Thursday 24 September 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.

The interactive irrigation scheduling tool helps you determine the most productive timing for the next surface irrigation for your unique circumstances.  It uses the same ETo information in the tables below.

IRRIGATION COMMENTARY

Keeping an eye on plant water requirements

• The water requirement for our reference crop of pasture (ETo) over the past 7 days was 18mm (0.18ML /ha), which is 1mm less than the previous week, at Kyabram.  See the tables and graphs below
• For most locations, similar ETo rates are forecast for the next 7 days.  See tables and graphs below
• Some rain is forecast tomorrow and for Wednesday.  See tables and graphs below

Water trading webinar – Wednesday

• An AgVic “Water Trading 101 – Situation and Seasonal Outlook” webinar will be held at 1pm this coming Wednesday, 30 September
• The webinar will discuss key features of the connected southern Murray Darling Basin water market and the current situation and seasonal outlook for water availability and price
• For more details and to register click here

Upcoming sMDB water availability – factors to consider

• Even though the BoM Seasonal Climate Outlook (updated today) favours above ‘average’ rainfall across our main catchment areas, the BoM Streamflow Forecast for September to November shows probabilities for ‘average’ to below average inflows for Lake Eildon, Lake Dartmouth and Hume Dam
• The Seasonal Streamflow Forecast factors the catchment conditions into the streamflow forecast.  Root zone soil moisture in the upper Goulburn and upper Murray catchments is currently below average.  It’s also worth noting that the Seasonal Streamflow Forecast has moderate to high skill score this time of the year for these catchments
• The Water Allocation Outlook for February 2021 for an average inflow scenario, is 100% HRWS for both the Victorian Murray and Goulburn systems and 47% General Security (GS) for the NSW Murray
• For a “below average inflow scenario” (exceeded 70 to 75 years in 100), the Water Allocation Outlook for February 2021, is 100% HRWS on the Victorian Murray system, 87% HRWS on the Goulburn and 31% GS on the NSW Murray
• For comparison, last irrigation season, Vic Murray finished on 66% HRWS, the Goulburn finished on 80% HRWS and NSW Murray finished on 0% GS
• This irrigation season, there would be an extra 790GL (1,674GL x 47% = 790GL) or 520GL (1,674GL x 31% = 520GL) allocated on the NSW Murray system alone, under “average” (47% GS) or “below average” (31% GS) inflow scenarios respectively, compared to last season (0% GS)
• An extra 500GL was also carried over in the sMDB into this season compared to last season (1,316GL vs 812GL)
• As always, we are really in the ‘hands of the Gods’ in terms of future rainfall, inflow, water allocations and to a large extent, the price of water allocation
• In the meantime, monitoring the key drivers of water availability and price, assists in gauging where the market may go and what early actions might be needed to secure water (and stock feed) requirements, depending on your unique farm business circumstances

SOIL TEMPERATURE

• The soil temperature as measured by a capacitance probe at 9am today (Thursday 24 Sept) under ‘surface irrigated pasture’ at 10cm depth at Lancaster was 11oC, the same as this time last week (17 Sept)
• This time last year (17 Sept 2019), the soil temperature was 10oC

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

Weekly Totals – Thurs 17 Sept to Wed 23 Sept 2020

Tip:  To convert mm to megalitres /ha, divide by 100.  Eg. 50mm = 0.5ML /ha

Important:  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 (Sept) for each 3mm 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 24 Sept to Wed 30 Sept 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.

If you would like 7-day forecast ETo (and rainfall, temperature and wind) for any selected location/s emailed to you daily, click here.

⁺ 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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