The catch-cup test: measure what your sprinklers actually put out

Sprinkler run times are usually guesses. A catch-cup test replaces the guess with a measurement. It takes about half an hour per zone and tells you three things: how fast the zone applies water, how long to run it, and how evenly it covers the lawn. It’s the single most useful thing you can do before summer, and it costs nothing but a few containers.

Why measure?

Two sprinkler systems with the same run time can put down very different amounts of water. Pop-up spray heads often apply water quickly; rotors apply it slowly. Pressure, nozzle size, head spacing and wear all change the result. Without measuring, a “15-minute” setting could be giving one lawn too little and another far too much.

The test also finds coverage problems. Water Corporation notes that dry brown patches often mean uneven watering, and recommends catch cups to check. If one corner of a zone gets half as much water as the rest, running the whole zone longer won’t fix it; it just overwaters everything else.

What you need

• Six or more straight-sided containers, all the same. Straight sides matter: the depth of water then equals the millimetres applied. Food tins with the labels removed work well, as do straight-sided plastic cups.
• A ruler marked in millimetres.
• A timer.
• Something to note the readings, such as a notebook or your phone.

If your containers have sloping sides, you can still do the test by measuring volume instead of depth; see the FAQs below.

Before you start

• Check your local water rules for when you can run sprinklers. Water Corporation, for example, only allows testing on rostered days at set times; your area may have its own rules.
• Pick a calm morning. Wind blows water away from some cups and into others.
• Run each zone briefly first to check for broken heads, blocked nozzles and leaks. Fix those first, or the test will just measure the fault.
• Test at the time you normally water. Water pressure can vary through the day.
• Mow first. Short grass is less likely to block the spray or tip the cups over.

Step by step

1. Pick one zone to start with.
2. Place the containers evenly across the zone in a rough grid: some near the sprinklers, some between them, some at the edges and corners. Keep them clear of the spray heads themselves and make sure nothing blocks the spray.
3. Run the zone for a set time, such as 15 minutes. Use the same time for every zone so the results are easy to compare.
4. Measure the depth in each container with the ruler, in millimetres. Hold the ruler upright on the bottom of the container and read at eye level.
5. Write them all down, with a rough note of where each cup was.
6. Enter the readings in the Sprinkler Run-Time Calculator with the test time and how much water you want to apply.
7. Repeat for each zone.

Reading the results

Average depth and application rate. The calculator turns your average into millimetres per hour. Pop-up sprays often apply water faster than rotors; your own number is what counts.

Run time. From the rate, the calculator gives the minutes to apply your target. For example, if a zone collected an average of 5 mm in 15 minutes, it would need about 30 minutes to apply 10 mm.

Evenness. The calculator compares the lowest quarter of your cups with the average. The closer the two are, the more even the coverage. If some cups have much less than others, parts of the lawn are under-watered, and running the zone longer would overwater the rest.

Choosing your target

Your target is how many millimetres you want to apply each watering. Water Corporation’s benchmark for Perth and the South West is 10 mm twice a week in summer, and much less in cooler months. Elsewhere, use your water authority’s guidance if it has some, or start from that benchmark and adjust by watching the lawn. The how much water guide explains how soil, season and grass type change the answer.

If coverage is uneven

Look for the cause before changing run times:

• blocked, broken or tilted heads;
• heads sunk below the grass, or grass grown over them;
• mismatched nozzles or sprinkler types in one zone;
• heads too far apart;
• low water pressure, especially when too many heads share one zone;
• wind, if you tested on a breezy day.

Fix what you find, then test again. Small fixes, such as raising a sunken head or clearing a blocked nozzle, often make a large difference.

Run-off

If water starts running off before the run time is up, the soil can’t absorb it that fast. This is common on clay, compacted soil and slopes. Split the run into two or three shorter cycles with a break of about an hour in between, so each cycle soaks in before the next. Many controllers have a “cycle and soak” setting for this.

On sandy soil, run-off is less likely, but water may bead on dry patches that have turned water-repellent. A wetting agent can help; see the hydrophobic soil guide.

Measuring by volume

If your containers don’t have straight sides, or the water is too shallow to read accurately with a ruler, measure the volume instead.

1. Measure the diameter of each container’s opening in centimetres.
2. Work out the opening’s area: half the diameter, squared, multiplied by 3.14. A 10 cm opening has an area of about 78.5 cm².
3. After the test, pour each container into a small measuring jug and note the volume in millilitres.
4. Divide the volume by the opening’s area, then multiply by ten. The answer is millimetres of water.

For example, 40 mL collected in a container with a 78.5 cm² opening is about 5 mm. Use the same containers for every zone so the results compare fairly.

Large zones and odd shapes

On a big zone, more cups give a better picture: as a rough guide, one cup for every few square metres. Lay them out in rows across the zone rather than scattering them, so you can see where the pattern falls off. On narrow strips, such as a nature strip, run a single line of cups down the middle and another along each edge.

If a zone covers both sun and shade, or lawn and garden bed, note which cups were where. That may be a sign the zone should be split; see the DIY irrigation design guide.

Keeping a record

A simple table for each zone makes next season’s test quicker:

Copy the results into your Lawn Journal so they sit alongside your watering log.

Testing a hose-end sprinkler

You don’t need an in-ground system to use this test. Set up your portable sprinkler where you normally put it, place cups across its pattern, and run it for a set time. The average tells you how long to leave the sprinkler in each spot to reach your target, and the pattern of readings shows how far apart to move it so the edges overlap.

Worked example

Say you place eight cups in your back lawn zone and run it for 15 minutes. The readings in millimetres are 6, 5, 6, 4, 6, 5, 2 and 6.

• The average is about 5 mm in 15 minutes, or about 20 mm an hour.
• To apply 10 mm, the zone needs about 30 minutes.
• The cup reading 2 mm is far below the rest. It was in the far corner, so that corner gets less than half the water of the rest of the zone. Check the nearest head for a blockage or a wrong nozzle before you run the zone longer.

Adjusting through the year

The test tells you how fast each zone applies water. That rate doesn’t change much through the year unless you change the system, but your target does. Reduce run times as the weather cools and increase them as it warms, using the same application rate. Many controllers have a seasonal adjustment that scales every zone by a percentage, which makes this quick.

Repeat it

Test each zone, and re-test at the start of each summer or after changing anything in the system. Keep the results in your Lawn Journal so you can see whether a zone’s output is drifting, which usually points to worn nozzles or a pressure problem.

Common mistakes

• Using cups with sloping sides. The depth no longer equals millimetres applied.
• Testing on a windy day, which blows water away from some cups and into others.
• Putting all the cups in the middle. Edges and corners are where coverage is usually worst.
• Testing at a different time of day from when the system normally runs. Water pressure can vary through the day.
• Running every zone longer to fix one dry corner, instead of fixing the head that serves it.