Agriculture and fishing have different needs for water, as do manufacturing industries, cities, and wildlife . Water allocation is the process of distributing water supplies to meet the various requirements of a community.
Determining how to allocate water supplies requires the consideration of certain factors, including the source of the water and methods for obtaining it. The cost of the water supply and water treatment systems are also taken into account, and the intended uses are reviewed. Water use is classified by whether it is an instream or a withdrawal use and by whether it is a consumptive or nonconsumptive use. Instream uses include navigation, hydroelectric power, and fish and wildlife habitats, while withdrawal uses remove water from the source. Consumptive uses make the water unavailable, either through evaporation or transpiration , or through incorporation into products or saltwater bodies. Water withdrawn but not consumed can be treated and returned to the water supply for reuse downstream.
The demand for water is determined by combining the sum of the amount of water required for instream needs with the sum of the amount of water needed for withdrawal uses. This sum is compared to the amount in the water supply. If it is equal to or greater than the sum, then the supply will meet the demand, but if this is not the case, measures to reduce consumption or increase the water supply need to taken.
About 70% of water supplies worldwide go to food production. Industry uses 23% and cities use about 7% of water supplies. In the western United States, for example, about 85% of the water supplies are currently used by farming operations at government-subsidized rates. Cities and industry as well as populations of fish and wildlife split the remaining 15%. Some politicians and environmentalists consider this division unfair, and assert that water allocation should be controlled by environmental awareness. They suggest seeking new ways of getting more water to more people while protecting wildlife and wetlands .
Government agencies own most of the water supplies in the western United States, and they own about 65% of the supplies in the east. Those who control the water supplies govern its use, though regulations on ownership vary by state. In western states, supplies are on a first-owned, first-served basis, even if others later run short. The distribution of supplies and shortages is shared more equally in the east.
Some water allocation programs look toward conservation to meet water needs. Certain ordinances, in California for example, restrict and limit water usage. One method of enforcing compliance that has been used there is publishing the names of people who do not abide by the ordinances to embarrass them. Another method uses a special section of the police force for checking water use. These officers give first-time offenders information on water conservation . They fine repeat offenders.
Wells , dams , reservoirs, conveyance systems, and artificial ponds physically control water supplies. Wells draw out groundwater . The other constructions manage surface water. Some areas use desalinization techniques to get fresh water from seawater.
A well taps groundwater through a hole sunk to an aquifer .An aquifer is a permeable layer of rock containing water trapped by layers of impermeable rock. If the pressure is sufficient, the water will come to the surface under its force, a type of well known as an artesian well . If the pressure is not sufficient for an artesian well, a pump is used to mechanically drag the water to the surface.
Dams are constructed to halt some or all of a river's flow. The region behind the dams, where water collects, is called an impounding, or water collecting, reservoir . Each dam is designed for a specific use, though dam types may be combined depending on the river and the community's needs. Storage dams hold water during times of surplus for periods of deficit, such as holding the spring runoff for the summer dry season. Along with providing a water supply, storage dams can improve (or harm) habitats for fish and wildlife, create electricity, and help in irrigation and flood control. Diversion dams affect the course of a river, preventing it from flowing forward so that a conveyance system can redirect it to irrigate fields or to fill a distant storage reservoir. These are often used in the western United States, resulting in damage to downstream areas. Lack of water can destroy fisheries and wildfowl habitats, and shorelines and deltas erode when no sediment comes downstream to replace what the ocean washes away. Detention dams control floods and trap sediment. Floods are controlled to prevent destruction of communities and habitats downstream from the dam. Trapping sediment causes the downstream portion of the river to be cleaner.
Conveyance systems divert streams so they flow where humans most need them. Conveyance systems include ditches, canals, and aqueducts (also known as transmission stations). Artificial ponds store water that is pumped in using a conveyance system. Aqueducts consist of closed conduits or pipes. A closed system helps to prevent contamination. A pump system helps the flow when the water level is not high enough for gravity to take over. Consumer demand for water varies by day and by season, and enclosed ground reservoirs and water towers may be used to store treated water and meet peak demands.
Only about 3% of the earth's water is fresh, and of that, 23% is available for use as surface water and ground-water. The rest is in the unusable form of glacial ice. The amount of freshwater is not increasing with the global population, and about a third of the people in the world now live in countries with water problems. These facts make the equitable allocation of water more difficult, and the need for wise water management more acute. Worldwide shortages also make it important for water management to control the quality of water. Farming runoff, urbanization, and industry all reduce water quality , which causes health and environmental problems, including algal blooms caused by nitrogen-rich fertilizers, and acid rain . One survey done by the Environmental Protection Agency (EPA) found that over half of the rivers, lakes, and streams in use in the United States have been nearly or totally destroyed by pollution . Pollutants have also made it difficult and expensive to recover and purify groundwater. Purification treatments in general can be expensive or difficult to obtain, and this causes poor water quality in many parts of the world.
Competing needs have also created severe problems; including lowered water pressure, decreased stream flow, land subsidence and increased salinity . Additionally, over-grazing , deforestation , loss of water-retaining topsoil , and strip mining decrease the amount of available water.
Treating sewage before returning it to the water supply and properly disposing of chemicals can improve water quality and increase the amount of usable water available. Reducing consumption can prevent water shortages. Households and industries can lower their water consumption by employing devices that decrease the amount of water used in, for example, showers, toilets , and dishwashers. Farmers can lower their water consumption by growing drought-resistant crops instead of ones requiring large quantities of water, such as alfalfa, and by using more efficient watering techniques. These methods not only stabilize water supplies, they also reduce the cost to the consumer. Another method for use in water shortages is desalinization of sea water. Desalinization plants remove the salt from sea water and purify it. Although such plants can be expensive, the expense can prove to be less than attempting to pipe fresh water in from sources already strained.
[Nikola Vrtis ]
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