How to bucket test a centre pivot, step by step

A bucket test is the simplest way to find out what your pivot is really putting on the paddock. You set out a line of buckets, let the pivot pass over them, and measure what lands in each one. From that, you get two numbers that matter: how much water you're applying, and how evenly you're applying it.

Do a start-up check first

There's no point testing a pivot with a blocked nozzle or a leaking boot. Before you set out a single bucket, run the system and walk the length of it. Look for blocked or missing nozzles, damaged sprinklers, leaks, and pressure that isn't where it should be. Fix what you find, then test. Otherwise your result just tells you something you could have seen by walking it.

Pick the right day

Wind is the biggest enemy of a good bucket test. Choose a calm day, ideally early morning or evening, and avoid testing straight after rain. Note the conditions on the day, because they matter when you compare results later.

Set out your buckets

Lay a straight line of buckets out from the pivot centre towards the end of the machine, across the path the pivot will travel. Space them evenly and follow the spacing set out in the Irrigation New Zealand method. Use identical containers, set each one level and on firm ground, and keep them clear of crop that could shield them from spray. Record the distance of each bucket from the centre.

Start the line far enough in front of the pivot that it's running at normal speed and pressure by the time it reaches the buckets.

Let the pivot pass

Run the pivot at its normal speed setting and pressure, and let it pass completely over the whole line. Stay well clear of the moving machine and the end gun. Don't collect the buckets until the spray has fully passed every one.

Measure promptly

Measure each bucket straight away, before evaporation takes its share. You can measure by volume with a measuring cylinder and convert to depth using the bucket's opening area, or read depth directly if your containers are designed for it. Write down every reading, including any that look odd.

Work out the results

There are two main results:

• Application depth: the average amount of water applied, in millimetres. Compare it to what you think you're putting on at that speed setting.
• Uniformity: how evenly the water was spread, usually reported as distribution uniformity (DU) or Christiansen's uniformity coefficient (CU).

One thing catches people out on pivots. A bucket near the end of the machine represents far more paddock than one near the centre, because the outer spans cover a much bigger circle. A proper pivot calculation weights each bucket by its distance from the centre. A simple average of the buckets will give you the wrong answer.

Read the pattern, not just the number

Look at how the readings change along the line. A single low bucket often points to a blocked or damaged nozzle at that spot. A section that is consistently low or high suggests the wrong nozzle package, pressure problems, or worn sprinklers. If the outer buckets are low, check the end pressure and the end gun.

Keep the record

Record the date, the weather, the pivot speed setting, the pressure, and every reading. A single test is a snapshot. A series of tests over several seasons shows you whether the system is holding up or wearing out.

Try it: Enter your readings in the free Uniform Ag calculator. It handles the pivot weighting for you and gives you your application depth and uniformity, plus a basic PDF report.