Controlling diffuse nutrient loss from agricultural land

Like all land-uses, agriculture loses nutrients into the environment. Problems occur if the amount lost (particularly of nitrogen and phosphorus) gets too high for the environment to cope with. In many cases, the environment around farms includes freshwater ecosystems (rivers, lakes and groundwater) that support local communities.

Farm nutrient losses are spread out over the farm (diffuse losses). This loss is unlike factory or sewage treatment discharges, which can be measured at the end of the pipe. No practical or affordable way exists to accurately measure actual nutrient losses from farms.

In some other countries, where agricultural nutrient losses affect water quality, regulators try to limit farm nutrient losses by controlling farm practices and inputs (such as fertiliser use and stocking rate). However, this approach of controlling farm inputs is widely criticized for restricting innovation in nutrient use efficiency - while also not achieving the environmental outcomes being sought.

Using a model (such as OVERSEER) to predict the amount of nutrients lost annually from a farm lets local regulators put “effects-based” controls on nutrient outputs, rather than controls on farm inputs and practices. This output control approach enables farm decisions to be based on informed outcomes. Output controls also stimulate innovation to reduce nutrient losses, while maintaining a viable business. Output controls are preferable to input controls as they are more flexible, efficient and effective.

Why Regional Councils use OVERSEER

OVERSEER predicts nutrient losses as part of a farm's nutrient budget. It is the only tool available in New Zealand predicting annual average nutrient losses from farms. OVERSEER does not model the flow of pathogens present in farm animal dung and urine - only the flow of nutrients introduced by animal dung and urine.


Because OVERSEER uses readily available and auditable information, OVERSEER is increasingly being used by councils to inform regulation limiting nutrient losses from agricultural land – in an effort to protect water quality.


OVERSEER has currently used by seven councils in New Zealand to support water management policies as part of the government’s National Policy Statement for Freshwater Management.

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Challenges for councils in using OVERSEER

Like all models used in regulation, there are challenges for regional councils in incorporating OVERSEER numbers into their regulatory processes. Modelled information needs to be applied appropriately, according to the limitations of the model. For regional councils using OVERSEER, challenges include:

  • Understanding the model averaging methods.
  • Understanding the level of uncertainty in the modelled outputs.
  • How to deal with changes and updates to the model.
  • Managing input data quality and auditing.
  • Generation of large numbers of consistent and high quality budgets.
  • Setting user qualification requirements.
  • Understanding how often and when to collect information.

Guidance on the use of OVERSEER in regulation

OVERSEER Limited works with regional councils to ensure OVERSEER is used appropriately in regulation, given OVERSEER’s capabilities and assumptions.

In response to the above challenges, regional councils, central government, OVERSEER Limited and industry are working together to create guidance on the use of OVERSEER in regulation. The work is important, as it will produce guidance for councils on the use of OVERSEER in policy, planning and rules covering farm nutrient losses.

This Overseer Use in Regulation Guidance Project has three phases:

  1. Phase one produced a plain English description of OVERSEER for a non-technical audience - and a stocktake on current use of OVERSEER by regional councils.
  2. Phase two produced a document providing detailed technical considerations on the use of OVERSEER by regional councils.
  3. The third phase of the project is still underway and will produce user-friendly web-based guidance modules for the Regional Sector Information Portal.

For more information on the project, download the FAQs