What distribution uniformity means for your water and your yield

Every irrigation system puts more water in some places than others. Distribution uniformity (DU) is how we put a number on that. The closer your system gets to applying the same depth everywhere, the higher the DU.

How DU is worked out

The most common form is called low-quarter DU. You take the average of the driest quarter of your bucket readings and divide it by the average of all of them. If the driest quarter got 8 mm and the overall average was 10 mm, your DU is 0.8, or 80%.

Another measure you'll see is Christiansen's uniformity coefficient (CU). It looks at how far every reading sits from the average, not just the driest quarter. Both are useful, and both are in common use in New Zealand.

Why it hits your yield

Here's the problem with poor uniformity. You set your irrigation to meet the crop's needs, but the dry parts of the paddock never get that much. You've got two choices, and both cost you:

• Irrigate to the average. The dry patches stay short of water and grow less.
• Irrigate to the dry patches. Now the rest of the paddock gets more than it needs. You pay to pump water that drains below the roots, and it can take nitrogen with it.

Either way, the driest parts of the paddock set the limit on your production, and the wettest parts drive up your costs.

What good looks like

There's no single perfect number, because it depends on the system type and how it's run. Irrigation New Zealand's design standards say spray systems, including pivots, should be designed to achieve a minimum DUlq of 80% or CUc of 85%. For shallow-rooted crops and frost fighting, the bar rises to DUlq above 82% and CUc above 90%. These are design targets for new systems, not a pass or fail mark for an existing machine. More important than hitting a magic number is knowing yours, and seeing whether it's getting better or worse.

Common causes of poor DU

• Blocked, worn or wrongly placed nozzles
• Pressure too low, too high, or uneven along the machine
• Worn or damaged sprinklers
• Wind, especially with fine droplets
• A nozzle package that doesn't match the current pump or pressure

Most of these are cheap to fix once you know they're there.

Find your number

The only way to know your DU is to measure it. A bucket test takes a few hours and costs little more than some buckets and your time.

Try it: Run a bucket test and put your readings into the free Uniform Ag calculator to get your DU, CU and application depth.

Sources: Irrigation New Zealand, Piped Irrigation Systems Design Standards