Earth's water is always in movement, and the water cycle, also known as the hydrologic cycle, describes the continuous movement of water on, above, and below the surface of the Earth. Although the balance of water on Earth remains fairly constant over time, individual water molecules can come and go in a hurry. Since the water cycle is truly a "cycle," there is no beginning or end. Water can change states among liquid, vapor, and ice at various places in the water cycle, with these processes happening in the blink of an eye and over millions of years.
The water cycle has no starting point. But, we'll begin in the oceans, since that is where most of Earth's water exists. The sun, which drives the water cycle, heats water in the oceans. Some of it evaporates as vapor into the air. Ice and snow can sublimate directly into water vapor. Rising air currents take the vapor up into the atmosphere, along with water from evapotranspiration, which is water transpired from plants and evaporated from the soil. The vapor rises into the air where cooler temperatures cause it to condense into clouds.
Air currents move clouds around the globe, cloud particles collide, grow, and fall out of the sky as precipitation.
Some precipitation falls as snow and can accumulate as ice caps and glaciers, which can store frozen water for thousands of years.
Snowpacks in warmer climates often thaw and melt when spring arrives, and the melted water flows overland as snowmelt.
Most precipitation falls back into the oceans or onto land, where, due to gravity, the precipitation flows over the ground as surface runoff.
A portion of runoff enters rivers in valleys in the landscape, with streamflow moving water towards the oceans.
Runoff, and ground-water seepage, accumulate and are stored as freshwater in lakes. Not all runoff flows into rivers, though.
Much of it soaks into the ground as infiltration.
Some water infiltrates deep into the ground and replenishes aquifers (saturated subsurface rock), which store huge amounts of freshwater for long periods of time.
Some infiltration stays close to the land surface and can seep back into surface-water bodies (and the ocean) as groundwater discharge, and some ground water finds openings in the land surface and emerges as freshwater springs.
Over time, though, all of this water keeps moving, some to reenter the ocean, where the water cycle "ends" ... oops - I mean, where it "begins."
BIBLIOGRAPHY
http://ga.water.usgs.gov/edu/watercycle.html
