Water is one of the most vital natural resources for all life on Earth. The availability and quality of water always have played an important part in determining not only where people can live, but also their quality of life. Even though there always has been plenty of fresh water on Earth, water has not always been available when and where it is needed, nor is it always of suitable quality for all uses. Water must be considered as a finite resource that has limits and boundaries to its availability and suitability for use.
The balance between supply and demand for water is a delicate one. The availability of usable water has and will continue to dictate where and to what extent development will occur. Water must be in sufficient supply for an area to develop, and an area cannot continue to develop if water demand far outstrips available supply. Further, a water supply will be called upon to meet an array of offstream uses (in which the water is withdrawn from the source) in addition to instream uses (in which the water remains in place). Figure 1 represents the demands on water as a tug-of-war among the various offstream and instream uses.
Traditionally, water management in the United States focused on expanding or manipulating the country's supplies of fresh water to meet the needs of users. A number of large dams were built during the early twentieth century to increase the supply of fresh water for any given time and place. This era of building large dams has passed. In the twenty-first century, the finite water supply and established infrastructure require that demand be managed more effectively within the available sustainable supply. Water-use information can be used to evaluate the impacts of population growth and the effectiveness of alternative water management policies, regulations, and conservation activities.
Water is constantly in motion by way of the hydrologic cycle. Water evaporates as vapor from oceans, lakes, and rivers; is transpired from plants; condenses in the air and falls as precipitation; and then moves over and through the ground into waterbodies, where the cycle begins again. *
The water-use cycle is composed of the water cycle with the added influence of human activity. Dams, reservoirs, canals, aqueducts , withdrawal pipes in rivers, and groundwater wells all reveal that humans have a major impact on the water cycle. In the water-use cycle, water moves from a source to a point of use, and then to a point of disposition. The sources of water are either surface water or groundwater. Water is withdrawn and moved from a source to a point of use, such as an industry, restaurant, home, or farm. After water is used, it must be disposed of (or sometimes, reused). Used water is either directly returned to the environment or passes through a treatment processing plant before being returned.
U.S. Water Withdrawals in 1995
The U.S. Geological Survey compiles nationwide water-use data every 5 years. The year 1995 is the last year for which full datasets are available.
As Figure 2 shows, the United States in 1995 used about 402,000 million gallons of water per day (MGD). Most of this water use was fresh water, about 341,000 MGD, or about 85 percent, while 60,800 MGD was saline water. The majority of fresh-water withdrawals, about 264,000 MGD, or about 77 percent, came from surface-water sources, while about 76,400 MGD came from groundwater. The source of saline water is surface water.
Even though discussions of water use typically focus on fresh-water use, saline water use also is important in the United States. Some categories of water use, such as thermoelectric, industrial, and mining, use saline water, mainly for cooling generators in thermoelectric power plants. Industries and mines use saline water to cool machinery and to wash and transport products, mainly for cooling of machinery.
Categories of Water use
The U.S. Geological Survey categorizes water use for analyzing current patterns and predicting future trends.
Commercial water use includes fresh water for motels, hotels, restaurants, office buildings, other commercial facilities, and civilian and military institutions. Domestic water use is probably the most important daily use of water for most people.
Domestic use includes water that is used in the home every day, including water for normal household purposes, such as drinking, food preparation, bathing, washing clothes and dishes, flushing toilets, and watering lawns and gardens.
Industrial water use is a valuable resource to the nation's industries for such purposes as processing, cleaning, transportation, dilution, and cooling in manufacturing facilities. Major water-using industries include steel, chemical, paper, and petroleum refining. Industries often reuse the same water over and over for more than one purpose.
Irrigation water use is water artificially applied to farm, orchard, pasture, and horticultural crops, as well as water used to irrigate pastures, for frost and freeze protection, chemical application, crop cooling, harvesting, and for the leaching of salts from the crop root zone. Nonagricultural activities include self-supplied water to irrigate public and private golf courses, parks,