Thursday 15 Aug 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.
Keeping an eye on plant water requirements
- The water requirement for our reference crop of pasture (ETo) for the past 7 days was 9mm, which is 2mm more than the previous week (7mm) and below the long term August weekly average (12mm), at Kyabram
- In many locations, rainfall exceeded pasture water use last week
- Slightly higher ETo rates are forecast for the next 7 days, with some rain (see table below)
Estimating Irrigation Start-up
- The soil profile under irrigated perennial pasture at Lancaster was refilled with 19mm of rain last week
- Cumulative ‘ETo minus Rainfall’ (ETo-R) since the rain is 5mm
- Soil cores taken this morning (Thursday 15 Aug) on the Lancaster perennial pasture paddock confirm the soil remains moist to a depth of at least 70cm
- Currently water trade down the Barmah Choke and the Goulburn IVT is effectively closed
- As a result of the closures, a difference in allocation price has emerged
- The reported allocation price on the Goulburn system is $540/ML or less, and on the Vic Murray system $618/ML – with prices slightly higher below the Barmah Choke than above the Choke
- The soil temperature as measured by a capacitance probe at 9am this morning (Thursday 15 Aug) under irrigated pasture at 10cm depth at Lancaster was 8oC, the same as last week (8 Aug)
- This time last year (15 Aug 2018), the soil temperature was 7oC
LAST WEEK’S REFERENCE EVAPOTRANSPIRATION+ – Pasture Irrigation Requirement$
Weekly Totals – Thurs 8 Aug to Wed 14 Aug, 2019
|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, 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 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 15 Aug to Wed 21 Aug, 2019
|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 service please don’t hesitate to contact me, Rob O’Connor at email@example.com.
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.
+ 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.