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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