Thursday 18 October 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 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 operates with Microsoft Excel.
Keeping an eye on plant water requirements
- The weekly water requirement for our reference crop of pasture (ETo) over the last 7 days was 30mm (excluding rainfall), 5mm more than the previous week and equal to the longer term monthly October average (30mm), at Kyabram
- Tomorrow is forecast to be a day with relatively high ETo for this time of the year
- Forecast ETo next week is 34mm with no real significant rain, at Kyabram
Rainfall over the past week
- The BoM map below shows rainfall totals over the past week.
Water allocations, outlooks and carryover volumes – updated last Monday 15 October
- HRWS allocations increased by 3% to 77% and by 2% to 84% (Ctrl click) on the Vic Goulburn and Murray systems respectively
- Under a “dry” inflow scenario (90 in 100, climate adjusted), HRWS allocations are forecast to reach 88% and 100% by 15 February 2019 on the Vic Goulburn and Murray systems respectively
- Carryover volumes were reported to be 434GL and 485GL on 1 July 2018 on the Vic Goulburn and Murray systems respectively
- The break-down of carryover ownership is
– Goulburn system: Private 293GL, Environment 97GL and Water Corps 43GL
– Murray system: Private 180GL, Environment 289GL and Water corps 23GL
- GS allocations remained unchanged at 0% and 7% for the NSW Murray and Murrumbidgee systems respectively
- Under a “dry” inflow scenario (75 in 100), GS allocations are forecast to reach 5% and 7% by 1 February 2019 on the NSW Murray and Murrumbidgee systems respectively
- Carryover volumes were reported to be 520GL (“31%”) and 419GL (“22%”) on the NSW Murray and Murrumbidgee systems
Seasonal outlook videos
- AgVic and Plan2Farm hosted a series of Seasonal Update workshops across the Goulburn Murray district last month
- The sessions were videoed and can be viewed online, including a hay and grain outlook, irrigated cropping options, water market drivers and water availability
- The soil temperature (as measured by a capacitance probe) at 9am this morning under irrigated pasture at 10cm depth at Lancaster was 15oC, 3oC higher than last week
- This time last year the soil temperature was 15oC
Last Week’s Reference Evapotranspiration+ – Pasture Irrigation Requirement$
Weekly Totals – Thurs 11 Oct to Wed 17 Oct, 2018
|ETo – R
(mm irrigation required)
|Dairy x pasture crop coefficient||Average^ ETo
|Required@ Surface Irrigation Interval (days)||Spray> irrigators needed to apply (mm)|
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, but 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.”
Next Weeks Forecast Reference Evapotranspiration – Pasture Irrigation Requirement$¥
Weekly Totals – Thu 18 Oct to Wed 24 Oct, 2018
|ETo – R
(mm irrigation required)
|Required Surface Irrigation Interval (Days)||> Spray irrigators will need to apply (mm)|
Daily ETo and rainfall – last week & next week
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 email service please don’t hesitate to contact me, Rob O’Connor at firstname.lastname@example.org.
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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. 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. 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.