As with all land-uses, farming results in some loss of nutrients into the environment. This is not necessarily a problem unless the amount gets too high for the receiving environment to cope with. In many cases the receiving environment is freshwater ecosystems (including rivers, lakes and groundwater) that support communities.

Unlike factory and seweage treatment discharges that can be measured at the end of the pipe, agricultural nutrient discharges are predominantly diffuse and so there is no practical way to accurately measure annual losses from individual farms. In some places around the world where agricultural losses are considered problematic to water quality, controls have been put on farm practices and inputs (such as fertiliser use and stocking rate) in an attempt to limit the resulting nutrient losses. This approach has been widely criticized for restricting innovation that can drive nutrient use efficiency.

Using a model to predict the amount of nutrients that is lost in any one year from a farm enables “effects-based” controls on outputs, especially nitrogen, rather than controls on farm inputs and practices. This approach enables farm decisions to be based on informed outcomes and stimulates innovation to reduce losses while maintaining a viable business. Output controls are seen as preferable to input controls as they are regarded as more flexible, efficient and effective.

OVERSEER predicts nutrient losses as part of a farm's nutrient budget. It is the only tool available in New Zealand that produces annual nutrient loss information at a farm-scale across a wide-range of farm systems. Because of this, and because it uses readily available and auditable information, it is being increasingly used to inform regulation to limit nutrient losses from agricultural land to protect water quality. OVERSEER does not model the flow pathways of pathogens that may be present in farm animal excreta only the flow pathways of nutrients introduced through animal excreta.

This use is expected to increase as the regulators (regional councils) begin to implement the National Policy Statement for Freshwater Management, under New Zealand’s Resource Management Act.

Challenges in using OVERSEER information in regulations

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 OVERSEER this includes:

How to deal with changes and updates to the model

Managing input data quality and auditing

Setting user qualification requirements

Understanding how often and when to collect information

Understanding the model averaging methods

Understanding the level of uncertainty in the modelled outputs

A Project has been undertaken by regional councils, central government and industry to create guidance on the use of OVERSEER in regulation. The project comprises three phases, the first produced a plain english description of OVERSEER for a non-technical audience and a stocktake on current use by Regional Councils. The second phase produced a document that provides detailed technical considerations on the use of OVERSEER by regional councils. The third phase is still underway and is to produce user-friendly web-based guidance modules for the Regional Sector Information Portal.

For more information on the Project please download the FAQ's.

Phase 1 Documents:
Technical Description of OVERSEER for Regional Councils.pdf 
Stocktake Report

Phase 2 Document:  
Using OVERSEER in Regulation.pdf