Cistern

Cisterns are excavated holes designed to catch and store rainwater, or to collect from aqueducts that may only run seasonally. They are found in ranges such as barrens, steppe or desert, and may also be built in cities in preparation for long sieges. They aren't likely to be found in climates that experience sufficient rainfall to maintain a reliable water table, where water wells are effective.

Cisterns range in size from hundreds of gallons, which might sustain a farm or a large house, to tens of thousands of gallons that enables the survival of a city. The size and number of cisterns depends on how much of the year is expected to occur without rainfall.

Construction

Cisterns must be waterproofed to ensure supply throughout the dry months. This calls for an inner layer of lime plaster, even if the cistern is excavated from solid rock. Houses built atop cisterns can be designed to advantage the standing, subterranean water's cooling properties, allowing a house in a hot climate to remain comfortable even in the late afternoon.

Cisterns must be kept clean. They must be well-enclosed, regularly maintained when dry and occasionally emptied entirely to ensure that all the water within is pure and fresh.