WATER CONSERVATION

People require clean, fresh drinking water. People also use water for bathing, cooking, and cleaning. Farmers need water to irrigate dry croplands. Industries use water to produce electric power and in the manufacture of many products. Water is also important in recreation and transportation. The demand for water is constantly increasing as a result of population growth and the expansion of agriculture and industry. The earth has an abundant supply of water, but the water is unevenly distributed. Some areas do not receive enough rainfall, while others get more than they need. Many dry regions of the world, such as the Middle East, North Africa, and parts of western and central North America, face serious water shortages.

In some areas, people obtain water by drilling wells to tap underground supplies.

But in parts of the western and central United States, farmers have drilled so many wells to provide water for irrigation that the level of the ground water has been greatly lowered. Many cities have also used up much of their ground water. In some cases, underground reservoirs can be refilled by pumping in water during periods of heavy rainfall.

Some rural areas and cities obtain water by damming rivers to create reservoirs. Engineers also build dams to control flooding. But in many cases, the construction of new dams to meet ever-increasing demands for water or to reduce flooding threatens wildlife. For example, during the late 1970's, conservationists opposed the construction of the Tellico Dam on the Little Tennessee River because of its possible harmful effect on a rare species of fish called the snail darter. The dam was completed in 1979, and some snail darters were transferred to the Hiwassee River in Tennessee in an attempt to ensure their survival.

A dam may harm certain fish because it reduces the flow of water. Dams can also prevent such migrating fish as salmon from reaching their breeding grounds. As the land behind a dam becomes flooded, the water destroys some wildlife habitats. Certain streams should not be dammed because they carry too much silt. On such streams, reservoirs quickly fill up with silt and can no longer store water.

People also deplete lakes to provide water for irrigation. Such irrigation practices have caused the Aral Sea, a saltwater lake in central Asia, to shrink to about 40 percent of its original size.

Water supplies for cities and farms can be increased partly through watershed management (the management of vegetation to prevent rapid runoff of rain and melting snow). Trees and other plants keep water from running off the land and so enable it to filter into the ground. The water thus refills underground supplies and flows through underground channels into lakes and streams. The destruction of plant cover disturbs this natural cycle. Rain water runs off the land rapidly instead of filtering into the ground. Watershed management also helps reduce flooding and soil erosion.

Some cities near seacoasts meet part of their water needs by desalting seawater. This process, called desalination, requires massive amounts of energy and is thus expensive. For this reason, desalination is not yet practical on a large scale. But solar energy may one day provide enough cheap power for large-scale desalination.