr> </Table> <Tr> <Th> Reservoir </Th> <Th> Average residence time </Th> </Tr>

The total amount of water on the earth from year to year and the hydrologic cycle