Water is one of the most important natural resources, for without water, there would be no life on earth. Although there is plenty of water on earth, the supply of water available for use is limited because it may not always be in the right place at the right time in the right quantity or quality. We are constantly faced with increased consumption and demands on water supplies. In addition, evidence that record use and improper disposal of chemicals are making their way into available water supplies. The availability and future of water supplies are of great concern.
The water (hydrologic) cycle is a continuous process by which water is transported from the earth's surface (including the oceans) to the atmosphere and back to the land and oceans. The water cycle has no starting point for explanation, but since most of the earth’s water is in the oceans, that is a good place to begin. The sun is the driving force behind the water cycle. By heating the water in the oceans, some of the water evaporates as vapor into the air. As air heats and rises, it carries the vapor into the atmosphere. As vapor meets cooler temperatures, it condenses into clouds.
Additional vapors are added to the atmosphere as ice and snow are sublimated into water vapor. Sublimation occurs readily when conditions such as high altitudes, strong sunlight, low relative humidity and dry winds are present. Evapotranspiration (water transpired from plants and evaporated from the soil) is another method of water vapors entering the atmosphere. A large oak can evapotranspirate 110 gallons of water per day.
The vapor rises into the air where cooler temperatures cause it to condense into clouds. Air movement causes clouds to move around the earth. While moving, cloud particles collide, grow and fall out of the sky as precipitation in various forms. Snow and ice can accumulate as ice caps and glaciers which can store frozen water for thousands of years. Snowpacks in warmer areas can thaw in two ways. Slow melt allows water to seep into the ground and replenish ground water, while faster melt causes the water to flow overland as surface runnoff. Precipitation as rain also typically flows overland as surface runoff, but slow steady rains can also enter the ground to increase water table levels.
Some runoff enters rivers in valleys in the landscape, with streamflow moving water towards the oceans. As mentioned, some runoff 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 ground-water 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, with some eventually reentering the oceans.
There are many pathways the water may take in its continuous cycle of falling as rainfall or snowfall and returning to the atmosphere. They cycle for water may be short, or it may take millions of years. People tap the water cycle for their own uses. Water is diverted temporarily from one part of the cycle by pumping it from the ground or drawing it from a river or lake. It is used for a variety of activities such as households, businesses and industries; for irrigation of farms and parklands; and for production of electric power. After use, water is returned to another part of the cycle: perhaps discharged downstream or allowed to soak into the ground. Used water normally is lower in quality, even after treatment, which often poses a problem for downstream users.
Much of our water use is hidden. For example, it takes an estimated 1,500 gallons of water to produce the bread, meat, potatoes and drink in a fast-food meal. It takes an estimated 400 gallons of water to produce a pair of jeans. And, it takes approximately 32,000 gallons of water to produce an average car.
Not only people rely on water for life, but plants and animals do too. Many wildlife species live in, on, or near the water and require access to it throughout their lives. Other species may not use water as their primary habitat, but it is nonetheless essential to their well-being. In addition, many natural processes rely on water for their continuance. For example, Wind Cave would not have been developed were it not for water and its continual movement.
Hydrology: The Study of Water and Water Problems A Challenge for Today and Tomorrow; A publication of the Universities Council on Water Resources
USGS Water Resources
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