We are grateful to Rex N. Gibson QSM, M.Sc. for the following article:

Back in May 2018 the Stuff Media website raised a new issue of concern to all who enjoy the recreational and health benefits of clean waterways. Most people now recognise that industrial waste and faecal materials, whether from humans or ruminants, are leaving a “clean up” legacy for our children’s and grandchildren’s generations to clean up. The Stuff website article showed that our regulatory bodies, who are charged with preventing this, are almost certainly deluding themselves with irrelevant data resulting from inadequate computer modelling.

Sam Mahon pleading to the ‘Great Pollution Enabler’ – “Please stop crapping into our waterways” One picture says a thousand words. New Zealand’s degraded waterways are also our Drinking Water catchments. These two issues are intertwined/Interlocked. New Zealand has the highest rate of gastro illness, from the consumption of dangerous infected drinking water, in the OECD. That is a sad scientific medical fact.

The numbers of sea-run trout and salmon in Canterbury’s degraded rivers are the lowest they have ever been. That is a sad scientific ichthyological fact.

To make matters worse many experts are now saying that the Regional Councils use inadequate computer software programs to monitor water quality and impacts on ecosystems!

Specifically it is the use of the “Overseer” and “RHYHABSIM” programs that is causing concern, even anger. Overseer is a software tool jointly owned by the Ministry of Primary Industries, AgResearch, and the Fertiliser Association of New Zealand and receives over $1.7 million dollars of taxpayer funding per year (Corporate welfare?). It purports to provide an estimate on how a farm is using available nutrients, how much fertiliser is “needed”, and whether damaging nutrients are leaching into waterways. It is promoted by Dairy NZ., however its numbers (data) are based on estimates, and experts are saying that it is just “not sound enough” to use as a compliance tool.

A New Zealand soil scientist, Doug Edmeades, is one who believes Overseer was never designed as a regulatory tool. It was recently brought into the spotlight after mathematician Dr John Gamlen criticised a key Tasman District Council scientist Andrew Fenemor at a hearing involving special protection for Te Waikoropupu Springs, our iconic pure springs near Takaka.

Te Waikoropupu’s gin clear waters

There were upstream proposals to intensify dairying and consequent nitrate applications.  Gamien called Fenemor’s nitrate modelling at the Te Waikoropupu Springs “flawed”, resulting in wrong conclusions. Fenemor, a scientist at Landcare Research, used Overseer to conclude that it would not harm the springs if irrigation was doubled in the catchment.

Like Gamien, Professor Graeme Wake also believes Overseer is “flawed” and has significant mathematical fallacies. Wake says its flaws are easy to spot for mathematicians and professionals.

Most significantly, Overseer did not recognise that things could evolve from a static situation into a dynamic one (= ecology). “This is why this model fails…and users try to fix it by introducing fudge factors, which works for a while until circumstances change.” Wake said no mathematician had ever been allowed to look at how Overseer worked, despite asking many times.


Overseer is however a modelling system widely used by regional councils around the country. Scientists and mathematicians say that it was never designed to be used in that way. Key decisions based on it, around water use, may thus be very seriously flawed. It becomes pseudo-science if its’ methodology is not credible.

RHYHABSIM is another computer modelling system used by regional councils. Many applicants use it to sell their case and justify water takes.

A Danish critique of this system states:

  • It was relatively easy and non-labour intensive to apply,
  • However, it must be stressed that these types of habitat models do not include all factors that affect ecosystem functioning and therefore carrying capacities of streams for indicator species – such as fish. (They detail various examples)

A high powered New Zealand critique examined a widely used approach for evaluating effects of changing flow regimes, the Instream Flow Incremental Methodology (IFIM), in particular physical habitat simulation using RHYHABSIM (River Hydraulic Habitat Simulation). Despite increasingly elaborate statistical treatments and increased computing power, the model has failed to deliver results that can be verified or validated. It is based on a 1980s U.S. version called PHABSIM which has been strongly criticised in U.S. fisheries journals. A panel of experts in the Superior Court of California found PHASIM was not scientifically defensible.

N.Z.’s critique found that generic problems (of the system’s methodology) were compounded by:

  • limited knowledge of many aquatic species,
  • the lack of rigorous description of aquatic habitat requirements,
  • the fact that habitat suitability curves have been developed for a very limited range of conditions and a narrow view of flow requirements.
  • focussing on the effects of minimum stream flows on various life stages of a few species of fish, and on food production for a small reach of stream, they drew dubious conclusions about ecosystem health.

These bovine creatures think that this Rakaia catchment water is “swimmable”.

Rarely has consideration been given to transferability of results, or changes in water temperature or water quality with changing flow regimes. These are the points that the NZ Salmon Anglers Association, for example, has been making for years.

River mouth openings, flushing flow requirements, maintenance of lateral and longitudinal stream processes, and maintenance of river channel processes, are all essential components of an environmental flow assessment, but have often not been considered. They also found that the IFIM process is often confused with one of its basic steps — using models to simulate a relationship between streamflow and in-channel physical habitat.

A focus on minimum flow (“survival flows”) for target fish species (e.g. trout or salmon) will not help without consideration of critical elements such as flow variability, and maintenance of ecosystem processes. Stream temperature and water quality are two overriding constraints on habitat availability, both depend on flow conditions, and yet neither are normally rigorously considered by regional councils in setting stream flow regimes in New Zealand.

We should forget minimum flows; environmental flows should be the focus, from upland catchments to coastal waters factors. To satisfy the spirit and intent of the Resource Management Act, environmental flows, not minimum flows, are required. Will someone please remind our regional councils!

The “rape of the Rakaia”, and its national water conservation order, has unfortunately been significantly influenced by RHYHABSIM modelling. In the 2012 ECan hearing to amend the Rakaia NWCO the lead “expert” witness (and co-author of RHYHABISM) presented “evidence” that the model showed that the altered flows would have “minor or less than minor” effects on the “habitat” of fish and birds found in the Rakaia River. The evidence’s accuracy was questioned by another submitter and although the expert admitted his evidence was only an estimate the (non-elected) hearing commissioners were unmoved.

Rakaia mouth area – note the flow needs to keep the mouth permanently open, lately that has been in doubt.

Computer models are “in vitro” (= laboratory) studies. Evidence on over-riding water conservation orders should be done in “in vivo” studies (= in a natural setting), as occurred in the Glove and Duncan Rakaia analysis back in 1985.

Just as we now hear about the advances in “artificial intelligence” we must beware of “artificial data” when dealing with environmental matters. In our case when it affects our fisheries.

Since the 2012 hearing, the effects on the once famous Rakaia salmon fishery have not been “minor or less than minor”. The increased abstraction parallels the demise of the supposedly protected outstanding recreational fishery. It is easy to argue the direct correlation between the two; especially when the missing aspects of the model are viewed in the light of the current degraded state of the Rakaia; notably temperature and changing flow regimes. We need evidence; not “expert” assumptions when determining river water takes. Our aquatic wildlife, whether native or acclimatised, and all human river and aquifer users, deserve better.

Rex Gibson is a Freshwater Spokesman for the New Zealand Federation of Freshwater Anglers Inc. He qualified in Vertebrate Ecology and Parasitology and has an in-depth history as an educator in the environmental sciences.