Cisterns

As simple as a barrel, but more commonly an in-ground or basement tank, cisterns capture rainfall from rooftops and other hard surfaces. Often in abundant supply, this soft water lacks the mineral salts common to ground water and is ideal for irrigation. Cisterns provide a substantial proportion of monthly water requirements— especially in remote areas.

As much as 90 percent of the annual rainfall on the roof can be collected, depending on the gutter and downspout system. Every 2.5 centimeters of rainfall provides approximately 20 litters of water per square meter. The cistern is best constructed of high density concrete, vibrated as it is cast in place, and allowed to wet cure before being put to use. A tight, opaque cover should avoid problems with mosquitoes or algae. Using a crystallizing, cementitious sealer on the inside after curing will minimize leakage. Manufactured polyethylene tanks are also available and may be suitable for use as cisterns and storage tanks.

Water storage in a cistern also reduces the flooding impact of storms, lowering urban runoff. Cistern water storage can be an integral part of a storm water management plan. However, it does require a system of screens and sand filters to stop leaves and debris from entering the cistern.