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SNWA

at the Wash
Water Cycle Interactive
PlayWatch the water cycle in action. Press the play button to begin.

The Hydrologic Cycle
The Earth's water is continually in motion in a process known as the hydrologic cycle. The process consists of water entering the atmosphere through either evaporation or transpiration and returning to the Earth's surface through condensation and precipitation.

The waters of our long history are the waters of today. The same water has been transferred time and time again from the oceans into Earth's atmosphere and then dropped upon land and ultimately, moved back to the seas. It evaporates from oceans, streams, lakes, rivers and other forms of surface water and rises into the sky as water vapor.

Water vapor comes together in the form of precipitation. About 70 percent of this water evaporates back into the atmosphere. About 10 percent of the total amount remains on the surface and runs off into ponds, lakes and rivers. The remaining 20 percent seeps into the soil and becomes groundwater.

The sun supplies the energy to keep the water moving from the Earth to the atmosphere and back to Earth. At any given time, only about five gallons of every 100,000 gallons of water is in motion. Viewed as a cycle of nature, this process has neither a beginning nor an end. No water is actually gained or lost, but the amount of water available to any user may fluctuate and changes in water quality.

In our geologic past, changes in this cycle produced ice ages across continents and created vast deserts. Today, even small changes in local weather cycle patterns produce floods or droughts that affect many people. When turning on the faucet for a glass of water, a person is tapping into the Earth's hydrologic cycle.