An irrigation system is nothing more than a means to efficiently water a controlled area. Although designs differ from contractor to contractor and from area to area, most irrigation systems operate in basically the same manner.

Water Supply

Every irrigation system has a point at which it gets its water from, known as the point-of-connection (POC). This can be a tap off of an existing house or building water supply pipe, its own dedicated city or county water meter, or a pump, if pulling water from a well or lake. In either case, this is where water feeding the entire system comes from.

Before water can be fed to the irrigation system it must first pass through a backflow preventer. This device will stop water in the irrigation pipes from backing up into the city or county water system in the event of a drop in city or county water pressure. 

The water pipe which feeds the irrigation system is known as the main line. When the system is active during the summer months this pipe is under constant water pressure. If the main line gets damaged, water will continue to flow through the break until the water gets turned off at the POC.

Zones  

Throughout the property, electric valves are installed at various points along the main line. When these valves are turned on at the irrigation controller they allow water to flow from the main line, through lateral line, to feed the irrigation heads that comprise a particular zone. A zone is just another name for an irrigated area. Most systems have at least three or four zones, large commercial systems may have fifty or more zones. A system is designed with numerous zones so that areas with different water requirements can be watered independently. This also allows for the irrigation heads to operate at their optimum water pressure.

Controller

The irrigation controller, or clock, is the "brain" of any system. It controls when the system comes on, which days it comes on, and how long each zone will run. There is a wire that runs from the controller to every electric valve on the system. When a zone is activated at the controller, it sends voltage through the wire to the electric valve to open the valve. When the zone is de-activated, the voltage to the valve ceases and the valve will close.

If a system is equipped with a rain sensor (which every system should be), the sensor's wires are connected at the controller. The sensor can be mounted on a deck, gutter, a pole, or anywhere that will mimic the environmental conditions of the irrigated property. The sensor, when rained on, will absorb the rain water and disconnect the circuit feeding the electric zone valves. This is what will prevent the system from coming on in the middle of or following a summer thundershower. When the rain sensor dries out sufficiently, it will automatically allow the system to come on again at the next programmed start time, with no action on the part of the person maintaining the system.

Heads

There are basically three ways to irrigate any given area - rotor heads, spray heads, or drip irrigation. Rotor heads are used to irrigate large areas, spray heads are used to irrigate small or irregular shaped areas, and drip irrigation is most commonly used to irrigate plant beds, or areas where over spray is a concern.  Most systems will have a combination of these types of irrigation. It is important that only one of the types is used on any given zone. Having spray heads on the same zone as rotor heads will lead to mis-matched precipitation rates, which leads to wet spots or dry spots on a given zone. It is also a recipe for wasting water, because you'll have to over-water some areas in order to put down enough water in other areas. A properly designed, installed, and programmed irrigation system will put down the same amount of water across all zones of similar types, just like natural rainfall.