The Water Cycle

The hydrosphere is the entire area where water occurs on a planet. Most of the Earth's water supply is contained in salt water oceans, where individual water molecules may remain for as long as 3,000 years, a measurement known as residence time. Water molecules in critical freshwater sources, such as lakes and streams, cycle much faster.

The cycle begins as the sun's energy warms and evaporates surface water from oceans, streams and lakes into the atmosphere, where it condenses to form clouds. Water then returns to the earth's surface as precipitation.

Now it can flow back into the ocean as runoff via streams and lakes, seep into the soil to be stored in aquifers, flow beneath the surface as groundwater, or evaporate again through transpiration by terrestrial plants.