Weekly Irrigation Requirements – Northern Victoria




Thursday 9 May 2019

Weekly Irrigation Requirement$ Summary – Driving Water Productivity

This update provides ‘reference evapotranspiration’ (ETo) information to assist with improved irrigation scheduling for top yields and high water productivity.

This is the last email update for 2018-19.  Thanks for your support and may next irrigation season be a much more prosperous one!



  • The interactive scheduling tool helps you determine the most productive timing for the next irrigation for your unique circumstances. It uses the same ETo information in the tables below and operates with Microsoft Excel on mobile devices and desktop computers.
  • It can be found HERE



Keeping an eye on plant water requirements

  • Total ETo for the past 7 days was 11mm, which is 4mm less than the previous week (15mm) and equal to the May weekly average (11mm), at Kyabram
  • It appears rainfall exceeded ETo over the last 7 days for most locations
  • Typical May ETo rates are forecast for the next 7 days along with rainfall (see below)


2018-19 irrigation season in review

  • The total water requirement over the 2018-19 season (1 July 2018 to 8 May 2019) for our reference crop of pasture (ETo) was 1,301mm (13 ML/ha), at Kyabram
  • Over this period 242mm of rainfall was received at Kyabram, indicating 1,059mm (10.6 ML/ha) of irrigation was required on good pasture. 1,301mm – 242mm = 1,059mm. Note: These irrigation requirement estimates assume all rainfall and irrigation was effective and there was no deep drainage or net change in soil moisture over the full ‘year’ period
  • For the previous irrigation season (1 July 2017 to 8 May 2018), the total water requirement for the reference crop of pasture (ETo) was 1,246mm (12.5ML/ha), while rainfall was 331mm, indicating 915mm (9.1ML/ha) of irrigation was required, at Kyabram
  • Over the previous 8 financial years (1July to 30 June), annual ETo has averaged 1,376mm (13.8 ML/ha), rainfall has averaged 478mm (range: 268mm to 724mm) and the irrigation requirement for pasture has averaged 898mm (9.0 ML/ha), at Kyabram
  • Click for more detail on the Variation in irrigation requirements of forages in northern Victoria

Recent rainfall  

  • May 2019 rainfall totals are shown in the map below
  • Some areas in “the hills” have received over 100mm for the month to date, with Falls Creek (“Rocky Valley” 173mm), Dederang (136mm), Dartmouth Reservoir (110mm) and Mt Buller (100mm) amongst the highest totals
  • More rain is forecast tonight (Thursday) and tomorrow, with 10mm to 15mm forecast across a lot of the plains
  • May the rain keep falling!

Have you put in your water order?

  • If you want to fill up your farm dam or tanks for the winter period or think you may need to apply some irrigation water, don’t forget to put in your water order by tomorrow (Friday, 10th May)
  • Tomorrow is the last day to submit water orders to GMW to ensure delivery


  • The soil temperature as measured by a capacitance probe at 9am this morning (Thursday 9 May) under irrigated pasture at 10cm depth at Lancaster was 12oC, 2oC less than last week (2 May)
  • This time last year (9 May 2018), the soil temperature was 11oC



 Weekly Totals –  Thurs 2 May to Wed 8 May, 2019

Location ETo+





ETo – R

(mm irrigation required) 

Dairy x pasture crop coefficient Average^ ETo
 (mm /DAY)
Required@ Surface Irrigation Interval (days) Spray> irrigators needed to apply (mm)
Deniliquin* 16 27 -11 1 2.3 19
Kyabram* 11 37 -26 1 1.6 27
Tatura* 12 43 -31 1 1.7 26
Kerang# 12 18 -6 1 1.8 24
Echuca# 11 19 -8 1 1.5 27
Yarrawalla# 12 19 -7 1 1.8 24
Numurkah# 11 34 -23 1 1.5 27

Tip:  To convert mm to megalitres /ha, divide by 100.  Eg. 50mm = 0.5ML /ha

The “required surface irrigation interval” shown in these tables is calculated only using cumulative ETo, and needs to be considered in conjunction with rainfall on your farm.  For example, this time of the year (May) for each 2mm of effective rain on your property, typically an extra day can be added to the “required surface irrigation interval.”



Weekly Totals – Thurs 9 May to Wed 15 May, 2019

Location ETo





ETo – R

(mm irrigation required)

Average ETo
(mm /DAY)
Required Surface Irrigation Interval (Days) > Spray irrigators will need to apply (mm)
Deniliquin 12 13 -1 1.7 26
Kyabram 10 21 -11 1.5 29
Tatura 10 21 -12 1.4 31
Kerang 13 12 1 1.9 23 1
Echuca 12 20 -8 1.7 26
Yarrawalla 12 11 1 1.8 25 1
Numurkah 10 23 -13 1.5 29


DAILY ETo and rainfall – last week & next week


You can follow on Twitter and Facebook or to receive this weekly post as a weekly email contact  robert.oconnor@ecodev.vic.gov.au


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.

The intention of this service is not for this information to be used in isolation when making decisions about irrigation timing.  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.

If you have any questions, comments or feedback related to this service please don’t hesitate to contact me, Rob O’Connor at robert.oconnor@ecodev.vic.gov.au

irriSAT (Ctrl click) is a free Australian web based irrigation scheduling tool that uses satellite ‘vision’ to tailor irrigation scheduling for your paddocks and crops.


+ Reference evapotranspiration (“ETo”).  Refers to the evapotranspiration from an actively growing, well-watered, grass stand 120mm in height.

* ETo and rainfall data sourced from the  Bureau of Meteorology

# ETo and rainfall data sourced from the Queensland government’s SILO web site based on climate data provided by the Bureau of Meteorology.  This is “triangulated” data – based on calculations and measurements from the closest weather stations which are not on-site.  Comment: SILO ETo data generally appears to be slightly lower than BOM ETo data.

~Rainfall R.”  Where a rainfall event is less than 2mm it is unlikely to be listed.  Any rain in excess of that which fills the soil profile (or reduces ‘cumulative ETo-R’ back below the estimated RAW) also needs to be disregarded in terms of plant available moisture.  Because rainfall can vary over short distances, irrigators are encouraged to collect and use their own rainfall figures for their farm.

x  “Dairy pasture crop coefficient.”  For crops other than pasture refer to this fact sheet for discussion of an appropriate crop coefficient “What is evapotranspiration and how do I use it to schedule irrigations?”

^ 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.  eg.  Required surface interval =  40mm  ÷  average (daily) ETo, plus 2 days to allow for excess water to drain away following a surface irrigation event.  (Where SILO ETo data is used – Kerang, Echuca, Yarrawalla and Numurkah, 1day is added rather than 2 days because SILO ETo figures are appearing to be slightly lower than BOM ETo figures.)
> Spray irrigators need/ed to apply.”  This figure is based only on evapotranspiration minus rainfall (“ETo – R”) and assumes a good spray uniformity and application efficiency.  If a spray irrigation system has compromised spray uniformity or application efficiency, then more water will need to be applied to best meet plant needs.   Typically with pressurised irrigation systems, irrigations will be applied more regularly than once per week in summer to optimise plant growth.

¥Next weeks forecast reference evapotranspiration – pasture irrigation requirement.”  These figures are sourced from the BOM.  Unfortunately this is a ‘fee for service’ arrangement and the forecast ETo figures are not publically available.


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