<P> In most areas of the United States, contractors who need temporary water may purchase permits to use hydrants . The permit will generally require a hydrant meter, a gate valve and sometimes a clapper valve (if not designed into the hydrant already) to prevent back - flow into the hydrant . Additionally, residents who wish to use the hydrant to fill their in - ground swimming pool are commonly permitted to do so, provided they pay for the water and agree to allow firefighters to draft from their pool in the case of an emergency . </P> <P> Municipal services, such as street sweepers and tank trucks, may also be allowed to use hydrants to fill their water tanks . Often sewer maintenance trucks need water to flush out sewerage lines, and fill their tanks on site from a hydrant . If necessary, the municipal workers will record the amount of water they used, or use a meter . </P> <P> Since fire hydrants are one of the most accessible parts of a water distribution system, they are often used for attaching pressure gauges or loggers or monitor system water pressure . Automatic flushing devices are often attached to hydrants to maintain chlorination levels in areas of low usage . Hydrants are also used as an easy above - ground access point by leak detection devices to locate leaks from the sound they make . </P> <P> Fire hydrants may be used to supply water to riot control vehicles . These vehicles use a high pressure water cannon to discourage rioting . </P>

Where does the water from a fire hydrant come from