Weekly Irrigation Requirements – North East Victoria
This update provides ‘reference evapotranspiration’ (ETo) data to assist with improved irrigation scheduling for top yields and high water productivity.Note: To convert mm to megalitres /ha, divide by 100. Eg. 50mm = 0.5ML /ha
What is Potential Evaporanspiration?
Potential evapotranspiration is calculated from a number of other measurements such as solar radiation, wind, humidity, temperature, etc. It measures the potential water use of a healthy well watered crop.
How to use the Potential Evapotranspiration Figures
The potential Evaporation rates can be used to help determine the application rate required and how frequently irrigation should occur.
Application rate required can be calculated from the equation below
Application rate = ETo x Crop coefficient / irrigation efficiency
The crop coefficient can be a little bit subjective, but if you imagine a healthy, well fertilised, managed and watered pasture (ie no limiting factors) such as the most productive dairy pastures this will have a crop coefficient of 1.0. You need to make an honest estimate of where your pasture is likely to sit, an average irrigated pasture is likely to be at about 0.8. A healthy thick Lucerne stand at flowering stage or a tall actively growing crop of maize or sorghum is likely to use more water and the crop coefficient will be approximately 1.2
The irrigation efficiency for spray irrigation is usually determined from the Distribution Uniformity which should be above 75%. The irrigation efficiency for flood irrigation depends on the amount of run-off which occurs and leaching past the root zone.
Example: a reasonable ryegrass pasture (crop coefficient of 0.8), irrigated with a centre pivot with a 90% uniformity for a day with a potential ETo of 8mm would require 7.1mm to be applied for that day.
The frequency of irrigation is determined from the water holding capacity of the soil. Typically in a pasture situation 40 mm can be removed from the soil before plant stress occurs. Assuming the soil profile is full each days potential evapotranspiration multiplied by the crop coefficient can be subtracted from the 40 mm till it is close to zero, which indicated it is time to irrigate.
These figures will need to used in conjunction with rain fall.
The intention of this service is not for the information to be used in isolation when making decisions about irrigation scheduling.
ETo provides a relatively objective estimate of plant water use and provides another handy ‘tool in the irrigator’s toolkit.’
ETo should be effectively used in conjunction with other preferred methods of working out when to irrigate. By regularly using ETo data, the idea is that irrigators will be better informed about their current pasture irrigation requirements and will also be able to ‘sharpen up’ their scheduling skills and tools they have developed over time and use regularly (eg. the use of the electric fence post).
This publication is provided solely for information purposes and no representation or warranty is made as to its quality, accuracy, completeness, suitability or fitness for any particular purpose. You should make your own inquiries as to its appropriateness and suitability for your particular circumstances. The State of Victoria as represented by its Department of Economic Development, Jobs, Transport and Resources accepts no duty of care and disclaims all liability in relation to use of this publication.
AgVic also provides weekly ET email updates for Northern Victoria (contact email@example.com), the Mallee (firstname.lastname@example.org) and the Macalister District (email@example.com).
For more detail about using evapotranspiration data refer to this Agriculture Victoria fact sheet “What is evapotranspiration and how do I use it to schedule irrigations”or this scientific paper.
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