Science modelling

The health of Southland’s freshwater and estuaries has declined due to excess nutrients, pathogens, sediment and other contaminants. See more about the current state assessment here.

Environment Southland has completed scientific modelling, using the latest information and data, to better understand the gap between where our water quality is now and where it needs to be to meet our goals for improved freshwater quality - the Murihiku Southland draft freshwater objectives.

We have undertaken modelling of some key contaminants (nitrogen, phosphorus, sediment and E. coli) to understand the gap between the current state and the outcomes sought by the community and iwi of hauora, or health and wellbeing, of freshwater and estuaries.

This modelling shows that levels of particular nutrients and pathogens need to be significantly reduced.

To meet the draft Murihiku Southland freshwater objectives would require reductions on average across Southland’s developed catchments of nitrogen by an estimated 70%, of phosphorus by an estimated 70% and of E. Coli by an estimated 90% over the next 25 years.

The estimated reductions required are different in each catchment, but all show that significant reductions are required in both urban and intensive farming areas. Find out more about individual catchments here.

Southland with science reporting catchments

We are proposing to set out five-yearly steps towards improving the state of waterways.

These percentages provide a sense of the scale of reductions needed but are not limits that an individual farm, property or business needs to meet over any particular timeframe.

Find out about the next steps in the process here.

These findings have been reviewed by technical experts from other organisations and have gone through official peer review processes.

About the modelling

We have calculated the reductions required to reach:

  • The national bottom lines set out by central Government (link)
  • The standards in the 2016 proposed Southland Water and Land Plan
  • The draft objectives proposed to provide for hauora – the health and well-being of waterbodies, in a generation.

For all of these targets, sizeable reductions are required across most of the developed river catchments in the region.

This work has been completed using the best available science, and these models have been published in peer-reviewed international science journals.

With all models of this type, there is some uncertainty around the absolute size of the percentage reductions needed. Uncertainty is unavoidable. We have estimated the amount of uncertainty by calculating the upper and lower bounds of the 90% confidence interval around our best estimates. The methods we’ve used and the approach to handling uncertainty is consistent with the Ministry for the Environment guidelines. Scientists from other organisations have reviewed this information, and while some views on the specific numbers may vary, there is agreement that the scale of change needed is significant.

How to achieve these reductions

These numbers are not the limits that each business or farm will need to operate within, but will inform the setting of limits.

This information will also inform consideration of actions for achieving reductions that help us reach a state of hauora in our waterbodies across the region. This is on the agenda for the Regional Forum. The types of issues the Regional Forum have been grappling with include repurposing land that is currently being used in a way that ultimately negatively impacts our waterways; looking at ways of mitigating risks; building resilience in the face of climate change; and what incentives and regulatory frameworks might work best so landowners, and the wider Southland community, are encouraged to do everything they can to improve the effects on waterways.

Find out more about the process and timeline here.