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Las Vegas WashThe Hydrologic Cycle
The hydrologic cycle is made up of four parts:
  1. Evaporation
  2. Transpiration
  3. Condensation
  4. Precipitation

Earth's waters are 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.

Hydrologic Cycle Interactive
See the water cycle in action.
Watershed Interactive
Study the parts of the watershed process.
The same water throughout history has been transferred from the oceans into Earth's atmosphere and then dropped upon land where it eventually moves 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 which then falls to the Earth. About 70 percent of this water evaporates back into the atmosphere. The water that doesn't evaporate remains on the surface and runs off into ponds, lakes and rivers or seeps into the soil to become 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, about five gallons of every 100,000 gallons of water on Earth 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.