A water tower is a structure designed to hold a reservoir of water and also to help maintain the water pressure in a municipal water system. These structures have been used for centuries to hold supplies of water, and they are capable of providing water even in the event of a power outage or pump failure, because they are elevated, meaning that the water will pressurize the pipes with the assistance of gravity. Many areas use water towers as backup water supply systems to ensure that citizens have access to running water, and most small towns can store up to a day's worth of water in this way.
For every foot (0.3 meters) above the ground, a water tower is capable of generating more pressure. Typically, one is installed on raised ground, and the tank of the tower is elevated to increase the potential pressure. In a small town, a single tower can pressurize enough water to supply the entire town. In large cities, tall private buildings sometimes use them to supply their tenants, since the city's water system is not pressurized enough to get water to the top floors in peak periods of demand. When used in combination with a municipal water system, the pressurizing ability of this structure serves two primary functions.
The first function is to maintain constant pressure in the system and prevent negative pressure, which can suck groundwater or other sources of pollution into the water supply, contaminating the town's water. A town with a water tower, however, can rest assured that the water lines will be fully pressurized at all times, keeping the water safe to drink. In some areas, a tower must be attached to the municipal water system for this reason.
The second function is more economic. A water tower can deliver water at periods of peak demand relatively easily, while a pump may have difficulties. By using a pump and a tower, a town can save costs by pumping for average demand. When demands for water outstrip the abilities of the pump, the tower kicks in to supply more water. When the pump is providing more water than the town needs, as often happens at night, the excess fills the storage tank so that it is ready for the next period of peak demand.