San Francisco still guards against fire with 177 secret underground water tanks
A cistern is simply a sealed tank for storing water, usually rain, and the idea is at least as old as Neolithic villages in the Levant. The same technology sits under Roman courtyards, medieval castles, Bermuda's white roofs, every flushing toilet, and the streets of San Francisco.
What separates a cistern from a well is its waterproof lining, often hydraulic plaster. The word travelled from a Greek term for a basket through Latin cisterna, from cista, a box. Some of the earliest known examples are lime-plastered pits sunk into house floors at Neolithic sites such as Ramad and Lebwe; Jawa had them too before the fourth millennium BC was out. For dry-land farmers they were a basic tool of water management. At Tel Hazor in Israel a large cistern was carved into bedrock beneath a Late Bronze Age palace, and Iron Age royal centres added underground water systems that count among the earliest feats of urban engineering. Roman houses usually had one beneath the impluvium, which gathered, filtered, stored and cooled rainwater while also ventilating the home.
Medieval hill castles relied on them where wells could not reach deep enough. At the Riegersburg in Austrian Styria a filter cistern was cut into lava rock, with rainwater trickling through sand that cleaned it and added minerals.
San Francisco, long prone to catastrophic fires, voted money in 1850 to dig more than 100 cisterns for firefighting. Today its Auxiliary Water Supply System maintains 177 of them, each holding from 75,000 to over 200,000 US gallons, entirely separate from the mains so that they survive an earthquake that breaks ordinary pipes. Brick circles marked CISTERN S.F.F.D give them away at street level. Bermuda, Flanders and the U.S. Virgin Islands require rainwater harvesting in new buildings, and Bermuda's stepped white roofs funnel rain to tanks below.
In northeastern Brazil, the One Million Cisterns Program has helped smallholders in the semi-arid interior capture rain from rooftops into covered tanks of 16,000 litres, enough gathered over three or four wet months to cover drinking and cooking through the dry season. The government adopted the policy in 2013, and by 2016 some 1.2 million such cisterns served human consumption. Closer to home, every modern toilet uses a small cistern to measure out each flush; older designs mounted it high on the wall with a pull chain.
Source: Cistern
