Earth's Water Cycle

Water Cycle Definition and Reservoirs

The water cycle, also called the hydrologic or hydrological cycle, is the continuous movement and transformation of water on, above, and below Earth's surface. Water moves between reservoirs through processes such as evaporation, transpiration, condensation, precipitation, infiltration, runoff, sublimation, and subsurface flow. During this cycle, water changes among liquid, solid, and vapor phases, while Earth's total water mass remains relatively constant. The major reservoirs are oceans, glaciers and ice caps, groundwater, lakes, rivers, soil moisture, the atmosphere, and living organisms. Oceans contain about 97% of Earth's water and provide most global evaporation, while ice sheets and glaciers store most of the planet's freshwater. Water may remain in reservoirs for very different lengths of time, from about nine days in the atmosphere to thousands of years in deep groundwater and ice. Solar energy drives evaporation and other phase changes, while gravity drives precipitation, runoff, infiltration, and groundwater movement.

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Solar Energy and Phase Changes

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