Thursday 13 December 2018

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 (Ctrl click, then click on the Excel icon that appears on the bottom left of your screen) 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 is designed to operate in Microsoft Excel on mobile devices and desktop computers.

IRRIGATION COMMENTARY

Keeping an eye on plant water requirements

• Reference evapotranspiration (ETo) over the last 7 days was 48mm, only 2mm more than the previous week (46mm) and 1mm more than the longer term monthly December average (47mm), at Kyabram
• 11mm of “effective” rain was recorded over the last week at Kyabram
• Over the next 7 days, ETo is forecast to be lower than last week at 40mm, with lower ETo rates forecast for the next few days and increasing to more typical December ETo rates for the remainder of the 7 day outlook, at Kyabram
• Relatively heavy rain is forecast today (Thursday)

Dry seasons support

• Dry seasons support provided by the Victorian Government (Ctrl click) includes technical, financial, personal and regional support
• On-farm infrastructure grants of up to $5000 are available for works such as water and fence infrastructure improvements, or stock containment areas, for eligible farm businesses in relevant shires. Dollar for dollar matching co-contribution is required
• Plan2Farn provides free help to farmers to manage dry times and make decisions about all aspects of the farming enterprise
• Energy grants are also available to eligible farm businesses for projects that achieve energy efficiency or provide energy productivity improvements

SOIL TEMPERATURE

• The soil temperature as measured by a capacitance probe at 9am this morning (Thursday 13 Dec) under irrigated pasture at 10cm depth at Lancaster was 20oC, 3oC warmer than week
• This time last year (13 Dec 2017) the soil temperature was 19oC. It increased by 3oC during the previous week and continued to increase by a further 2oC to 21oC, over the following 7 days

Last Week’s Reference Evapotranspiration⁺ – Pasture Irrigation Requirement$

 Weekly Totals –  Thurs 6 Dec to Wed 12 Dec, 2018

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 Weeks Forecast Reference Evapotranspiration – Pasture Irrigation Requirement$^¥

Weekly Totals –  Thurs 13 Dec to Wed 19 Dec, 2018

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

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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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