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On and below the earth's surface, water flows in a continuous cycle from the ocean to the land and back to the ocean again. The total amount of water flowing through this cycle is estimated to be over 100 million billion gallons per year.

Beginning with the ocean, water vapor is evaporated from the ocean surface and is carried by the circulation of the atmosphere over land surfaces. The water vapor is then transferred to the land surface by precipitation where one of many things can happen to it:
It can:
-evaporate back into the atmosphere
-be intercepted and used by vegetation
-be transpired back into the atmosphere by vegetation.
-infiltrate into the ground
-be stored temporarily in glaciers
-run off into streams
The water eventually returns to the oceans to repeat the cycle over again.