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4.1.4.2 - When should I irrigate?

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Ideally, pasture is irrigated just before its growth rate declines as the soil dries out. Many farmers judge this by experience, with varying degrees of success. The recommended best management practice for border-check irrigated pasture is to irrigate after 50 mm of “evaporation less rainfall” (E – R) has occurred since the previous irrigation. This is equivalent to about 40 mm of pasture water use, which is about the maximum that pasture can extract from typical SIR soils before its growth slows due to moisture stress.

Extending the irrigation interval as a means of saving water is not recommended. Pasture growth slows with moisture stress, and productivity per megalitre is reduced.

For more information on irrigation scheduling, see Step 2.3 - How well is my current irrigation system performing?

Bays can be irrigated in any order, but it is practical to irrigate adjacent areas in sequence, depending on the channel system.
      Photo:  4.1.4.2
      Evaporation data or evapotranspiration data can help you decide when to irrigate

There are three broad approaches used to decide when to irrigate. These are
  • Irrigators subjective judgement
  • Use of evapotranspiration (ET) and rainfall (R)
  • Soil moisture measurement
Irrigators’ judgement

Irrigators' judgment, based on the number of days since the previous irrigation, the current appearance of the crop, the weather and predicted weather conditions is still the most common method used to schedule irrigations. However, with scarcer and increasingly expensive water, more precise and objective approaches are being increasingly used.

Use of evapotranspiration estimates and rainfall data

This method uses either an evaporation pan or weather station data to estimate potential evapotranspiration, and then rely on crop factors based on crop type and stage of growth to arrive at an estimate of actual ET. By then subtracting the effective rainfall, an estimate of the irrigation requirement can be made.
A longstanding rule of thumb for scheduling border-check irrigation of perennial pasture in northern Victoria has been to irrigate when evaporation minus rainfall (E - R) equals 50 mm. Converting evaporation (E) data to potential evapotranspiration (ET0) requires a pan coefficient of 0.7, meaning that the rule of thumb (E – R) value of 50 mm corresponds to an (ET0 – R) value of 35 mm. These days ET0 data are available directly for a large number of regional locations from the Bureau of Meteorology website.

To estimate the crop water requirement, we need crop evapotranspiration (ETc) which is equal to ETo Kc. For an established pasture, Kc can range in value from 0.85 to 1.05, depending on the species mix and height. If a typical ryegrass dominant pasture is assumed to have a Kc value of 0.95, crop water requirement will be 35 0.95 = 33 mm, which is about the maximum depth of water that ryegrass dominant pasture can extract from typical SIR soils before its growth slows due to moisture stress.

Recent developments in the application of remote sensing in the science of crop water use will provide new tools that allow reliable, cost effective, real time monitoring of actual crop ETo.

For further detail about ETo and the estimation of actual ETo, see FAO Irrigation and Drainage paper 56 “Crop evapotranspiration-Guidelines for computing crop water requirements” (Allen et al 1998).

Soil moisture measurement

Soil moisture measurement can also be used to decide when to apply irrigation and how much to apply. For this purpose soil moisture monitoring equipment installed in the field can be used to continuously monitor the amount of soil water available to plants and trigger irrigation when soil moisture is depleted. There is a wide range of soil moisture monitoring tools in the market, using a range of technologies for measuring soil moisture.

For further information on the flow rates see the DEPI Agnote Border-check Irrigation Design
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