Thursday 9 August 2018

Weekly Irrigation Requirement$ Summary – Driving Water Productivity

Welcome to the first Irrigation Requirement$ Summary for the 2018-19 season.

I hope you found some time over the last few months to relax!

It seems this irrigation season has come around relatively quickly.

Irrigation commentary

Irrigation Start-Up

• There are already reports of people surface irrigating and spray irrigating in northern Victoria and southern NSW
• Soil cores taken yesterday afternoon from a perennial pasture bay that was last surface irrigated on 1 May 2018 at Lancaster (near Kyabram) indicated there was adequate moisture for optimal growth on this bay – it was relatively easy to form “ribbons” using my hands with soil samples taken at 10cm increments along the length of the cores (to 40cm depth)
• Soil moisture measured by a capacitance probe in this bay also indicates there is currently adequate moisture for optimal pasture growth
• The capacitance probe data shows the soil profile was last full of plant available moisture on 18 June, 2018. Cumulative ‘evapotranspiration minus rainfall’ (ETo-R) since this date is 30mm.  Given the recommended surface irrigation interval for pasture is 40mm to 50mm (cumulative ETo-R), this method also suggests there is still some soil moisture available to sustain optimal pasture growth rates
• Without the recent rainfall, it is likely to have been a different story
• Looking ahead, with current ETo rates at about 2 mm/day and on the rise, it’s going to pay to remain vigilant with monitoring plant water requirements and irrigation timing
• ‘Meteye’ provides 7-day forecast rainfall data which needs to be factored in to irrigation decisions particularly at this time of the year  http://www.bom.gov.au/australia/meteye/

Rainfall in the past week

• Highlight: Mt Buller 115mm

http://www.bom.gov.au/jsp/awap/rain/index.jsp

 Soil temp

• The soil temperature (as measured by a capacitance probe) at 9am this morning under irrigated pasture at Lancaster at 10cm depth was 7oC

Last Week’s Reference Evapotranspiration – Pasture Irrigation Requirement$

Weekly Totals –  Thu 2 Aug to Wed 8 Aug, 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 based only on cumulative ETo and needs to be considered in conjunction with rainfall on your farm.  For example, this time of the year (August) for each 2mm of effective rain that falls on your property, typically an extra day needs to be added to the “required surface irrigation interval.”

Next Weeks Forecast Reference Evapotranspiration – Pasture Irrigation Requirement$^¥

Weekly Totals –  Thu 9 Aug to Wed 15 Aug, 2018

 Daily ETo & rainfall – last week & next week

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You can follow on Twitter and Facebook or to receive this Weekly Update as an 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?“

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* 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.

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

^~ “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.  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 on the use 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.  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 appear 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.