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A cistern is a large vessel which is used to hold a reserve of water. Cisterns can be either above or below ground, and they come in a range of sizes and shapes, with varying features. Many people in dry or rural areas have cisterns to back up their regular water supply, and in some cases, a cistern is used as the primary source of water for a household. Several companies sell ready-made cisterns for people who want to install them, and others will custom build a cistern to exact specifications.
People have been storing water in cisterns for thousands of years. Many early cultures realized the value of saving rainwater, rather than allowing it to run off, and they built large jars and later big containers for the purpose of storing rainwater. Some cisterns could also be filled from springs or rivers. The cistern could be used to supplement water supplies during the dry seasons, and to water gardens.
In many regions of the world, you can still see gravity-fed cisterns. These cisterns collect rainwater with open tops or gutter systems, and they are mounted on platforms or towers which elevate them above a home or garden. The pull of gravity sucks water into irrigation or home plumbing, allowing the cistern to supply water without using power. A gravity-fed cistern's rate flow can be controlled with valves, or increased with the assistance of a pump.
When a cistern is used to store water for household use, it may have filters installed to clean the water before it reaches the home. Otherwise, people may need to boil the water or use water treatment tablets to make it drinkable. When used for irrigation, filters are not typically included, since plants are not as vulnerable to algae and other water contaminants.
A below ground cistern can be quite large, and tends to be more efficient than an above ground cistern, since it does not lose water through evaporation as the earth insulates it. However, setting up a gravity feed system for a below ground cistern is rather challenging, and many people must install pumps to get water out of a below ground cistern; in regions without electricity, people may use buckets to haul water up from the cistern. In both cases, the classic shape for a cistern is round or rectangular, although other shapes may be made to conform with unusual spaces. The cistern also typically has a cap or cover to keep animals, plants, and insects away from the water.