Water towers are an engineering solution for increasing an area's water pressure. The basic principle is that water is pumped up into the water tower (essentially an elevated water tank) at night and then distributed to homes with an additional pressure boost from gravity. Large water towers are found in all sizes of large towns and small cities.

A Detailed Description

As stated before, water towers exist mainly to provide a pressure boost for municipal water systems. The force of gravity provides a boost of 0.43 PSI for each foot of height. A typical municipal water system runs at a pressure of 50 to 100 PSI (with 30 PSI as a bare minimum)¹. A few quick calculations suggest that a water tower needs to be at least 116 to 232 feet above groundlevel to provide adequate pressure. In order to minimize the required height of the actual structure, water towers are often located near high points².

A water tower's tank is typically sized to hold a day's supply of water. This allows for the water supply to an area to continue flowing with consistent pressure, even if the pumps fail temporarily. As a result, water towers can vary quite widely in volume. However, even smaller towns usually have a water tower with at least 1,000,000 gallons of water capacity.

Water towers also are necessary to handle the demands of peak usage on the system. A city's rate of water usage may vary significantly during a day. There are often surges in the mornings and evenings contrasted with minimal usage at night. Without a water tower's reserves, a city would need a pumping station that works above the rate of peak consumption to handle demand. With a water tower in place, a city can instead use a much slower (and cheaper) pumping system. Peak demand is supplied directly from the water tower, and the pump refills the water tower at night.

Stuff excerpted to another entry.