Pouring With Answers

By Adam Thoms

Although high summer temperatures just ended, anytime is a good time to make sure your irrigation system is running at its peak potential. An irrigation audit ensures you are not wasting water but also supplying an even coverage of water.

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Pre-Inspection
This is also a great time to review run times for programs and heads on an irrigation controller. Before beginning an audit, watch all of the irrigation heads run for any issues, including:

  • Low heads that can’t spray through the turf canopy

  • Blocked nozzles

  • Irrigation heads that don’t run

  • Heads that are not turning or are watering concrete

  • Heads with too much or too little pressure for proper operation.

After watching each head run, make any corrections necessary.

This is also a great time to evaluate the site and note changes in soils or slopes that may affect water-infiltration rates, or different plant material covered by each irrigation head. Also document any abnormalities between irrigation heads, like different manufacturers or nozzles in heads that will change water output or irrigation throw distance. Finally, note if the irrigation heads are uniformly spaced, and mark any spacing differences. If heads are spaced too far apart, some areas will receive half-rates of water; if the heads are too close together, the result will be an excessive overlap of water. To correct these spacing issues, you may have to adjust nozzles, change irrigation heads, or reinstall the irrigation lines.

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The Audit
After doing a pre-inspection of the system, it is time to gather output information from the irrigation heads. The wind speed should be five miles per hour or less. Make sure to check the wind speed every five minutes during the audit. High wind speeds will distort the irrigation pattern and give skewed results. After determining the wind speed is acceptable, use a pressure gauge, like a pitot gauge, to check the operating pressure of every head. Pressures should be within manufacturer specifications for every irrigation head. Variance in pressure between heads will change the precipitation rate, which can affect run times for various zones.

Now it is time to determine precipitation rates and irrigation uniformity. You will need at least 24 identical containers or catch cans to capture water from each irrigation head. Containers with wide throats allow for shorter run times of the heads. Catch-can spacing will vary on how far apart the sprinkler heads are spaced. If the heads are 15 to 40 feet apart, the cans should be a third of the sprinkler spacing apart, and if the heads are 40 feet apart, then the cans should be a fourth of the sprinkler spacing apart. So for example, if a sprinkler is 40 feet apart, a can should be every ten feet. If the head is a full-circle head, the first row should be in line with the sprinkler head being tested, but if the head is a partial-circle, the can should be offset two feet in the direction of the spray pattern. Once the catch cans are set, it is time to run the test. Make sure to only run one zone at a time; this will help identify any problems.

A spray sprinkler should run for three to six minutes, a rotary for 10 to 20, and a larger sprinkler for five revolutions. A good rule of thumb is to catch 1.5 times the area of the catch device in milliliters. The goal should be to have minimal variation between irrigation heads. If you have more than 10 percent variation between catch cans, consider changing nozzles. Keep in mind that an irrigation head that only turns 90 degrees should have a nozzle that is one quarter of the gallons per minute as a 360-degree irrigation head if on the same zone. In the amount of time it takes the full-circle head to complete one rotation, the 90-degree irrigation head will have covered the same area four times.

Analyze The Data
After collecting an average amount of water captured, find out how much precipitation is being applied in a set amount of time:

Precipitation rate in inches per hour = (3.66 which is a constant X average catch can volume in milliliters).

(Amount of time the system was running in minutes X Area of the throat of the catch device in square inches).

This will give you an idea of how many inches of water an irrigation system is applying per hour. Keep in mind that most turfgrasses need about one inch of water per week to keep active growth. Those water needs can vary depending on soil, weather, height of cut, and other variables. Use this information to schedule irrigation so you are applying the correct amount of water.

It is always good to keep records so you can see when it is time to replace irrigation components, and to answer any questions the public may have on how much water you are applying. Ensuring your irrigation system is performing properly can minimize wasted water, and improve the health of the plants.

Adam Thoms, Ph.D., is an assistant professor specializing in commercial turfgrass management for Iowa State University’s Department of Horticulture. Reach him at athoms@iastate.edu.