Wednesday 6 November 2024
Weekly Irrigation Requirement$ Summary – Driving Water Productivity
Northern Victoria
This update provides ‘reference evapotranspiration’ (ETo) information to assist with improved irrigation scheduling for top yields and high water productivity.
Similar weather and irrigation scheduling information provided in this email that is updated daily for any Vic location, including an interactive irrigation spreadsheet, is available on ExtensionAus.
No login or password is required to use this tool.
In this update:
- Weekly irrigation requirements – summary
- Commentary:
– October 2024 irrigation requirements
– Southern Basin water allocation update
– Risk of spill announcement anticipated
– Seasonal outlook update
– Whats on: Farm emissions workshop for livestock producers
– Link: Impacts of new National Water Agreement anticipated by Bundaberg farmers - Forewarned is forearmed
– Recent rain: Irrigated plains and headwaters
– Forecast rain: 6-9 November. 10-13 November. Next 4 weeks – Tragowel
– Chance of at least 15mm beyond 8 days
– Forecast maximum temperatures: Next 4 weeks – Tragowel - Last week’s irrigation requirement
- Next week’s irrigation requirement
- Daily ETo and rainfall
- Soil temperature
- Seven-day forecast: Wind, temperature, humidity, rain and ETo – Tragowel
1. Weekly irrigation requirements – summary
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- Dashboard estimate: The required pasture surface-irrigation interval (for a 40mm refill point) is 10 days based only on cumulative ETo for the past week (not including any rainfall), at Kyabram.
- The plant water requirement (irrigation and rainfall) for our reference crop of pasture (cumulative ETo) for the past 7 days was 35mm at Kyabram.
- Plant water requirements for the past week (35mm) is higher than previous weeks.
- The weekly ETo forecast continues to climb. Cumulative ETo for the next 7 days is forecast to be 41mm at Kyabram. Use the online ETo Tool for updates.
- Today (Wednesday) is forecast to be a high ETo day, with high temperatures, windy, relatively sunny, and lower humidity.
- The forecast is for 1-10mm rain across the GMID and 10-25mm in the upper catchments in the next 8 days, at this stage.
‘Water requirement’ for our reference crop of pasture (ETo) at Kyabram*:
* See the tables and graphs below for ETo (and rainfall) at more locations.
** Rainfall is not included in these estimates. The figures presented are cumulative daily short crop reference evapotranspiration (ETo).
2. Commentary
October 2024 irrigation requirements
The total pasture water requirement (ETo, irrigation and rain) for October 2024 was 137mm (1.37ML/ha), which is 51mm more than the previous month (September 86mm), and 10mm more than the longer-term October average (127mm, 2012-2023), at Kyabram.
Total rainfall in October 2024 was only 18mm, which is 5mm more than the previous month (September, 13mm), and 25mm less than the longer-term October average (43mm, 2010-2023), at Kyabram.
Based only on this information, the net pasture water requirement for October 2024 was 119mm (or 1.19ML/ha, 137mm minus 18mm), at Kyabram.
Long-term average ETo rates increase to 38mm/week in November, from 29mm/week in October, at Kyabram.
Southern Basin water allocation update
Water allocations were updated on Friday last week (1 November). Of note in the key Southern Basin irrigation systems was a 4% increase in the Victorian Murray. No changes were made to the NSW Murray or Murrumbidgee. Some small allocation increases were also made in some of the smaller Victorian irrigation systems. See the table below.
Southern Basin water allocations (%)
*HRWS: High-reliability water share. HS: High Security. LRWS: Low-reliability water share. GS: General Security.
Sources: NVRM. NSW government. MDBA.
Risk of spill announcement anticipated
The next risk of spill announcement will be made on Monday, 11 November. In the last announcement made on 10 October, the risk of spill of Victoria’s share of Lake Hume was 45%, at Lake Eildon it was 30%, and at Lake Eppalock it was 25%. So far this season, irrigation deliveries have been very high, whilst storage inflows have been very low, reducing the risk of spill.
According to the Victorian Water Register, there is currently 298GL in Murray spillable accounts, 381GL in Goulburn spillable accounts, and 16GL in Campaspe spillable accounts.
When the risk of spill is determined to be below the 10% threshold, water is released from spillable accounts for use and trade.
Seasonal outlook update
The Fast Break reports that the outlook from 12 global models for November 2024 to January 2025 is split between likely wetter and neutral across Victoria. Confidence in wetter predictions are lower, based on the poor form to date. A warmer than average outlook is given for the 3 months ahead.
What’s on
- Farm emissions: the ins and outs for livestock producers. 14 November. Tatura.
Link
3. Forewarned is Forearmed
Recent rain
The Bureau’s rainfall map below for the past 7 days shows no rain across the Southern Basin irrigated plains and upper-Murray and upper-Goulburn.
Source: Bureau.
Forecast rain 6-9 November
Source: Bureau.
Forecast rain 10-13 November
Source: Bureau.
Forecast rain, next 4 weeks – Tragowel
The Bureau rainfall outlook shown in the graph below favours average to wetter conditions for each of the next 4 weeks (2-5mm /week medians) for Tragowel, at this stage.
To generate a similar graph to the one above for your location, click on the Forewarned is Forearmed link immediately below, then on the bureau page click on the location icon, enter your location, and select the ‘Rainfall climate summary’ graph icon. Source: Forewarned is Forearmed. Bureau.
Chance of at least 15mm, 15-21 November (‘beyond 8 days’)
The Bureau rainfall outlook shown in the map below currently indicates a 10-25% chance of receiving at least 15mm of rain in the week 15-21 November across the GMID, at this stage.
Chance of at least 15mm rainfall for 15-21 November
Source: Forearmed is Forearmed. Bureau.
Forecast maximum temperatures. Next 4 weeks – Tragowel
The Bureau outlook shown in the graph below currently favours above-average to average maximum temperatures (27-30oC weekly medians) for each of the next 4 weeks at Tragowel, at this stage.
Source: Forearmed is Forearmed. Bureau.
4. Last week’s reference evapotranspiration (ETo)
Pasture Irrigation Requirement$
Weekly totals – Wednesday 30 October – Tuesday 5 November
5. Next week’s forecast reference evapotranspiration (ETo)
Weekly totals – Wednesday 6 November – Tuesday 12 November
6. Daily evapotranspiration (ETo) and rainfall – last week and next week
7. Soil temperature and moisture
The soil temperature measured by a capacitance probe at 9 am today (Wednesday 30 October) under surface irrigated annual pasture at 10cm depth at Wee Wee Rup (near Leitchville) was 19.8°C.
Soil temperature at Wee Wee Rup (oC)
At Wandella (near Kerang) at 9 am this morning the soil temperature was 16.6°C at 10cm depth under irrigated lucerne.
The soil temperature at the Tatura weather observation station at 10cm depth at 9 am today was 18.8oC.
Soil moisture at Wandella under irrigated lucerne
8. Seven-day forecast – wind, temperature, humidity, rain, and ETo – Tragowel
Source: SWAN.
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Disclaimer
The intention of this service is not for the information to be used in isolation when making decisions about irrigation. 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.
This publication is provided solely for information purposes and no representation or warranty is made as to its quality, accuracy, completeness, suitability or fitness for any particular purpose.
You should make your own inquiries as to its appropriateness and suitability for your circumstances.
The State of Victoria as represented by the Department of Energy, Environment and Climate Action (DEECA) accepts no duty of care and disclaims all liability in relation to use of this publication.
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+ Reference evapotranspiration (‘ETo’). Refers to the evapotranspiration from an actively growing, well-watered, grass stand 120mm in height. Sourced from AgVic Online ETo Tool, which uses bureau AWO data. Bureau AWO gridded daily historical ETo figures may change significantly over time, particularly in the first three days, because improvements are made to the dataset as more data is received by the BoM from different weather stations.
~ ‘Rainfall R.’ Historical rainfall data is sourced from bureau AWO. Bureau AWO gridded daily historical rainfall figures may change significantly over time, particularly in the first three days, because improvements are made to the dataset as more data is received by the BoM from different weather stations. Where a rainfall event is less than 2 mm it is usually not ‘effective’ in terms of adding to the soil moisture reserve (because it is unlikely to penetrate through the crop canopy) and is unlikely to be listed in the tables, or graphs. Also be aware that any rain more than that which fills the soil root profile (run-off or deep percolation) is also not ‘effective’ from an irrigation perspective. Because rainfall can vary over short distances, you are encouraged to collect and use your own rainfall figures for your farm.
x ‘Dairy pasture crop coefficient.’ A crop coefficient refers to your crops relative water use potential. For crops other than pasture refer to Tech Note for discussion of an appropriate crop coefficient. Alternatively, you can use AgVic’s Online ETo Tool Interactive Spreadsheet which provides guidance on which crop coefficient to use for your crop and its stage of development.
^ 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. e.g. Required surface irrigation interval = 40 mm ÷ average (daily) ETo, plus 1 day to allow for excess water to drain away following a surface irrigation event. The ‘Required surface irrigation interval’ is only a dashboard estimate based only on cumulative ETo over the recent 7 days, to suit the purpose of a weekly update.
For more accurate irrigation scheduling particularly in spring and autumn, the cumulative deficit (‘ETo – R’) since the last irrigation usually needs to be calculated over a period longer than 7 days and the irrigation timed when the readily available water (40mm in this case) is depleted.
> ‘Spray irrigators need/ed to apply.’ This figure is based only on evapotranspiration minus rainfall (‘ETo – R’) for the given week and assumes excellent accuracy in terms of mm water applied, spray uniformity, and application efficiency.
Typically spray uniformity will not be 100% and extra water will need to be applied to best meet plant needs. The ‘spray irrigators need/ed to apply’ figure given in the table also assumes the rainfall is effective and contributes equivalently to the soil moisture reserve, which may not be the case.
In practice, the moisture deficit (or reserve) carried into the season also needs to be considered to ensure there is adequate soil moisture for maximum plant growth and to avoid a ‘green drought.’ It is best to use rainfall measurements collected on your property. Typically, with pressurised irrigation systems, irrigations occur more regularly than with surface irrigation to optimise plant growth.
Due to the cumulative error involved with using ETo data over a period, it is important that other scheduling methods are used in conjunction with ETo data for spray irrigation systems.
¥ ‘Next week’s forecast reference evapotranspiration – pasture irrigation requirement.’ Forecast ETo and rainfall data is sourced from AgVic’s online ETo tool, which uses forecast daily ETo data from the bureau’s Gridded Evapotranspiration Products derived from the Australian Digital Forecast Database (ADFD). Forecast rainfall is also from the bureau’s ADFD.
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ETo data and associated irrigation information for the locations shown in this newsletter is updated daily and can be accessed on the Online ETo Tool.
Historic daily ETo and rainfall data is sourced from the bureau’s gridded Australian Water Outlook service.
Historical daily ETo and rainfall figures may change significantly over time, particularly in the first three days, because improvements are made to the dataset as more data is received by the bureau from different weather stations.
Forecast daily ETo data is from the bureau’s Gridded Evapotranspiration Products derived from the Australian Digital Forecast Database (ADFD), which AgVic receives on a ‘fee for service’ basis.
Forecast rainfall is also from the ADFD. Please visit AgVic’s online ETo Tool for updated ETo and rainfall data for your location.
Tip: To convert mm to megalitres /ha, divide by 100. E.g. 50mm = 0.5ML /ha.
Important: The ‘required surface irrigation interval’ shown in these tables is a ‘dashboard estimate’ and is calculated using cumulative ETo only over seven days and needs to be considered in conjunction with rainfall on your farm.
Because rainfall can vary over short distances, please collect and use your own rainfall measurements. The rainfall figures in the tables are only a guide.
For more information about selecting an appropriate crop coefficient for your particular crop and its stage of development refer to this Tech Note What is evapotranspiration and how do I use it to schedule irrigations?, Ag Vic’s Online ETo Tool Interactive Spreadsheet, or the scientific booklet (FAO56) – Table 12, Chapter 6.
