Veterinarian and ecologist Alison Dewes questioned the model’s ability to account for nutrients and contaminants in its calculations.
Overseer general manager Caroline Read said the aim of modelling longer-term nutrient trends limited its capacity to also account for shorter-term bacterial run-off and contaminant losses.
The model assumed farms operated in a system where all bacterial run-off was contained.
“There is the potential there to expand Overseer to look at other contaminants but they would need to be assessed with respect to priorities before we would invest in that.”
She cautioned the software programme that was originally developed as a fertiliser budgeting tool in the 1990s might not be the silver-bullet system for all farm losses.
More could be done with more money but funds could not be simply thrown into a problem.
One weakness in Overseer’s modelling was the nutrient flow modelling for Canterbury, a region with vulnerable aquifers and elevated nitrogen levels in the water table.
Modelling was based on Lincoln University dairy farm soils, typically heavier than most in the region where two-thirds were classed as stony.
Read acknowledged the limited data sources for those soils, with extrapolation for vulnerable stony soil areas modified based on theory rather than actual evidence.
“Now we are trying to get many lysimeter trials going to measure nitrogen losses over a number of soils with Landcare to provide real data and make any changes we need to make.”
Overseer’s ability to predict losses was also limited to its modelling capacity working only down as far as the end of the root zone in farm soils
But the solution to better measurement of E. coli and contaminant losses might already be close at hand.
Ballance was in the final stages of establishing a business case for its MitAgator software that estimated the risk of nitrogen, phosphate, sediment and E. coli losses in a farm catchment.
Combining soil maps, digitised models and aerial photos of a farm, likely flow paths for bacterial and nutrient losses could be determined and mitigating action taken to reduce them.
Ballance head of research and development Warwick Catto said the system leveraged off Overseer’s nitrogen and phosphate ability but the E. coli and sediment predicators would be increasingly critical as attention focused on groundwater impacts and quality.
Research by Landcare scientists established that, on certain soil types, downward movement of E. coli and campylobacter was possible through pastoral soil under heavy rainfall or irrigation.
