Thursday 5 Dec 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.

IRRIGATION SCHEDULING TOOL

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

IRRIGATION COMMENTARY

Keeping an eye on plant water requirements

• The water requirement for our reference crop of pasture (ETo) for the past 7 days was 37mm, 13mm less than the previous week (50mm) and 10mm less than the long-term December weekly average (47mm), at Kyabram
• The next 7-day period is forecast to have a higher cumulative ETo, similar to typical December ETo rates. See tables and graphs below

Water for fodder program

• Farmers in the southern connected Murray–Darling Basin will be able to apply to purchase water at a discounted rate
• Eligible farmers will be able to apply to purchase an allocation of 50 megalitres from the South Australian Government for $5,000
• You must use this water to grow fodder or pasture in the water year it is allocated
• Round 1 of the program will launch in the week of 9 December 2019 and will remain open until it is oversubscribed
• For more information and to prepare yourself for next week’s launch, click here

Water availability

• Currently there is 865GL of unused available water in the Vic Murray system, compared to 1363GL at the same time last year. More detail
• Currently there is 628GL of unused available water in the Goulburn system, compared to 1024GL at the same time last year. More detail

SOIL TEMPERATURE

• The soil temperature as measured by a capacitance probe at 9am this morning (Thursday 5 Dec) under ‘surface irrigated pasture’ (last irrigated 24 Oct 2019) at 10cm depth at Lancaster was 15oC, 1oC warmer than this time last week (28 Nov)
• This time last year (6 Dec 2018), the soil temperature was 18oC

LAST WEEK’S REFERENCE EVAPOTRANSPIRATION⁺ – Pasture Irrigation Requirement$

Weekly Totals –  Thurs 28 Nov to Wed 4 Dec, 2019

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 (December) for each 7mm of effective rain on your property, typically an extra day can be added to the “required surface irrigation interval.”

NEXT WEEK’S FORECAST REFERENCE EVAPOTRANSPIRATION – Pasture Irrigation Requirement$^¥

Weekly Totals – Thurs 5 Dec to Wed 11 Dec, 2019

 DAILY ETo and rainfall – last week & next week

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

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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.

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⁺ 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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