Distribution uniformity is a measure of how evenly the system applies the water to the crop. Typically, distribution uniformity measurements are made at the top of the crop canopy in field crops, and above the soil surface in any case. This means there are opportunities for the plants and the soil itself to further influence the actual evenness.

It is common to report the lower quartile distribution uniformity value, DU_LQ. This is the ratio of the average of the lowest quarter of measured depths to the average of the whole field. We can use the DU_LQ to adjust the depth of irrigation we apply to ensure at least 7/8ths of the crop gets enough water.

If our aim is to adequately water our crop, improving the distribution uniformity gives great increases in efficiency and has many other benefits too. If we can increase the DU_LQ from ~0.7 to ~0.9 we can effectively irrigate 50% more land with the same amount of water. Alternatively, we can water the same area using a third less water and power. A high DU pays handsomely.

When considering uniformity, it is very important to think what the evenness is applying to. We take individual plants as the unit to be considered, as they are what we are trying to manage.  For a spray irrigated vegetable or wheat crops that means the whole soil volume across the field needs to be evenly watered. But for drip irrigated grapes or fruit trees, only the main root volume of each plant needs be considered. Knowing the irrigation target is one of the main factors used to decide what to measure when we are assessing uniformity.

It is not economically viable to aim perfect uniformity in most cases, although some drip irrigation systems can be getting very close especially when new. But for sprinkler systems on vegetables or broad acre field crops, achieving the best uniformity you can through good design, correct maintenance and smart decision making will increase profitability.

Let’s step through some maths….

Let’s say we went out to measure an irrigation system, and we set out 16 buckets to catch water.

Here’s what we might get when we measure how much water we caught in each bucket. Some variation between them – is it a good or poor result?

Calculating the DU_LQ helps us decide.

Here are the same data, but this time arranged with the smallest volume on the left and the largest volume on the right.

DU_LQ is the ratio of the average of the lowest quarter of volumes over the average of all volumes. We have 16 buckets, and the average of the lowest four is 540 mL. The average of all 16 is 600 mL.

540/600 = 0.90 which is high for a DU.

This is a good result.

Here is another set of readings from the block before the system was given a good overhaul. We’ve ordered the readings small to large again and we do the same maths.

In this case, the lowest four average is only 300 mL, even though the average of all is still 600 mL.

300/600 = 0.50. This is very low for a DU.

Don’t neglect maintenance.

How do we use the results to make decisions?

We use DU_LQ to decide how much extra irrigation we should apply to be sure the 7/8^ths of the crop gets at least the target application depth. Let’s say we want to apply 30 mm of irrigation to our crop. We simply divide the target application depth by the calculated DU_LQ.

In the first case: DU = 0.90, so 30 mm / 0.90 = 33.3 mm.

We set the irrigation system to apply an average of 33.3 mm and 7/8^ths will get at least that. 1/8^th will get a little less, and some will be a bit more.

In the second case: DU = 0.50, so 30 mm/ 0.50 = 60 mm.

We have to set the irrigation system to apply twice as much water as we want so 7/8^ths will get at least our target depth. 1/8^th will still get a quite a lot less, and some will get a huge amount more – too much – and we can expect plant saturation problems as a result. And a high power bill. And we’ll run out of water allocation before the season is done.

Applied Depth

The actual Applied Depth will vary from our Target Depth – the distribution uniformity story shows us that. It might also vary because our average setting is incorrect, and we are not actually applying what we think. We measure to check.

The graph below shows 16 measurements laid out in order across an irrigator. We have converted volumes caught to Applied Depth for this graph. (Caught volume divided by bucket area equals applied depth). Our target depth in this case was 14.5 mm. How did we do?

What we see is that our actually applied depth of 14.0 mm is close to the target of 14.5 mm.

Unfortunately, we also see that the system has poor uniformity, and some areas are getting a lot more than others. In fact, some areas are getting 2.5 times more than others. Not good for even crops.

Application Intensity

We have put Application Intensity in here too. Application intensity is how hard the rain is falling – how fast the irrigation is being applied. We measure it as mm/hour. Many travelling irrigation systems apply at least 25 mm of water an hour, some much more. If you go out in a thunderstorm that is anywhere near that you get soaked, and you’ll water running everywhere. It’s the same with irrigation. If the water falls faster than the soil can soak it up, you’ll get surface ponding. If you get surface ponding, the water will run to the low spots, and you will have areas that end up dry even though the right amount of irrigation landed there. And you’ll have drowned out areas where the excess ended up.

This is noticeable on pivot irrigators, where the intensity increases away from the pivot centre. The end has to move much faster than the middle, so the application intensity has to be higher to deliver the same depth of water. Think about it.

This image shows a corn crop where the irrigation intensity has been higher than the soil’s infiltration rate. It doesn’t take much for water to move from one area to another.

Keep an eye out for ponding, it is a sign that water is being redistributed. You can help by looking after the soil because a well-structured soil will have better macroporosity and is less likely to cap. That helps infiltration and makes your irrigation more efficient and more effective.