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Wells and cisterns both provide water, but they do it in different ways. A cistern is a container which is used for the purpose of storing water; cisterns can be found in various sizes all over the world, both above and below ground. A well, on the other hand, taps into a supply of groundwater, such as a spring or underground stream. Wells are built by digging into the Earth and inserting reinforcements so that the sides of the well do not collapse.
Both cisterns and wells have been used for centuries. Many people historically took advantage of the sites of natural springs to build settlements, and as the springs started to fail, they dug out around them to tap the vein of groundwater which supplied the spring, creating early wells. To access the water in the well, people lowered buckets which could be filled with water and pulled up. Ultimately, electric pumps were developed to extract water from wells. Later, people sunk wells in areas that they thought would yield a good supply of ground water, with or without surface springs.
Classically, wells were hand-dug and lined with stone to prevent cave-ins. They were capped with small structures to prevent people and animals from falling in; aside from being unfortunate for the victim, a dead animal in a well would also spoil the water. Modern wells are dug with the assistance of drills, and they are usually lined with concrete rings which are pre-cast to match the diameters of the drills used to make wells. After a well is dug and lined, it is capped and topped with a pump to access the water inside.
A cistern, on the other hand, is designed to store water, not to tap an existing water source. Cisterns have classically been filled with rainwater, although people can also fill them by pumping wells into them or trucking in water. Cisterns have historically been used in dry regions to store water in the event that wells fail or rainfall becomes extremely irregular, and some people use them as a backup water supply which can be utilized in the event of an extended power outage which restricts access to the contents of a well.
Historically, cisterns were built above ground, and they used a gravity feeding system to supply water. In some cultures, cisterns are sunk into the earth, but although they are underground, they do not act as wells; someone still has to fill the cistern with water. Underground cisterns tend to lose less water through evaporation, since they are covered and insulated by soil, but they require the use of an electric pump or lowered bucket to access the water inside.
People in cities take advantage of a centralized plumbing system which is connected to a reservoir or river which is used as a source of water. Typically, the city's plumbing is attached to a water treatment plant which purifies the water before it reaches individual homes, making it safe to drink. Many of these systems rely on gravity and pressure to move water supplies, giving citizens access to water with or without electricity.
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