The Water Cycle - Nature's Recycling System
The definition of recycling is "to pass through a cycle or a part of cycle again" (Webster's New World Dictionary - Third College Edition, 1988, Simon and Schuster, NY). Nothing fits that definition better than the HYDROLOGIC, or water, cycle. The hydrologic cycle is a perpetual motion--a natural process of water molecules recycling from the land, to the air, and back to the land.
The sun's energy warms water, which is transferred as vapor from the oceans, seas, and land masses into the atmosphere. Once in the atmosphere, the vapor is formed into clouds. The clouds are carried by weather patterns, which are influenced by the topography of the Earth's surface. Sometimes vapor condenses as fog, mist, or clouds, and sometimes it falls to the Earth as precipitation, where it is accumulated in surface water and the soil. Then the process of recycling, or returning the water back into the atmosphere, continues.
The key processes in the hydrologic cycle are evaporation, transpiration, precipitation, and infiltration. Other processes are respiration and combustion.
To trace the movement of the water through this cycle, begin at the far right where the sun's energy is evaporating water from the sea into the atmosphere. As vapor leaves the oceans and the drier lands, it leaves behind minerals, like salts, which can make land inhospitable. But in the oceans, it is only a part of a natural process without damage to the sea life.
The unseen vapor then joins the procession of water molecules on a journey that will lead them back to the land or water as precipitation. The precipitation may take any one of several forms, but will always start out as frozen water. The molecule attaches itself to some sort of airborne debris and is then tumbled and tossed toward the Earth's surface. Whether the water ends up as a raindrop, snow crystal, or hail, is dependent upon the season, location, and climate.
Not all the water will reach the Earth. Some will evaporate on the way between the sky and the land and then return to the atmosphere to start the process over again.
Once the water reaches the Earth, it will either run off across the land surface, infiltrate (fill the pore spaces between individual soil particles), or fall into bodies of water.
Runoff is intercepted by conservation practices, such as small dams, terraces, and grassed waterways. These practices allow the water to either infiltrate or be retained as surface water.
Small amounts of water are retained and held by plants, buildings, automobiles, machines, and other structures until they evaporate back into the atmosphere. As automotive and other engines do their job of powering vehicles, some of their waste material is also water vapor being expelled into the atmosphere through the process of combustion or burning. And animals respire water vapor when they breathe.
Most of the water infiltrates into the soil. Some water will be drawn up by plant roots, then transpired, or given off through the leaves, back into the air as water vapor. Other water will move slowly into underground aquifers, percolating through the soil into the bedrock. Eventually, through wells or irrigation, the groundwater may be drawn back up and used.
Other water will move slowly through the soil and bedrock until it reaches the surface in the form of a seep, spring, or artisan well.
Excess water will run off across the land's surface into various bodies of water, taking with it precious soil and anything that was attached to individual soil particles. Then the evaporation process--like transpiration, respiration, and combustion--begins anew. And the never-ending recycling of water continues.
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