Thursday, 07 October 2021 09:55

Simplistic water rules not useable

Written by  Jacqueline Rowarth
Dr Jacqueline Rowarth Dr Jacqueline Rowarth

OPINION: What is simple is always wrong. What is not is unusable.

French philosopher and poet Paul Valery wrote those sentences in 1942. We should remember the words in our struggles to find a way forward for agriculture.

Around New Zealand the regional and local authorities are dealing with the National Policy Statements, particularly those for freshwater. The goal is to find an indicator of water quality and apply a regulation.

It is not an easy task.

Soil scientist Emeritus Professor Ian Cornforth, Lincoln University, worked on indicators in the 1990s. He concluded that they need to be sensitive, respond predictably to a change in management and influence the area of concern in a predictable way (either through a functional relationship or through threshold values).

Indicators also need to correlate well with ecosystem processes and be scientifically valid.

Correlations are great. The relationship indicates that as one factor changes, so does another. Statistics helps establish validity. The nearer the correlation to 1, the better the cause and effect and predictability. Below 1 the predictability decreases. At 0.5 only 50% of the variability is being accounted for by the factor being examined.

Local Government New Zealand (LGNZ) has been trying to explain these difficulties. In 'Initial Economic Advisory Report on the Essential Freshwater Package', released in July 2019, its authors stated: "Not only is there little evidence of a casual link, there's little evidence of a strong correlation between nutrients and the Macroinvertebrate Community Index (MCI). Other factors may be more important in determinants of environmental quality."

Australian and New Zealand researchers quantified the problem this year. Their paper 'Nutrient criteria to achieve New Zealand's riverine macroinvertebrate targets' derives criteria "to support national aspiration in improving water quality". However, the relationships explain very little (0.11 for Dissolved Inorganic Nitrogen (DIN) and 0.10 for Dissolved Reactive Phosphate (DRP)) of the change in MCI.

Periphyton (algae) mass is another example. Various reports have failed to produce a convincing relationship between DIN and algal growth. Some have shown that the nature of the river bottom (stony or soft) and magnitude of flow (fast or slow) have greater effect than nutrient concentration.

The problem with sorting out cause and effect is that many of these factors are intertwined - rivers near the source tend to be fast, have stony bottoms and low nutrient concentration. They pick up sediment and nutrients as they flow. Nearer the sea on flatter land, they slow down, and sediment drops, clothing the bottom. In between, different forms of life are supported and the MCI has been modelled as excellent or good for 78% of New Zealand's total river length (

Less than 1% of total river length had a poor MCI score in the 2020 report. Around 70% of the river length flowing through the native land cover class had an excellent MCI score. Less than 1% flowing through the urban land cover class (land with urban cover exceeding 15% ofthe catchment) scored 'excellent'.

Human activities including urban development and agriculture have affected water quality globally. Trying to find a simple solution will not achieve what is necessary and a complex solution will be unusable. Worse, the implications of implementation will be a negative effect on the economy with no guarantees in improvements in water quality.

LGNZ has said this: "Since cost-effectiveness is a necessary condition for efficiency, the choice of DIN and Dissolved Reactive Phosphate as policy targets would be expected to fail the efficiency test for policy if nutrient reducing actions are not cost effective."

Treasury has estimated that achieving nitrogen load reductions driven by the lake nitrogen and periphyton attributes in the existing national policy statement for freshwater management would cost $394 million per annum by 2050.

That is a lot of money to forgo with no certainty that doing what is suggested will have any effect at all on water quality.

Clearly, more research is necessary.

Dr Jacqueline Rowarth, Adjunct Professor Lincoln University, is a farmer-elected director of DairyNZ and Ravensdown. The analysis and conclusions above are her own. This email address is being protected from spambots. You need JavaScript enabled to view it. 

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