Example of calculating irrigation run-time with the jar test

If you have a spray or rotor system, you can determine the precipitation rate using a simple “catch can” method. The simplest method is to use 15 to 20 jars with wide mouths and straight sides, such as cat food cans, tuna cans, or pint-size wide-mouth mason jars. Many containers will work, as long as the opening is the same size as the rest of the container and you have a large number of matching containers (see image). One convenient option is to buy a 12-pack of wide-mouth pint size (NOT quart size) mason jars. To test your precipitation rate, place the identical jars or cans in roughly a grid in one zone of your yard, ensuring that some cans are placed close to the sprinklers and that some are placed in the center of the irrigation areas. Run the corresponding zone of your irrigation system for 10 minutes. After 10 minutes, measure and record the depth of water in each container (in inches or part-inches) by using a ruler either along the side of the jar, or dipped into the can. Round to the nearest one-eighth of an inch. Add up all the recorded numbers (keeping units in inches) and divide the total inches by the number of jars or cans you put out to get the average depth. Divide that number by 10 (because the system ran for 10 minutes) to calculate the precipitation rate in inches per minute. See below for how to use this information to calculate your run-time. Before getting started, briefly run the irrigation zone you will be measuring to make sure there are no clogged or broken sprinklers or leaks. If you find any, they must be fixed first to get accurate measurements.

We will find the precipitation rate for this zone in a young, mostly California native plant landscape that was installed into an existing spray irrigation zone.

We will use a 12-pack of pint-size wide mouth mason jars which cost about $11 at a local hardware store and can be used for many things after the test is done.

Containers used to capture and measure the water falling need a “full opening” and straight sides, making wide-mouth quart sized mason jars a good choice. They are clear and easy to measure water once partially filled.

Step 1: Place containers around the landscape zone that is irrigated by the one valve you are currently testing. The jars can be placed somewhat randomly, but make sure that they cover all the various areas of the zone, and that some are closer to the sprinklers, and some are farther from the sprinklers in more central areas. Large zones may benefit from additional jars. Note: This irrigation system previously watered a turf area. As this garden grows in, it will block the sprays, so they will need to be put on higher solid-pipe risers or converted to drip irrigation to ensure even water coverage.

Above is a rough diagram of the main landscape area being tested and the layout of the jars showing some closer to sprinklers and landscape edges and some in more central areas.

Step 2: Manually run the irrigation zone for at least 10 minutes. In this example, after 10 minutes, the jars had little water in them so would have been hard to measure. We ran the zone for another 10 minutes (for a total of 20 minutes), accumulating enough water in the jars to be easy to measure. Make sure you write down the total number of minutes the system ran. That number will be very important later. Most irrigation controllers (timers) have a manual run function. If you need to look up how to use it, check your manual. If you don’t have your manual, most manufacturers have downloadable versions of their manuals online because people often loose them! Write down the make and model number of your controller and do an internet search of the make and model and the word manual.

Step 3: When done, gather the containers and bring them somewhere easy to measure them. It is much easier to measure at a table than crouching down trying to read a ruler on the ground.

Step 4: Measure and write down the depth of each jar. Round to the nearest 1/8 of an inch.

Example Jar Test Measurement Records

Jar #Inches (in eighths)Inches (decimal)
17/80.875
26/80.75
35/80.625
46/80.75
57/80.875
68/81.0
77/80.875
86/80.75
97/80.875
106/80.75
117/80.875
125/80.625

Step 5: Create a chart with the measurements, converting the records for each jar (originally noted in eighths of an inch) into decimals. You can use the chart below as a quick reference to help.

Converting eighth-inches to decimals

Inches (in eights)inches (decimal)
7/80.875
6/80.75
5/80.625
4/80.5
3/80.375
2/80.25
1/80.125
Use this table to convert your ruler measurements to decimals to input into the table below.

You are now ready to enter your information into the run-time calculator by jar test, here, to get the run-time for this irrigation zone.