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Irrigation planning for a dry season

Irrigation efficiency

Making sure that irrigation equipment is functioning properly and scheduling application to meet crop demand will provide savings through:

  • Reducing water wastage from over-application, leaks, and unevenness
  • Reducing energy and labour costs
  • Helping maximise crop yield and nutrient uptake
  • Reducing nutrient losses

A few basic actions can help improve the irrigation efficiency on your farm:

  • Regular maintenance and repairs of equipment
  • Understanding your soils and their ability to hold water
  • Monitor soil moisture levels
  • Know how much water you’re putting on and keep records
  • “Bucket test” irrigators to make sure water goes on evenly
  • Understand the crop’s root depth, water demand, and trigger point
  • Train staff to operate irrigation equipment properly

Target inputs to expected yield

There are many differences between farming dryland and irrigated crops, but nitrogen (N) management is probably the most important one to consider. If reduced irrigation capacity means a lower crop yield potential, N application should be targeted to the lower yield. Timing of N application also becomes a bit trickier.

Timing of N application

Optimum N timing in dryland crops is slightly more difficult to achieve compared to a crop where water is readily available to ensure uptake in periods of very dry weather. Timing N application to just before, or even during rainfall will help reduce N loss through volatilisation to the atmosphere. If there is uncertainty with the weather forecast, consider the use of a nitrogen fertiliser coated with urease inhibitors based on Agrotain® (eg SustaiN). These have given yield increases in some trials compared with urea, when the fertiliser hasn’t been washed in within 24 hours of application. Remember that the larger the soil mineral N reserve, the greater flexibility you have in timing windows for nitrogen application i.e. N timings can be delayed.

Overall rate of N

When converting a crop from irrigated to dryland, it is important to review the agronomy of the whole crop, including the overall rate of N. It is much easier to work out how much N you need to apply with the certainty of irrigation. A summary of results from ten FAR autumn sown feed wheat N trials with irrigation suggests a total of 300 kg N/ha (soil mineral plus applied N) will be close to the mark. Dryland yields are much more variable due to the reliance on rainfall, therefore calculating the optimum quantity of N is more difficult. As the dryland yield potential increases, the optimum N rate will increase toward the optimum under irrigation. The FAR Cropping Strategy- Nitrogen Application in Wheat offers a guide to targeting N rates for dryland and irrigated wheat. Any changes to N rate will need to be reviewed right up to ear emergence, based on spring rainfall events.

Prioritising irrigation

As the soil dries from the trigger point toward wilting point, potential yield decreases in a straight line with the effects varying by crop species (Table 1). The bigger the potential yields, the greater the relative reductions in yield. The form of yield loss can change. For instance, early drought stress in barley reduces grain number mostly through the loss of tillers, whereas late drought stress results in small, pinched grain with high screening losses.

Crop species

% decrease in potential yield for each mm below the trigger point

Maize*, oats, potatoes, sweetcorn


Ryegrass seed


Barley, peas, wheat


*maize sensitivity to drought decreases with time because the trigger point increases as the root system develops

The goal is to avoid a situation where the trigger point is exceeded. As an example, a 50 mm irrigation on a healthy wheat crop (yield potential 13 t/ha) applied just before trigger point is reached is 13 t/ha x 50 mm x 0.25% = 1.63 t/ha. Assuming the cost of irrigation is $100/ha ($2/mm) and the wheat is worth $300/t, the return would be $480/ha, a net gain of $380/ha.

For the same irrigation on a 2.2t/ha ryegrass seed crop, the yield response would be 2.2 t/ha x 50 mm x 0.14% = 0.14 t/ha. At $2,200/t the return would be $338/ha and the net return to irrigation $238/ha. In this example, if there was insufficient water, it would pay to irrigate the wheat ahead of the ryegrass.

Irrigation Cost Calculator

Click here to open the Irrigation cost calculator on the FAR website. This tool allows you to do the above calculation on a range of crops.

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