Thursday 16 Jan 2020

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 52mm, only 2mm more than the long-term January weekly average (50mm), at Kyabram
• Thankfully, generally lower daily ETo rates are forecast over the next 7 days. See tables and graphs below

Recent rainfall totals

• Rainfall for the last 7 days to 9am today, Thursday 16 January (BoM): Cobram 4mm, Echuca 15mm, Kerang 12mm, Deniliquin 7mm, Finley 0.4mm, Kyabram 5mm, Rochester 6mm, Elmore 17mm, Shepparton 3mm, Stanhope 12mm, Tatura 10mm, Albury 0mm, Benalla 1mm, Lake Dartmouth 3mm, Archerton 24mm, Jamieson 21mm, Mt Buffalo 5mm, Mt Buller 21mm, Mt Hotham 13mm,  Lake Eildon 32mm
• The BoM map below shows rainfall totals for the past week and can be accessed on the web here

SOIL TEMPERATURE

• The soil temperature as measured by a capacitance probe at 9am this morning (Thursday 16 Jan) under ‘surface irrigated pasture’ (last irrigated 24 Oct 2019) at 10cm depth at Lancaster was 20oC, the same as last week (9 Jan)
• This time last year (16 Jan 2019), the soil temperature was 21oC

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

Weekly Totals –  Thurs 9 Jan to Wed 15 Jan 2020

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 (January) 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 16 Jan to Wed 22 Jan, 2020

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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