The protection, development, and efficient management of water resources for beneficial purposes. Nearly every human activity—from agriculture to transportation to daily living—relies on water resources and affects the availability and quality of those resources. Water resource development has played a role in flood control, agricultural production, industrial and energy development, fish and wildlife resource management, navigation, and a host of other activities. As a result of these impacts, natural hydrologic features have changed through time, pollution has decreased the quality of remaining water resources, and global climate change may affect the distribution of water in the future. Hydrology
Water availability varies substantially between geographic regions, but it is also affected strongly by the population of the region. Asia, for example, has an extremely large total runoff but the lowest per-capita water availability. In addition, nearly 40% of the world's population lives in areas that experience severe to moderate water stress. Thus the combination of water and population distribution has resulted in a large difference in per-capita water use between countries.
Worldwide, nearly 4000 km3 of water is withdrawn every year from surface and ground waters. This is a sixfold increase from the levels withdrawn in 1900 (since which time population has increased four times). Agriculture accounts for the greatest proportion of water use, with about two-thirds of water withdrawals and 85% of water consumption. It also accounts for a great proportion of the increase in water use, with irrigated cropland more than doubling globally since 1960. However, in Europe and North America particularly, industry consumes a large proportion of available water; industrial uses for water are anticipated to grow on other continents as well.
Land development has substantially affected the distribution of water resources. It is estimated that one-half of the natural wetlands in the world have been lost in the last century. In some areas, such as California, wetland loss is estimated to be greater than 90%. The vast majority of wetlands loss has been associated with agricultural development, but urban and industrial changes have reduced wetlands as well. River channels have also been altered to enhance irrigation, navigation, power production, and a variety of other human activities.
Ground-water resources have been depleted in the last century, with many aquifers or artesian sources being depleted more rapidly than they can be recharged. This is called ground-water overdraft. In the United States, ground-water overdraft is a serious problem in the High Plains from Nebraska to as and in parts of California and Arizona. Ground-water hydrology
Streams have traditionally served for waste disposal. Towns and cities, industries, and mines provide thousands of pollution sources. Pollution dilution requires large amounts of water. Treatment at the source is safer and less wasteful than flushing untreated or poorly treated wastes downstream. However, sufficient flows must be released to permit the streams to dilute, assimilate, and carry away the treated effluents. Water pollution
The availability of fresh water is also likely to be affected by global climate change. There is substantial evidence that global temperatures have risen and will continue to rise. Although the precise effects of this temperature risk on water distribution are challenging to predict, most models of climate change do anticipate increased global precipitation. It is likely that some areas, particularly those at mid to high latitudes, will become wetter, but the increased precipitation will be more seasonal than current patterns. Other areas are likely to receive less precipitation than they do currently. In addition, many models predict increases in the intensity and frequency of severe droughts and floods in at least some regions. These changes will affect natural stream flow patterns, soil moisture, ground-water recharge, and thus the timing and intensity of human demands for fresh-water supplies. Global climate change
Land management vitally influences the distribution and character of runoff. Inadequate vegetation or surface organic matter; compaction of farm, ranch, or forest soils by heavy vehicles; frequent crop-harvesting operations; repeated burning; or excessive trampling by livestock or wild ungulates all expose the soil to the destructive energy of rainfall or rapid snowmelt. On such lands little water enters the soil, soil particles are dislodged and quickly washed into watercourses, and gullies may form. Land-use planning Soil conservation
There are a variety of measures that can be taken to reduce water consumption. In the United States, for example, per-capita water usage dropped 20% from 1980 to 1995. In many cases, improvements to existing systems would contribute to additional water savings. In the United States, an average of 15% of the water in public supply systems (for cities with populations greater than 10,000) is unaccounted for, and presumably lost.
Improvements can also be achieved by changing industrial and agricultural practices. Agricultural water consumption has an estimated overall water use efficiency of 40%. More effective use of water in agricultural systems can be achieved, for example, with more efficient delivery methods such as drip irrigation. More accurate assessment of soil and plant moisture can allow targeted delivery of water at appropriate times. In industrial settings, recycling and more efficient water use has tremendous potential to reduce water consumption. Overall, industrial water usage dropped by 30% in California between 1980 and 1990, with some sectors achieving even greater reductions. Japan has achieved a 25% reduction in industrial water use since the 1970s. Additional potential to reduce this usage still exists even in locations where many conservation measures are already in place.
Residential water consumption can also be reduced through conservation measures. High-efficiency, low-flow toilets can reduce the water required to flush by 70% or more. Additional savings are possible with efficient faucet fixtures and appliances.
Water conservation in the United States faces a number of institutional as well as technological challenges. States must administer the regulatory provisions of their pollution-control laws, develop water quality standards and waste-treatment requirements, and supervise construction and maintenance standards of public service water systems. Some states can also regulate ground-water use to prevent serious overdrafts. Artesian wells may have to be capped, permits may be required for drilling new wells, or reasonable use may have to be demonstrated. Federal responsibilities consist largely of financial support or other stimulation of state and local water management. Federal legislation permits court action on suits involving interstate streams where states fail to take corrective action following persistent failure of a community or industry to comply with minimum waste-treatment requirements.
The watershed control approach to planning, development, and management rests on the established interdependence of water, land, and people. Coordination of structures and land-use practices is sought to prevent erosion, promote infiltration, and retard high flows (to prevent flooding). The Natural Resources Conservation and Forest Services of the Department of Agriculture administer the program. The Natural Resources Conservation Service cooperates with other federal and state agencies and operates primarily through the more than 2000 soil conservation districts.
Because watersheds often span political boundaries, many efforts to conserve and manage water require cooperation between states and countries. Many countries currently have international treaties addressing water allocation and utilization. In 1997, the United Nations adopted the Convention on the Law of the Non-navigational Uses of International Watercourses, which includes an obligation not to cause significant harm to other watercourse states, as well as provisions for dispute resolution. In addition, in 1996 the Global Water Partnership and the World Water Council were formed for the purpose of addressing ongoing international water concerns.
The increasing utilization of the continental shelf for oil drilling and transport, siting of nuclear power plants, and various types of planned and inadvertent waste disposal, as well as for food and recreation, requires careful management of human activities in this ecosystem. Nearshore waters are presently subject to both atmospheric and coastal input of pollutants in the form of heavy metals, synthetic chemicals, petroleum hydrocarbons, radionuclides, and other urban wastes. Overfishing is an additional human-induced stress. Physical transport of pollutants, their modification by the coastal food web, and demonstration of transfer to humans are sequential problems of increasing complexity on the continental shelf.
One approach to quantitatively assess the above pollutant impacts is to construct simulation models of the coastal food web in a systems analysis of the continental shelf. Models of physical transport of pollutants have been the most successful, for example, as in studies of beach fouling by oil. Incorporation of additional biological and chemical terms in a simulation model, however, requires dosage response functions of the natural organisms to each class of pollutants, as well as a quantitative description of the “normal” food web interactions of the continental shelf. Ecological modeling Food web
In addition to toxic materials introduced by oil spills, sewage, and agricultural and industrial run-off, coastal waters are vulnerable to thermal pollution. Thermal pollution is caused by the discharge of hot water from power plants or factories and from desalination plants. A large power installation may pump in 106 gal/min (63 m3/s) of seawater to act as a coolant and discharge it at a temperature approximately 18°F (10°C) above that of the ambient water. In a shallow bay with restricted tidal flow, the rise in temperature can cause gross alterations to the natural ecology. Federal standards prohibit heating of coastal waters by more than 0.9°F (0.5°C). Thermal ecology
Finally, dredging waters to fill wetlands for house lots, parking lots, or industrial sites destroys the marshes that provide sanctuary for waterfowl and for the young of estuarine fishes. As the bay bottom is torn up, the loosened sediments shift about with the current and settle in thick masses on the bottom, suffocating animals and plants. In this way, the marshes are eliminated and the adjoining bays are degraded as aquatic life zones. The northeast Atlantic states have lost 45,000 acres (182 km2) of coastal wetlands in only 10 years, and San Francisco Bay has been nearly half obliterated by filling. Dredging to remove sand and gravel has the same disruptive effects as dredging for landfill or other purposes, whether the sand and gravel are sold for profit or used to replenish beach sand eroded away by storms. The dredging of boat channels adds to the siltation problem, and disposal of dredge spoils is being regulated in coastal areas.