A water truck is simply a large carrier truck that has a water cistern attached to it. These water trucks are usually used for cleaning (such as spraying water down gutters), or for extinguishing fires (by hooking up hoses and spraying water from the cistern).

Proper Positioning

The most important step for filling a water truck is to make sure that it's positioned properly. The fill pipe comes down from the ceiling, and the manhole on the water truck is on top of the cistern. These two things need to be lined up as closely as possible. It may be helpful to stretch a rope down from the fill pipe to the manhole cover in order to check that they're properly lined up. It's also a good idea to have an outside observer watching the water truck as it's moved into position. The driver should keep the observer in view via the side mirror as well so that he knows when to stop the truck.

Filling the Cistern

Once the truck is in place it's time to fill up the cistern. The first part of this process is to make sure that the manhole on the top of the water truck is opened and that the cover is out of the way. The fill pipe needs to be opened slowly, and as the water falls you should watch to make sure that the water from the pipe is going mostly (if not all) into the cistern. There should be a fill gauge on the side of the water truck, and when that gauge reaches roughly 3/4, the fill pipe should be slowly closed. Ideally, by the time the rest of the water is drained from the pipe, there will be minimal spillover from the water truck's cistern.

The Hammer Effect

One thing to be very careful of when filling a water truck is known as the hammer effect. It refers to the fact that when water is moving down a pipe it builds up a great deal of force. If the fill pipe is opened too quickly then the water will be let out much too quickly. More importantly, if the fill pipe is closed too quickly, then the force of the water flow will be cut off all at once. That will mean that the built up force of the water will have nowhere to go. This could lead to burst pipes, damage to the water truck, and other building damage if the water is cut off too quickly.

Source:

Msha.gov: 768C Water Truck

Publiquip.com: Heavy Equipment Lexicon Water Truck