The simple way to use ET (evapotranspiration) information to more accurately schedule your irrigations if you are in the Mallee, Macalister Irrigation District, Goulburn Murray Region, or North East Victoria, is to use the Weekly Irrigation Requirements Updates posted on this ExtensionAUS website.  These Updates provide either an interpretation of the ET data or instructions on how to use the data, for irrigation scheduling.

Apps are also available such as irriSAT, that can provide irrigation scheduling advice for your specific circumstances based on ET measurements and your inputs in to the app.

Raw ET data can also be sourced from local Bureau of Meteorology weather stations or from the SILO web site.  Typically, daily ET figures are added up starting from the date of the last irrigation (known as “Cumulative ET”), as a way of determining the water use of the actively growing pasture or crop and associated soil moisture depletion.   

For the reference crop of pasture that is surface irrigated, when Cumulative ET equals the amount of water that was originally readily available soon after the last irrigation, it indicates readily available water has become depleted, assuming no rainfall.  At this point it’s time to irrigate again.

For a lot of soils that are surface irrigated, the amount of water that is readily available to pasture soon after irrigation, is in the range of 30mm to 50mm (0.3ML/ha to 0.5ML/ha) depending largely on soil characteristics.   
Typically for spray irrigation systems, irrigations are timed when ‘cumulative ET’ equals the last irrigation application in millimetres (allowing for rainfall, the efficiency of the irrigation system and relative crop water use).

ET provides objective information that can help better match irrigation applications with plant water requirements in order to maximise yields and water use efficiency.
For more information about ET and how it is used to schedule irrigations on pasture and crops, refer to this Fact Sheet or Tech Note.