Wednesday, 14 May 2025
Weekly Irrigation Requirements Summary
Driving Water Productivity
Northern Victoria
This update provides ‘reference evapotranspiration’ (ETo) information to assist with improved irrigation scheduling for top yields and high water productivity.
Similar weather and irrigation scheduling information provided in this email that is updated daily for any Victorian location, including an interactive irrigation spreadsheet, is available on ExtensionAus.
No login or password is required to use this tool.
In this update:
- Weekly irrigation requirements summary
- Water commentary
– Climate forecasts
– Seasonal determinations and irrigation forecasts - Last week’s irrigation requirement
- Next week’s irrigation requirement
- Daily ETo and rainfall
- Soil temperature and moisture
- Seven-day forecast – Cohuna
1. Weekly irrigation requirements – summary
- Dashboard estimate: The required pasture surface-irrigation interval (40mm refill point) is 28 days based only on cumulative ETo for the past week (not including any rainfall) at Kyabram.
- The plant water requirement (irrigation and rainfall) for our reference crop of pasture (cumulative ETo) for the past 7 days was 11 mm at Kyabram.
- Cumulative ETo over the next 7 days is forecast to be 13 mm at Kyabram. Use the online ETo Tool for updates for your location.
‘Water requirement’ for our reference crop of pasture (ETo) at Kyabram*
* See the tables and graphs below for ETo (and rainfall) at more locations.
** Rainfall is not included in these estimates. The figures presented are cumulative daily short crop reference evapotranspiration (ETo).
2. Water commentary
Climate forecasts
Get an update on soil moisture, seasonal climate drivers and outlooks with the Very Fast Break seasonal climate update video for May.
You can also read The Fast Break newsletter to get a more in depth description of climate drivers and seasonal forecasts including model forecasts for the next 3 months and the next 4 to 6 months.
Seasonal determinations and irrigation forecasts
Tomorrow 15 May is the end of the irrigation season. Updated seasonal determinations and outlooks for the 2025/26 season will also be released for Victoria and NSW.
3. Last week’s reference evapotranspiration (ETo)
Pasture Irrigation Requirements
Wednesday 7 May – Tuesday 13 May
4. Next week’s forecast reference evapotranspiration (ETo)
Wednesday 14 May – Tuesday 20 May
5. Daily evapotranspiration (ETo) and rainfall – last week and next week
6. Soil temperature
The soil temperature measured by a capacitance probe at 9 am today (Wednesday 9 April) under surface irrigated annual pasture at 10cm depth at Wee Wee Rup (near Leitchville) was 13.9°C.
Soil temperature at 10cm depth Wee Wee Rup (oC) for the past 12 months.
Soil temperature is about 5 degrees C warmer than this time last year.
At Wandella (near Kerang) at 9 am this morning the soil temperature was 13.9°C at 15cm depth under irrigated lucerne.
Soil moisture at Wee Wee Rup under irrigated pasture for the past 12 months.
Soil moisture is at about the same level as this time last year.
7. Seven-day forecast for Cohuna
Source: Weatherzone.
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Disclaimer
The intention of this service is not for the information to be used in isolation when making decisions about irrigation. ETo data provides another handy tool in your irrigation ‘tool box’ and needs to be used in conjunction with other preferred scheduling methods and updated weather information.
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 circumstances.
The State of Victoria as represented by the Department of Energy, Environment and Climate Action (DEECA) accepts no duty of care and disclaims all liability in relation to use of this publication.
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+ Reference evapotranspiration (‘ETo’). Refers to the evapotranspiration from an actively growing, well-watered, grass stand 120mm in height. Sourced from AgVic Online ETo Tool, which uses BoM AWO data. BoM AWO gridded daily historical ETo figures may change significantly over time, particularly in the first three days, because improvements are made to the dataset as more data is received by the BoM from different weather stations.
~ ‘Rainfall R.’ Historical rainfall data is sourced from BoM AWO. BoM AWO gridded daily historical rainfall figures may change significantly over time, particularly in the first three days, because improvements are made to the dataset as more data is received by the BoM from different weather stations. Where a rainfall event is less than 2 mm it is usually not ‘effective’ in terms of adding to the soil moisture reserve (because it is unlikely to penetrate through the crop canopy) and is unlikely to be listed in the tables,or graphs. Also be aware that any rain more than that which fills the soil root profile (run-off or deep percolation) is also not ‘effective’ from an irrigation perspective. Because rainfall can vary over short distances, you are encouraged to collect and use your own rainfall figures for your farm.
x ‘Dairy pasture crop coefficient.’ A crop coefficient refers to your crops relative water use potential. For crops other than pasture refer to Tech Note for discussion of an appropriate crop coefficient. Alternatively, you can use AgVic’s Online ETo Tool Interactive Spreadsheet which provides guidance on which crop coefficient to use for your crop and its stage of development.
^ The ‘Average (daily) ETo’ is simply calculated by dividing the total weekly cumulative ETo by 7 days.
@ To calculate the ‘Required surface irrigation interval’ it is assumed there is 40mm of ‘readily available water’ in the pasture root zone. e.g. Required surface irrigation interval = 40 mm ÷ average (daily) ETo, plus 1 day to allow for excess water to drain away following a surface irrigation event. The ‘Required surface irrigation interval’ is only a dashboard estimate based only on cumulative ETo over the recent 7 days, to suit the purpose of a weekly update.
For more accurate irrigation scheduling particularly in spring and autumn, the cumulative deficit (‘ETo – R’) since the last irrigation usually needs to be calculated over a period longer than 7 days and the irrigation timed when the readily available water (40mm in this case) is depleted.
> ‘Spray irrigators need/ed to apply.’ This figure is based only on evapotranspiration minus rainfall (‘ETo – R’) for the given week and assumes excellent accuracy in terms of mm water applied, spray uniformity, and application efficiency.
Typically spray uniformity will not be 100% and extra water will need to be applied to best meet plant needs. The ‘spray irrigators need/ed to apply’ figure given in the table also assumes the rainfall is effective and contributes equivalently to the soil moisture reserve, which may not be the case.
In practice, the moisture deficit (or reserve) carried into the season also needs to be considered to ensure there is adequate soil moisture for maximum plant growth and to avoid a ‘green drought.’ It is best to use rainfall measurements collected on your property. Typically, with pressurised irrigation systems, irrigations occur more regularly than with surface irrigation to optimise plant growth.
Due to the cumulative error involved with using ETo data over a period, it is important that other scheduling methods are used in conjunction with ETo data for spray irrigation systems.
¥ ‘Next week’s forecast reference evapotranspiration – pasture irrigation requirement.’ Forecast ETo and rainfall data is sourced from AgVic’s online ETo tool, which uses forecast daily ETo data from BoM’s Gridded Evapotranspiration Products derived from the Australian Digital Forecast Database (ADFD). Forecast rainfall is also from BoM’s ADFD.
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ETo data and associated irrigation information for the locations shown in these updates is updated daily and can be accessed on the Online ETo Tool.
Historic daily ETo and rainfall data is sourced from BoM’s gridded Australian Water Outlook service.
Historical daily ETo and rainfall figures may change significantly over time, particularly in the first three days, because improvements are made to the dataset as more data is received by the BoM from different weather stations.
Forecast daily ETo data is from BoM’s Gridded Evapotranspiration Products derived from the Australian Digital Forecast Database (ADFD), which AgVic receives on a ‘fee for service’ basis.
Forecast rainfall is also from the ADFD. Please visit AgVic’s online ETo Tool for updated ETo and rainfall data for your location.
Tip: To convert mm to megalitres /ha, divide by 100. E.g. 50mm = 0.5ML /ha.
Important: The ‘required surface irrigation interval’ shown in these tables is a ‘dashboard estimate’ and is calculated using cumulative ETo only over seven days and needs to be considered in conjunction with rainfall on your farm.
For more information about selecting an appropriate crop coefficient for your particular crop and its stage of development refer to this Tech Note What is evapotranspiration and how do I use it to schedule irrigations?, Ag Vic’s Online ETo Tool Interactive Spreadsheet, or the scientific booklet (FAO56) – Table 12, Chapter 6.
