The Overseer model was designed to help users understand how nutrients move around a farm system and how changes impact the system. It was expected to inform periodic decisions that would lead to more efficient use of nutrients. Unexpectedly, it has proved incredibly important as an indicator of a farm’s impact on the environment. It does not model all the ways a farm can impact the environment, but it offers satisfactory indicators of nitrogen and phosphorus lost to waterways and of greenhouse gas emissions.
Overseer is useful as an indicator of farm sustainability because there is no way to actually measure and prove what impact a farm is having on the environment. Overseer estimates can help users understand how a farm is likely to compare to similar farms and the potential to achieve efficiencies by changing how a farm is managed.
Overseer is ideal for comparisons because it was not designed to estimate real nutrient losses in any given year, but rather to estimate how a farm system is likely to perform typically. It helpfully eliminates the impacts of temporary events because these make it difficult to understand progress in terms of improved management practices or to compare different systems.
Another unanticipated benefit of OverseerFM is the wealth of information now available to understand trends in farm management practices and how those practices appear to be impacting the environment.
How does it do it? Overseer brings together decades of scientific research into New Zealand farming, and where research is not available it applies logic. As far as field measurements are available, Overseer estimates compare well enough to provide useful results for most New Zealand farms. Overseer is an advantage for the agricultural sector but not every industry can afford the research required by such a model. For atypical farms, the modelled results may be less useful.
Can I trust Overseer? The answer depends on what you trust it to do. Overseer does not pretend to model all risks to the environment, e.g. sediment and pathogens, or every risk and mitigating factor relevant to N, P and GHG losses. It purposely does not attempt to estimate nutrients lost during extreme weather. Other limiting factors are the soil and climate data available, as soil and climate have a significant impact on Overseer results. The model uses data sourced from NIWA and Manaaki Whenua S-Maps Online, and those inputs contribute, on average, around 30% uncertainty to Overseer results. Where S-Maps coverage is poor, uncertainty will increase.
You can trust Overseer to consider most risks and mitigations relevant to most farms in New Zealand and to provide useful comparisons most of the time.
If you have any doubts about how to use Overseer or about using Overseer for a particular purpose then it is best to take advice from a suitably qualified person with a thorough understanding of how the model works, its assumptions and its limitations.