I don't know how it's done in other municipalities, but...
In New York City, you have to go to a neighborhood firehouse and get a "sprinkler cap" (essentially a nozzle cover with holes bored into it) attached to the hydrant you want to use.
Some hydrants (especially in residential neighborhoods) have special locks on the valve stem so that only the fire dept can un-do them. Others, you can un-do with a simple spanner.
The purpose of the sprinkler cap is to keep the pressure in the acqueducts from going down (and to avoid having small kids getting knocked on their kiesters by the high-pressure jets of water).
