Cisterns are hollow excavations designed for the purpose of capturing and retaining rainwater, or for gathering water from aqueducts that flow intermittently. These structures are found in ranges such as barrens, steppe or desert, and are also constructed within urban areas in anticipation of extended sieges. They're not commonly found in regions with consistent and adequate rainfall, where water wells are common.
Cisterns vary in size, ranging from hundreds of gallons, sufficient to support a farm or a large residence, to tens of thousands of gallons, essential for the survival of an entire city. The size and quantity of cisterns are determined by the duration of the year when rain isn't expected to be a reliable water source.
Cisterns require waterproofing to guarantee a consistent water supply during the dry months. This involves the application of an inner layer of lime plaster, even when the cistern is carved out of solid rock. Homes constructed above cisterns can be designed to harness the cooling properties of the stored subterranean water, making it possible for houses in hot climates to maintain a comfortable temperature even during late afternoons.
Furthermore, the maintenance of cisterns is crucial. They must be securely enclosed, regularly attended to during dry periods, and periodically emptied entirely to ensure the purity and freshness of the water they contain.