Overseer nutrient budgets and irrigation
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OVERSEER® Nutrient budgets and Irrigation. David Wheeler 1 , John Bright 2 1 AgResearch, Hamilton 2 Aqualinc, Christchurch. Background. Overseer is a nutrient management tool Drainage is an important driver of leaching Irrigation inputs accounted for in the drainage calculation

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Overseer nutrient budgets and irrigation
OVERSEER® Nutrient budgets and Irrigation

David Wheeler1, John Bright2

1AgResearch, Hamilton

2Aqualinc, Christchurch


  • Overseer is a nutrient management tool

  • Drainage is an important driver of leaching

    • Irrigation inputs accounted for in the drainage calculation

  • Concerns about use of irrigation in Overseer

    • Aligning irrigation inputs to climate data inputs

    • Type of management options within Overseer

Drainage model
Drainage model

  • Each day:

  • SMt = SMt-1 + rainfall + irrigation

  • – AET – drainage – runoff

  • Drainage occurs when soils exceed field capacity

  • The amount and timing of irrigation can affect drainage

  • Climate data determines rainfall pattern, potential evapotranspiration

Inputting irrigation data into ovs
INPUTTING irrigation data into OVS

‘Method only’

‘Method plus rate’

Using method only
USING ‘Method only’

  • Overseer automatically calculates the irrigation amount required to maintain high soil moisture content while minimising drainage



Field capacity






Wilting point

Using method only1
USING ‘Method only’

  • Includes an estimate of ‘system losses’

    • Losses due to leaks, overlaps, inefficiencies, etc

    • Added to drainage

    • Border dyke also includes outwash loss

      Active management check box

    • Calculation then based on:

      • No system losses assumed

      • No irrigation applied within 5 days of rainfall

        Requires nutrient concentrations in water

    • Border dyke outwash can be recycled

Using method plus rate
USING ‘Method plus rate’

  • A three step process:

  • Irrigation amount estimated as per ‘method only’

  • Applied rate estimated from user-entered rate data

  • If estimated applied rate > method only rate

    • Difference added to drainage

      ESSENTIAL that entered rate and climate data align

Climate data
Climate data

  • User inputs to describe climate are:

    • Annual rainfall

    • Annual PET

    • Annual average temperature

  • Default PET and temperature based on long-term climate data set

  • User can define period annual data is average of, but

    • Data must be commensurate with the objectives of setting up the Overseer file, and with management input data

Climate patterns
Climate patterns

  • Overseer climate database uses a ‘typical’ distribution of rainfall, PET and temperature:

    • Currently typical long-term patterns

    • Moving to site-specific long-term patterns

  • Important to align climate and irrigation rates

Illustration of long-term average rainfall patterns and variation with a single year

New work underway
New work underway

  • The following projects will add robustness to the use of irrigation in Overseer:

  • Comparison of drainage estimates under irrigation between Overseer and IrriCalc

    • Also looking into additional management options to describe irrigation use

  • Improving supporting databases:

    • Soil data

    • Default climate data

Effect of management rules on calculated drainage
EFFECT of Management rules on calculated drainage

800 mm rainfall, irrigated October to February central pivot


* Estimated irrigation rate (mm/yr)

** Estimated drainage (mm/yr)


  • Recommend use ‘method only’ for now

    • This leaves Overseer to estimate the irrigation rate

    • Minimises the risk of not aligning irrigation rate and rainfall

  • If using ‘method plus rate’, then align climate and irrigation rate

    • Not recommended

    • Using this option may lead to an overestimation of drainage

  • Irrigation management rules are important

    • Investigating how to increase range of rules in Overseer

  • Look out for further updates

    • As we work through the results of the Irricalc comparison project


  • OVERSEER® owners are Ministry for Primary Industries, Fertiliser Association of New Zealand and AgResearch