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| The Hydrologic Cycle. Photo Credit: Enchanted Learning |
The Hydrologic Cycle
The continuous exchange of water between the earth and the atmosphere is known as the hydrologic cycle or water cycle. The physical properties of water allow it to change form and move from the earth to the atmosphere and back again.
Most of the liquid water on earth is found in the oceans, amounting to 97.2% of all water. Approximately 2% of water is found in solid form as glaciers and ice caps. The remaining water, with the exception of a small amount of water vapor in the atmosphere, is found underground, or in lakes or streams. Of the fresh water on earth, only 3% is surface water. The rest is found underground and may be accessible only through wells.
Water moves through the hydrologic cycle by different processes:
- Precipitation occurs when water vapor in the atmosphere condenses on dust particles in the air forming water droplets. As these droplets concentrate in the air, clouds are formed. Precipitation occurs when the air becomes saturated and can no longer hold the water droplets. They may then fall to the earth in the form of rain, snow, sleet, or hail depending upon temperature.
- Runoff occurs when precipitation reaches the earth and is not absorbed into the ground. The precipitation will then run over the surface of the land.
- Storage of water occurs when runoff reaches the lowest point on the land surface. Lakes and streams may be formed and provide temporary storage of the surface runoff. Over time, the stored water may seep underground to be stored in aquifers.
- Evaporation and transpiration of water recharge the atmosphere with water that may then fall again to earth in the form of precipitation. This process completes the hydrologic cycle.
Additional Resources:
Northwest
Basic Concepts of Groundwater Hydrology
Drought/Conservation
Midwest
The Hydrologic Cycle
Ohio's Hydrologic Cycle
Southeast
Florida's Water Resources
Water Management
Southwest
Climate Change Impacts in the Southwestern U.S.
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