Storage
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Water storage facilities allow the later use of runoff water as supplemental irrigation water. They can partly overcome the problem of the unreliability of rainfall. This also allows the prolongation of the cropping season or a second crop. The storage can be aboveground or underground. Surface storage, for example in ponds, has the disadvantage of evaporation losses as well as siltation and pollution. These drawbacks can be avoided by underground storage in cisterns, that can keep the water losses by evaporation and percolation minimal, if they are covered and have plastered walls. Siltation can be minimised by reducing erosion, or by installing a silt-trap through which the runoff passes before it flows into the storage tank. Storage facilities not only prevent the loss of runoff during periods of heavy rainfall but also play an important part in reducing floods and controlling soil erosion. Storage reservoirs include various types of surface and sub-surface tanks, rock catchment dams, earth dams, and sub-surface or sand dams in sand rivers. Surface tanks can be made of ferrocement, bricks, reinforced concrete, metal, plastic, fibreglass and wood. Sub-surface tanks are usually made of ferrocement, concrete, brick and traditional clay linings. Furthermore, water harvesting dams can be built of soil, rock or sand.
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