Weekly Irrigation Requirements – Driving Water Productivity
This update provides ‘reference evapotranspiration’ (ETo) data to assist with improved irrigation scheduling for top growth rates and yields for the Mallee region.
ETo graph and readings to week ending 02/09/2018
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.
|Eto READINGS FOR||WEEK||27-Aug||to||2-Sep|
Weekly ETo data
|MIN. & MAX. TEMPERATURES|
|Eto Figures are calculated by AgVic with the assistance of|
|the Bureau of Meteorology for the Mildura Airport Data|
|All Figures are from 9am to 9am the following day|
The intention of this service is not for the information to be used in isolation when making decisions about irrigation scheduling.
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 North East Victoria (contact firstname.lastname@example.org), Northern Victoria (email@example.com) and the Macalister District (firstname.lastname@example.org).
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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