Lake Rotorua

Lake Rotorua

Calculating nitrogen loss

The Nitrogen Discharge Allowances (NDA) within the proposed rules is based on estimates of nitrogen loss using the Overseer® nutrient budget model. A NDA sets out how the amount of nitrogen loss that is allowed to occur from a property after 2032. It is expected that Overseer® will continue to be used to estimate nitrogen loss on farms in the Lake Rotorua catchment. 

Calculating nitrogen losses using Overseer®

Overseer®  is the only credible national tool that can estimate annual nutrient losses from farms, especially nitrogen leaching. Overseer®  enables the effects-based management of land use impacts on water. Physical measurement is cost-prohibitive and subject to sampling, temporal and spatial variability. Other computer models are available but are generally more suited to a research context (e.g. SPASMO, APSIM) or are crop-specific (e.g. AmaizeN).

There are limitations with the Overseer®  model that are well recognised. These include technical limitations due to a lack of calibration in certain farm systems and geophysical conditions, as well as the inherent errors that will occur in estimating losses from any complex biological system.

There are also limitations associated with how it is used – users need to enter actual and reasonable input values. This requires a good understanding of farm systems in general and the farm being modelled in particular.

About Overseer®

Overseer® is New Zealand’s leading farm-scale nutrient budget model. It assists farmers, fertiliser representatives and farm advisers to examine nutrient use and movement within a farm (inputs and outputs) to optimise production and environmental outcomes. It is supported by over two decades of farm system and nutrient research.

The key features of Overseer® are:

  • Annual nutrient budgets (inputs, outputs and soil storage) for N, P, K, S, Ca, Mg and Na; including predicted N loss from the root zone and P runoff and risk level
  • Annual maintenance fertiliser requirements and greenhouse gas emissions
  • Use of data that a farmer knows or that is readily available, including suitable default values
  • An expert system requiring credible production input values
  • Applicable to all major New Zealand farm systems and regions
  • Assumes a quasi-equilibrium or “steady state” farm system, and that good management practices are used
  • Produces long term annual average outputs

Rationale for using modelled nitrogen losses

If a model-based approach to estimating and managing nutrient losses is not used, then input-based controls would be needed where nutrients reductions are needed to meet water quality goals. Input-based controls potentially include allowable land use types, farm practices, stocking rates and fertiliser rates. Relative to model-based approaches, input controls are at odds with the RMA’s effects-based philosophy and produce uncertain outcomes while being inflexible and more costly for farmers. Input controls tend to limit innovation in farm systems as farmers strive to optimise profitability within their NDA.

The use of Overseer®  in Rotorua rules

Overseer®  uncertainty and version changes pose significant challenges for its regulatory use. Regional Council has sought expert advice on the use of Overseer®, looking at four questions in particular:

  1. Will Overseer®  estimates of N loss be “good enough” in terms compliance, N purchase (incentives) and N trading (between farmers)?
  2. How should NDA rules account for Overseer®  version changes?
  3. How should Overseer®  uncertainty be accounted for in the NDA rules?
  4. How should farm nutrient management plans be linked to Overseer®  and the NDA rules?

report has been prepared that addresses these questions. 

Data input protocols

OVERSEER® has developed Best Practice Data Input Standards. The Input Standards give a range of recommendations on what input data to use.

Best Practice Data Input Standards for Version 6.2.3

Best Practice Data Input Standards for Version 6.3.0

Bay of Plenty Regional Council has set out what input protocols it will use for implementing Lake Rotorua Nutrient Management Proposed Plan Change 10. Councils input protocols are based on the OVERSEER® Input Standards and enable a more consistent approach between properties across the Lake Rotorua catchment.

OVERSEER® will update these standards as new versions of OVERSEER® are released and Bay of Plenty Regional Council will update its data input protocols accordingly.

Final practice note Rotorua catchment OVERSEER v6.3.0 (March 2018)

For more information about Overseer®  visit their website.

For more information:

Report on Overseer

Visit the Overseer website