Knowing the amount of water your sprinkler system applies to your landscape is an important step in efficient water use. Most people irrigate their landscape for a given number of minutes without knowing how much water they are really applying. This may lead to over- or under-watering, neither of which will benefit the landscape nor the environment. A catch can test is used to determine how long to run an irrigation system or hose-end sprinkler and how well the water is distributed over the landscape.
The root zone (where water and nutrient absorbing roots grow) is typically 6 inches deep in clay soil and 8 to 10 inches in sandy soil. Usually 1 inch of water will fill this root zone but in many cases irrigation systems apply water faster than the ground can absorb.
Calibrating will help you to apply the correct amount of water to your landscape. Whether you have an in-ground system or a hose and a sprinkler, the following steps will calibrate your system:
Place 5 to 9 catch cans (tuna or cat food cans work great) in one zone at a time. Scatter the cans at random within the zone. Repeat the entire procedure in every zone because there may be differences in the irrigation rates. If you use a hose-end sprinkler to water your landscape, place the containers in a straight line from the sprinkler to the edge of the watering pattern. Space the containers evenly.
Run each zone for 15 minutes.
To determine how much water is applied in each zone measure the amount of water in each catch can. Use a ruler to measure the depth of water in each container. Note: The more precise the measurement, the better your calibration will be. For most cases, measurements to the nearest 1/8 inch are adequate.
Look for large differences in water amounts between cans. For example, if one has 1/2 inch or more and other cans are nearly empty, you know that your coverage is not uniform and your system needs to be inspected further.
Find the average depth of water collected in the containers (add up the depths and then divide by the number of containers).
To determine the irrigation rate in inches per hour, multiply the average depth of water times four. For example, if you collect an average of 1/4 inch in 15 minutes, and your target application rate is 1/2 inch, you will need to run your irrigation system for 30 minutes.
Typically 0.75 to 1 inch is enough for one watering cycle.