Water tower. The large tank on top of the tower is kept full of water to maintain water pressure in the water supply system in the surrounding area. The amount of pressure that is provided depends on the height of the tower. A typical urban water supply needs a pressure of 35 to 70 tonnes per square metre. This is equivalent to 35-70 metres of water, requiring a water tower of that height. Photographed in a suburban area of Milwaukee, Wisconsin, USA.