Irrigation System Design Concepts

One must have a grasp on a few basic irrigation design concepts to successfully design and evaluate irrigation systems. Attention to all these concepts is required for an irrigation system to be successful.

Where to Place Sprinkler Heads

It is important to understand how sprinklers work when designing irrigation systems. The circular area covered by a single sprinkler head does not irrigate uniformly. The amount of water applied near the outside of the circle is much less than that applied near the middle of the circle because as the sprinkler rotates, the stream near the outside of the circle covers much more ground than near the center. By the time a sprinkler applies 1/4^th inch of water on the outer edge of its circular coverage area, it might apply 3/4^th inch of water near the sprinkler head in the middle of its coverage area. For this reason, an ideal way to place sprinklers is to place them in a grid of squares with each head spaced a distance equal to the radius of the throw for the heads used. This is called “head to head” coverage because each head emits a stream all the way out to the four closest heads in the square grid.

Choosing Nozzles to Match Precipitation Rates

Assuming proper spacing of sprinkler and spray heads has already been determined, the next task should be choosing the right nozzle for each sprinkler and sprayer to achieve uniform coverage.

Matching precipitation rates of sprinkler heads is based on the combination of flow rate and irrigation pattern for each head. For example, a head in the middle of the yard that rotates 360° might need an 8 GPM nozzle. However, a head at the street might be adjusted to only rotate 180° so that you are not wasting half of its water on the road. In this case, the sprinkler head at the road will make two passes over the same semi-circular area in the time that it takes the sprinkler in the middle of the yard to go a full 360° applying twice the water to the semi-circular area bordering the road. Therefore, if the 360° head in the middle of the yard is an 8 GPM head, then the 180° head bordering the road should be a 4 GPM head to match precipitation rates for the two coverage areas. A head in the corner of the property set to only rotate 90° would only need to be a 2 GPM head so that precipitation rates in that corner area will match that of other heads.

Matching precipitation rates of spray head nozzles is easier as all the work is typically already done for you. Manufacturers offer Matched Precipitation Rate (MPR) nozzles allowing sprayers with different arcs and radii to be mixed on the same zone.

Pressure Losses in Irrigation Piping

There are many reasons that pressure within irrigation piping might change, but most pressure changes are due either to changes in the landscape elevation or friction in the pipes. Changes in elevation can lead to increase or decreases in water pressure within the piping. Friction in the pipes lead only to loss of pressure. While there are other causes of pressure loss such as valves, filters, backflow preventers, etc., elevation changes and friction in pipes account for the bulk of changes in pressure.

Pressure Losses from Elevation Changes

While the steepness of an elevation change in the landscape has no affect on pressure, the total rise or fall in elevation does. Piping water downhill will increase pressure within the pipe while piping it uphill will cause a loss in pressure. This loss or gain in pressure is not affected by the size of the pipe or flow rate.

Each vertical foot of change in elevation equates to 0.433 pounds per square inch (PSI) of water pressure. So regardless of whether there is a 10-foot drop in elevation over 50 feet or over 100 feet, there is still a 4.33 PSI pressure gain.

Pressure Losses from Piping Friction

While decreasing the size of a pipe will increase water velocity, it also increases the loss of pressure due to friction. For this reason, it is important for irrigation piping to be properly sized for adequate flow and pressure. There are equations such as Hazen-Williams or Scobey that can be used to determine pressure loss due to friction.

While not exact, the following table of recommended pipe sizes for a desired flow rate can be used but does not factor in pressure changes due to elevation or loss due to friction:

Once you know your desired flow rate, simply pick the first pipe size that can handle the desired flow rate. Based on the table above, a system with a desired flow rate of 32 gpm would require at least a 1.5 inch pipe. A system with a flow rate of 45 gpm would need a 2.0 inch pipe.