Weekly Irrigation Requirements – Northern Victoria



Thursday 17 Oct 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 interactive scheduling tool helps you determine the most productive timing for the next surface irrigation for your unique circumstances.



Keeping an eye on plant water requirements

  • The water requirement for our reference crop of pasture (ETo) for the past 7 days was 29mm, 2mm more than the previous week (27mm) and 1mm less than the long-term October weekly average (30mm), at Kyabram
  • Higher ETo rates are forecast next week and tomorrow (Friday) is expected to be a relatively high ETo day.  See table below


Victorian climate projections

  • Recently released Victorian climate projections to 2090 that are tailored for each region can be accessed by clicking here
  • Be prepared for a sobering read


Dry seasons support

  • For dry seasons support available click here



  • The soil temperature as measured by a capacitance probe at 9am this morning (Thursday 17 Oct) under irrigated pasture at 10cm depth at Lancaster was 13oC, 2oC more than last week (10 Oct)
  • This time last year (17 Oct 2018), the soil temperature was 15oC



 Weekly Totals –  Thurs 10 Oct to Wed 16 Oct, 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* 37 0 37 1 5.2 10 37
Kyabram* 29 0 29 1 4.1 12 29
Tatura* 29 0 29 1 4.1 12 29
Kerang# 30 0 30 1 4.3 10 30
Echuca# 26 0 26 1 3.7 12 26
Yarrawalla# 28 2 26 1 3.9 11 26
Numurkah# 27 0 27 1 3.8 11 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 (October) for each 4mm of effective rain on your property, typically an extra day can be added to the “required surface irrigation interval.”



Weekly Totals – Thurs 17 Oct to Wed 23 Oct, 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 40 0 40 5.8 9 40
Kyabram 34 2 32 4.8 10 32
Tatura 32 2 30 4.5 11 30
Kerang 39 0 39 5.5 9 39
Echuca 37 0 37 5.3 10 37
Yarrawalla 37 0 37 5.2 10 37
Numurkah 35 0 35 4.9 10 35


 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@agriculture.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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