How do water towers work?

A water tower is a large tank raised high above the ground. Pumps send treated water up into it, usually when demand is low, and gravity pushes that water back down through pipes to homes when it is needed. The tower therefore does two jobs at once: it stores water, and it holds the pressure that moves water through a town's pipes. [1] [2]

Water pressure rises with depth, so water held high up pushes hard on the pipes below it. One source gives the figure of 0.43 pounds per square inch for every vertical foot, and says towns usually keep pressure between 50 and 100 pounds per square inch. A simple calculation then tells engineers how tall a tower must be. [3]

Pumps lift water into the tank, which gives it stored energy, called potential energy. When water leaves the tank, that becomes energy of motion. The U.S. Geological Survey describes towers as often sitting on high points in a town, so gravity can carry water through the pipes as far as the last house in a subdivision. [1] [4]

Water use is uneven. Practical Engineering notes that peak hourly demand in large cities can reach about five times the daily average, usually in the morning or evening. Meeting that with pumps alone would take much larger or more numerous pumps, which would be switched on and off constantly and would sit underused most of the day. [5]

With a tower, the pumps can run steadily. They fill the tank overnight and in other quiet periods. During the morning rush, the stored water adds to what the treatment plant supplies. Storage also lets the treatment plant and other upstream equipment be sized for average demand rather than peak demand. In one Minnesota city, the tower water turns over about five times a day. [5] [1] [2]

Because the water in a tower already has its stored energy, it can keep flowing during a power outage, at least until the tank empties. After that, pumps are needed again. Towers also keep a reserve for firefighting, and many municipalities and building codes set minimum water amounts that must be available to firefighters. [1] [5]

Some utilities also run pumps at night, when electricity can be cheaper, and let the tower carry the system during the day. Not every city uses towers. Some draw from sources already at high elevation, and others rely on pumps alone. Towers are costly, take up space, and can let water stagnate if it is not circulated enough. [5]