A cistern is a covered tank in which rainwater and runoff are stored and can be constructed either above ground or beneath the ground depending on the water source and space limitations. In exceptionally dry areas where rainwater is scarce, cisterns are built beneath ground level using engineered collectors to divert, capture and collect rainwater runoff. Cisterns, like rainwater tanks need to be cleaned and maintained regularly, at least seasonally, with special attention to water quality and water treatment options for local users.