Weekly Irrigation Requirements




Friday 16 February 2018

Weekly Irrigation Requirement$ Summary – Driving Water Productivity

This update provides ‘reference evapotranspiration’ (ETo) data to assist with improved irrigation scheduling.

This Irrigation Scheduling Tool helps you determine the most productive timing for the next irrigation for your unique circumstances.

Irrigation commentary

Current water requirements (ETo)

  • A trend of generally moderate daily summer plant water requirements are forecast to continue next week

How much moisture is likely to be readily available for my actively growing crop?

  • The amount of plant Readily Available Water (RAW) will vary on factors including the crop and soil type
  • RAW for crops such as mature lucerne, maize or sorghum are likely to be higher than for pasture because of a deeper and stronger root system
  • For established pasture on surface irrigated soils in the GMID, typically there will be up to ‘30mm to 60mm’ (0.3 megalitres per hectare to 0.6ML /ha) of RAW
  • For the surface irrigation interval calculations used in these updates, it is assumed there is 40mm of pasture RAW (plus 1 to 2 days following irrigation for the soil to drain)
  • Mature actively growing maize can typically extract up to 60mm to 70mm of RAW
  • Mature actively growing lucerne can typically extract 70mm to 100mm of RAW
  • RAW will be lower for newly established plants with a smaller root system
  • If irrigation intervals are stretched out to a point where plant water uptake exceeds RAW, a “green drought” situation will start to occur and growth rates and yields will progressively decline
  • Paddock and crop assessments can be used over time to more accurately estimate the appropriate RAW value for your specific circumstances
  • You can use the attached Irrigation Scheduling Tool to see how the optimal irrigation interval is affect by the amount of RAW

Goulburn to Murray Trade Limit

  • The negative balance for the Goulburn to Murray trade limit is currently 12GL http://waterregister.vic.gov.au/
  • The negative balance has reduced by 62GL since it was last reported in this email update a month ago (74GL then)
  • It is expected that opportunity to trade allocation out of the Goulburn system will start up when the negative balance reaches zero

Yesterdays Water Announcements

  • NSW Murrumbidgee General Security water increased 1% to 34%
  • Allocations on the Vic Murray and Goulburn systems and NSW Murray system remain unchanged (100% HRWS, 100% HRWS and 49% GS respectively)
  • The shortfall to a low-reliability water share determination on the Vic Murray system is 420GL, while on the Goulburn it is 770GL
  • The outlook with average inflow conditions on 25 August 2018 for the Vic Murray is 76% HRWS while on the Goulburn it is 71% HRWS https://nvrm.net.au/outlooks

Soil temp

  • The soil temperature (as measured by a capacitance probe) at 9am this morning under irrigated pasture at Lancaster at 10cm depth was 19oC, 4oC cooler than last week
  • The soil temperature (as measured by a capacitance probe) at 9am this morning under dried-off perennial pasture at Yarrawalla at 10cm depth was 20oC, 3oC cooler than last week. This time last year the soil temperature at Yarrawalla was 18oC

Last Week’s Reference Evapotranspiration – Pasture Irrigation Requirement$

 Weekly Totals –  Fri 9 Feb to Thurs 15 Feb, 2018

Location ETo+





ETo – R

(mm irrigation required) 

Dairy x pasture crop coefficient Average^ ETo
 (mm /DAY)
Required@ Surface Irrigation Interval (days) Spray> irrigators needed to apply (mm)
Deniliquin* 55 0 55 1 7.8 7 55
Kyabram* 39 0 39 1 5.6 9 39
Tatura* 42 0 42 1 6.0 9 42
Kerang# 41 0 41 1 5.8 8 41
Echuca# 40 0 40 1 5.6 8 40
Yarrawalla# 40 0 40 1 5.7 8 40
Numurkah# 40 0 40 1 5.7 8 40

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 based only on cumulative ETo and needs to be considered in conjunction with rainfall on your farm.  For example, this time of the year (February) for each 6mm of effective rain that falls on your property, typically an extra day needs to be added to the “required surface irrigation interval.”

ETo during February at Kyabram over the previous 7 years has averaged 43mm /week or 6.1mm /day (BoM ETo).  Average ETo for the previous month of December (for the previous 7 years) is 50mm /week or 7.2mm /day.

Next Weeks Forecast Reference Evapotranspiration – Pasture Irrigation Requirement$¥

Weekly Totals –  Fri 16 Feb to Thurs 22 Feb, 2018

Location ETo





ETo – R

(mm irrigation required)

Average ETo
(mm /DAY)
Required Surface Irrigation Interval (Days) > Spray irrigators will need to apply  (mm)
Deniliquin 52 4 48 7.4 7 48
Kyabram 46 6 40 6.5 8 40
Tatura 44 6 38 6.2 8 38
Kerang 51 4 47 7.2 8 47
Echuca 49 5 44 7.0 8 44
Yarrawalla 50 5 45 7.2 8 45
Numurkah 46 5 41 6.6 8 41

 Daily ETo & rainfall – last week & next week



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* ETo and rainfall data sourced from Bureau of Meteorology http://www.bom.gov.au/watl/eto/

# 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. http://www.longpaddock.qld.gov.au/silo/index.html

+ Reference evapotranspiration (“ETo”).  Refer to the ‘Important Background Information’ section above.

~Rainfall R.”  Where a rainfall event is less than 2mm it is unlikely to be listed.  In practice it is not always be the case that all rain contributes to plant available moisture reserves and is available for plant growth.  Any rain in excess of that which fills the soil profile (or reduces ‘cumulative ETo-R’ back below the estimated RAW) 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.”  Refer to the section above “Crops Other Than Pasture”, for more information.

^ 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.)  The “required surface irrigation interval” is based only on cumulative ETo and needs to be considered in conjunction with past and forecast rainfall for your farm.
> 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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