Weekly Irrigation Requirements – Macalister Irrigation District

Macalister ETo Update


The weekly Macalister ETo update has now finished and will recommence at the start of the next irrigation season.


13th April to 19th April 2018

Cowwarr’s weather data is missing for this week so we have used our 2 local weather stations to calculate the ETo.

  • Flood irrigation interval for the last week was 14 days
  • Spray irrigators needed to have applied on average 18.2 mm of irrigation water for the week
  • The predicted weekly pasture water use for the MID is 16.3 mm

* These figures are approximate and do not take into account rainfall on farm*

The Odd Spot

Soil temperature at Tinamba decreasing

Soil temperature at Tinamba has decreased over the last few weeks from 19.1 to 16.2 degrees, with the average temperature for this week sitting at 15.4 degrees.

Glenmaggie storage level at 31.3%

Glenmaggie level is approximately 10% below average for this time of year, at 31.3% capacity. According to SRW’s MIA newsletter the cumulative deliveries to date are tracking at approximately 20,000 ML above average for this time of year.

  Still putting out ETo due to the dry conditions

Note that while it’s not raining, and thus you still need to irrigate, your irrigation intervals will be getting longer and longer as ETo rates fall. Flood irrigators particularly may find pasture growth/quality declining as pasture is waterlogged for longer at this time of year. Please note that due to the lack of rain, we will still be putting out the ETo until the end of the season.

ETo – Last week totals

Location ETo mm Effective rain* mm (R) Dairy pasture coefficient Required irrigation mm
 (ETₒ – R)
Tinamba 16.4 3 1 13.4
Boisdale 20.6 3 1 17.6

Tip:  To convert mm to megalitres/ha, divide by 100.  Eg. 50mm = 0.5ML /ha.
* Effective rainfall = daily rainfall minus 2mm.
ETₒ   and the “crop coefficient” (conversion factor) of 1.0 applies to a healthy productive pasture 12 cm high.

MID average daily pasture water use over the last week was 2.6 mm/day.

The approximate MID flood irrigation interval based on last week’s daily pasture water use rate was 14 days. (This figure assumes that 30 mm of readily available water is held in the pasture root zone divided by the 2.6mm/day = 12 days and then add an extra two days to account for water logging). This does not take into account any rain your farm may have got during the week which will extend the interval.

Spray irrigators in the MID needed to have applied on average 18.2 mm of irrigation water for the week to meet pasture water use (2.6 mm x 7 days). This irrigation figure does not take into account any rainfall on your farm, which must be subtracted from the required irrigation figure, and assumes that the spray irrigation system is applying water at 100% uniformity. This figure may be greater depending on specific spray irrigation system uniformity.


For Friday 20 April to Thursday 26 April

The predicted weekly pasture water use for the Macalister Irrigation District is 16.3 mm per hectare (0.16 ML/ha), or a pasture water use rate of 2.3 mm/day per hectare.

The intention of this service is not for the information to be used in isolation when making decisions about irrigation scheduling.

ETo provides a relatively objective estimate of plant water use and provides another handy ‘tool in the irrigation scheduling tool box.’ Information in this email is only a guide and should only be used in conjunction with other tools including updated weather information.

For improved accuracy, the collection and use of individual farm rainfall measurements is advised.


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