Weekly Irrigation Requirements

 

 

 

 

Friday 8 December 2017

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

Irrigation commentary

Timing the next irrigation

  • After last week-ends rainfall deluge, the forecast for the next 7 days is for further relatively fine weather
  • A series of high ET days (a bit higher than the average for December) are forecast to start early next week
  • The next surface irrigation on pasture is due this coming Monday at Kyabram – based on cumulative ETo-R figures (40mm) and assuming the soil profile was filled with last week-ends rainfall and paddocks had drained by Sunday night

Current growth rates and the grazing rotation

  • The soil temperature has dropped 4oC compared to the same time last week
  • Sunlight levels were relatively low for this time of the year over the past 7 days http://www.bom.gov.au/watl/eto/ due to cloud cover
  • Waterlogging may have also been an issue for some paddocks following the rain
  • These factors combined are likely to have reduced growth rates on irrigated paddocks and the grazing rotation may need to be extended in order to optimise growth and yields

 Soil temp

  • The soil temperature (as measured by a capacitance probe) at 9am this morning under irrigated pasture at Lancaster at 10cm depth was 18oC, 4oC cooler than last week
  • The soil temperature (as measured by a capacitance probe) at 9am this morning under irrigated pasture at Yarrawalla at 10cm depth was 15oC, 5oC cooler than last week. This time last year the soil temperature at Yarrawalla was 16oC

Last Week’s Reference Evapotranspiration – Pasture Irrigation Requirements

Weekly Totals –  Friday 1 December to Thursday 7 December, 2017

Location ETo+

(mm)

Rainfall~

R

(mm)

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* 36 61 -25 1 5.2 9
Kyabram* 31 91 -61 1 4.4 10
Tatura* 30 82 -52 1 4.3 10
Kerang# 30 40 -10 1 4.2 9
Echuca# 28 133 -105 1 4.0 10
Yarrawalla# 29 55 -26 1 4.1 10
Numurkah# 27 83 -56 1 3.9 10

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 (December) for each 7mm of effective rain that falls on your property, typically an extra day needs to be added to the “required surface irrigation interval.”

ETo during December at Kyabram over the previous 7 years has averaged 48mm /week or 6.8mm /day (BoM ETo).  Average ETo for the previous month of November (for the previous 8 years) is 39mm /week or 5.5mm /day.

Next Weeks Forecast Reference Evapotranspiration – Pasture Irrigation Requirement$

Weekly Totals –  Friday 8 Dec to Thursday 14 Dec, 2017

Location ETo

(mm)

Rainfall

R

(mm)

ETo – R

(mm irrigation required)

Average ETo
(mm /DAY)
Required Surface Irrigation Interval (Days) > Spray irrigators will need to apply  (mm)
Deniliquin 52 2 50 7.4 7 50
Kyabram 47 0 47 6.7 8 47
Tatura 45 0 45 6.4 8 45
Kerang 52 0 52 7.4 7 52
Echuca 49 0 49 7.0 8 49
Yarrawalla 51 0 51 7.3 7 51
Numurkah 48 0 48 6.8 8 48

 Daily ETo & rainfall – last week & next week

Content Sources and further information

* 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.  Unfortunately this web site is for registered users only.  Comment: SILO ET data generally appears to be slightly lower than BOM ET data. http://www.longpaddock.qld.gov.au/silo/index.html

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

~Rainfall R.”  To keep it simple, it is assumed all of this rain is available for plant growth – except where a rainfall event is less than 2mm, in which case it is unlikely to be listed. In practice it may 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 increases ‘cumulative ETo-R’ back above 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 needed 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 poor spray uniformity or application efficiency, then more water will need to be applied to best meet plant needs.  Because rainfall can vary substantially over short distances, it is advised to use rainfall figures from your farm in the “ETo – R” calculation rather than the ‘generic’ rainfall figures given in the tables.  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 ETo figures are not publically available.

 

 

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