Wednesday 11 September 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 now available on ExtensionAus.

Storage levels remain sound and higher allocations are likely to lead to another big irrigation season. Weather outlooks remain variable for spring, keep a close eye on the climate drivers.

In this update:

1. Weekly irrigation requirements summary
2. Commentary (upcoming irrigation water availability)
   – recent rainfall (plains and upper catchment)
   – southern Murray Darling Basin (MDB) water allocations
   – southern MDB allocation outlooks
   – current MDB water in store
   – current Murray system inflows
   – soil moisture profile – upper Murray and upper Goulburn
   – seasonal streamflow forecasts – Dartmouth, Hume and Eildon lakes
3. Last week’s irrigation requirement
4. Next week’s irrigation requirement
5. Daily ETo and rainfall
6. Soil temperature and moisture

1.  Weekly irrigation requirements – summary

• Dashboard estimate:  The required pasture surface-irrigation interval (for a 40mm refill point) is 20 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 (ETo) for the past 7 days was 16 mm at Kyabram.
• Cumulative ETo for the next 7 days is forecast to be 20 mm at Kyabram. Use the online ETo Tool for updates.

‘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 (upcoming water availability)

Recent rainfall (plains and upper-catchment)

The Bureau of Meteorology (BoM) rainfall map below shows that during the past 7 days 1-5mm of rain fell across large parts of the irrigated plains, and relatively light falls in most of the upper catchments with the Goulburn upper catchment receiving the heaviest falls.

Source: BoM.

The current forecast for the next 8 days according to the BoM:

Source: BoM.

For a more in-depth look at the climate drivers have a look at The Fast Break newsletter and The Very Fast Break YouTube channel.

The latest climate webinar from Agriculture Victoria on the spring seasonal outlook is now available.

MDB Water Allocations

Source:  MDBA River Murray Weekly Report .

For water allocations in other catchments visit NVRM.

Southern MDB Allocation Outlooks

In the last allocation outlook announcement made on the 15 August 2024, the Northern Victorian Resource Manager indicated that with average inflows both the Goulburn and Victorian Murray systems would reach 100% high-reliability water share by 15 October 2024.

For NSW, in the last allocation outlook announcement made on 15 August 2024, Department of Climate Change, Energy, the Environment and Water (DCCEEW) indicated that with average inflows the NSW Murray system could reach 110% general security allocation by 1 February 2025, while the Murrumbidgee system could reach 47% general security allocation by 1 February 2025.

For allocation outlooks in other Victorian catchments visit NVRM.

MDB water in store

Currently, the active MDBA storage (Dartmouth, Hume, Victoria and Menindees lakes) is above the long-term average for this time of year.  (See the MDBA graph below.)

Source:  MDBA River Murray Weekly Report

Current MDB inflows

Currently, Murray system daily inflows are below the long-term average for this time of year.  It’s worth noting this was also the case at the same time last year, and inflows increased markedly with subsequent rainfall. (See the MDBA graph below.)

Source:  MDBA River Murray Weekly Report.

Soil moisture profile – upper Murray and upper Goulburn

Currently the relative rootzone soil moisture (top 1m of the soil profile) for the upper Murray is largely below average, where-as for the upper Goulburn relative rootzone soil moisture ranges from average to very much below average.  (See the BoM soil moisture maps below.)

Source:  BoM.

For relative rootzone soil moisture information for other catchments, visit BoM.

Seasonal streamflow forecasts – Dartmouth, Hume and Eildon lakes

The BoM seasonal streamflow forecasts were released yesterday and show below average inflows for September to November 2024 for Dartmouth, Hume and Eildon lakes. See the BoM graphs below. Keep in mind the current lake storages are Dartmouth 96%, Hume 72% and Eildon 89% capacity.

Source:  BoM.

For seasonal streamflow forecasts for other catchments please visit BoM.

3.  Last week’s reference evapotranspiration (ETo)⁺

Pasture Irrigation Requirement$

Weekly totals – Wednesday 4 September – Tuesday 10 September

Location	ETo (mm)	Rainfall (R) mm	ETo – R (mm irrigation required)	Pasture Coefficient	Ave ETo (mm /DAY)	Required Surface Irrigation Interval (days)	Spray irrigation required (mm/week)
Deniliquin	19	2	17	1	2.7	17	17
Kyabram	16	1	15	1	2.3	20	15
Tatura	15	2	13	1	2.2	20	13
Kerang	20	2	18	1	2.9	15	18
Echuca	17	1	16	1	2.5	17	16
Yarrawalla	18	2	16	1	2.6	16	16
Numurkah	17	2	15	1	2.4	18	15

ETo data and associated irrigation information for these locations is updated daily and can be accessed on the Online ETo Tool.

The weather data used in this update is now sourced from BoM.

Historic daily ETo and rainfall data is sourced from BoM’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 BoM from different weather stations.

Forecast daily ETo data is from BoM’s Gridded Evapotranspiration Products derived from the Australian Digital Forecast Database (ADFD), which is ‘fee for service’.

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.

4.  Next week’s forecast reference evapotranspiration (ETo)

Pasture Irrigation Requirement$^¥

Weekly totals – Wednesday 11 September – Tuesday 17 September

Location	ETo (mm)	Rainfall (R) mm	ETo – R (mm irrigation required)	Ave ETo (mm /DAY)	Required Surface Irrigation Interval (days)	Required Spray Irrigation (mm)
Deniliquin	25	4	20	3.5	13	20
Kyabram	20	5	15	2.8	16	15
Tatura	19	6	13	2.7	17	13
Kerang	25	4	21	3.5	13	21
Echuca	21	4	17	3.0	15	17
Yarrawalla	22	4	18	3.2	15	18
Numurkah	21	5	16	3.0	15	16

^< Only daily rainfall events 2mm or greater are included in the above tables and in the ETo graphs below.

5.  Daily evapotranspiration (ETo) and rainfall – last week & next week

7.  Soil temperature

The soil temperature measured by a capacitance probe at 9 am– 28 August under surface irrigated annual pasture at 10 cm depth at Wee Wee Rup (near Leitchville) was 13.98°C.

At Wandella at the same time the soil temperature was 13.9°C under irrigated lucerne at 10 cm depth.

At Tatura at the same time the soil temperature was 11.2°C at 10cm depth.

 

Graph 1: Soil Temperature at Wee Wee Rup, measured by a capacitance probe

Soil moisture is being drawn down by active pastures at Wandella. It was irrigated 11 days ago.

Graph 2: Soil moisture graph under pasture at Wandella

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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.

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 BoM AWO data.  BoM 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 BoM AWO. BoM 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 BoM’s Gridded Evapotranspiration Products derived from the Australian Digital Forecast Database (ADFD).  Forecast rainfall is also from BoM’s ADFD.

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