Three reports released last year, and another commissioned by Overseer Ltd, confirm the technology was a decision support tool for farmers but was increasingly being used by regional councils as a tactical tool used especially for setting and measuring nitrogen leaching limits.
Overseer chief executive Caroline Read said she will work with regulators to get consensus on how to best use Overseer and to move away from it being used to provide an absolute number on the amount of nitrogen leaching.
Reports last year by the Biological Emissions Reference Group, Productivity Commission and the Parliamentary Commissioner for the Environment all raised doubts about the accuracy of Overseer in establishing absolute numbers on the level of nitrogen emissions.
Read said Overseer was more accurate at measuring trends and proportionate changes over time.
The Overseer-commissioned review, by Gerard Willis of Enfocus, described Overseer as a decision support tool for farmers.
“That is, it allows the user to understand the long-term impacts of system wide changes to a farm rather than day-to-day changes in N-loss.”
Actual N-loss from a farm can never be known because it cannot be reliably measured for a whole farm and Willis said as with any modelling, Overseer simplified complex processes and standardised localised variability.
Read said in an interview there were too many variables to accurately measure any nutrient losses from a farm and she wants councils to move away from having “hard numbers” on which farmers are judged to have either passed or failed to one where Overseer was used to determine trends and compare system and management changes.
As technology improves Read said measuring nutrient loss will become more accurate and provide farmers with a comparison of the impacts of different systems and management changes.
“That is what we are trying to achieve at Overseer, to give farmers the opportunity to understand that if they make changes, what it will mean to their system,” she said.
Willis said in his report that Overseer modelled rather than measured nutrient loss, and then only losses below plant root zones.
Councils have used it as a regulatory tool for more than a decade, principally as a compliance measurement tool and to set nutrient loss limits.
Over time its use has extended as regional councils look for technology to measure diffuse discharges and to meet freshwater quality targets.
Willis urged care with its use in planning effects-based water management.
“Overseer should not be considered as a substitute for a broad, multi-pronged approach to water management more generally.”
Willis said after N, phosphorous (P) was the other important nutrient for water quality and while regional councils did not set property-specific P-loss limits using Overseer, there have been calls for this happen.
He said Overseer assumed all farmers followed good management practices, but this ignored changes to improve those practices and similarly ignored those following poor management.
He warned that using Overseer to show compliance or compliance failure against specific N leaching limits could lead to inequities in the way farmers are treated relative to others, drive creative uses of Overseer and be difficult to justify and enforce if tested legally.
For that reason, he advocated Overseer be used to estimate farm performance against a target range and where failure to meet those standards triggers closer scrutiny of a farm operation.
This may not necessarily mean refusal or forfeiture of consent, because Willis said Overseer data should not be the only consideration for regulators.
“This approach contrasts with one that uses Overseer as part of a pass/fail test that sees a limit imposed and the activity unable to be authorised, even under a consenting regime, until such time as Overseer can demonstrate that the limit will not be exceeded.”
