Step 5. Calculate your irrigation requirement

You now have the data required to calculate your irrigation requirement.

Once you have the data, you will multiply by the farm size. This will help develop your irrigation plan, but for the hydraulic design, what you are looking for is the largest irrigation volume amount in the season, as this is the volume of water that the system needs to be able to apply (i.e. the maximum volume it will need to deliver). If your system cannot deliver this amount of water when needed, you will lose yield.

Metric – Example  
Stage Flowering
ETo 6
Kc 0.75
Water requirement (mm) (6 x 0.75) = 4.5
Less rainfall (Pe) 0
Irrigation requirement (mm) 4.5
Irrigation volume of water required (m3/ha) 45
Irrigated farm size (ha) 100
Total irrigation volume required (m3) 4,500
US – Example  
Stage Flowering
ETo 0.23 in
Kc 0.75
Water requirement (in) (0.23 x 0.75) = 0.17
Less rainfall (Pe) 0
Irrigation requirement (in) 0.17
Irrigation volume of water required (gal/ac) 4,616
Irrigated farm size (ac) 100
Total irrigation volume required (gallons) 461,600


Caution! Beware of averages.

The graph below shows the variance between the average water requirement and the peak water demand.

A common mistake is using average water requirement as a guide for your irrigation system. However if you do this, during peak water demand periods, there is a deficit of water, which leads to yield reduction … often at the most critical time.


The final irrigation design needs to be based on the peak water requirement, not the average water requirement.


Reduced deficit irrigation (RDI)?

The above works when you want the most bountiful, large crop. But that is not always the case. Think of table grapes, where you could want large full grapes, vs wine grapes, which are more concentrated. Here you want reduced deficit irrigation, which requires a different formula. As this a more specific subject, read more about RDI in our vineyards brochure.



  • Water consumption per growing stage (mm (in)/day/stage)

  • Peak consumption required (m3 (gal)/day)

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