A CSIRO review has confirmed water modelling on which the proposed $80 million Southern Forests Irrigation Scheme (SFIS), near Manjimup, is currently based, was flawed.
In his report released last week, CSIRO research scientist Justin Hughes confirmed the modelling prepared for the Department of Water and Environmental Regulation (DWER) in 2018 and peer reviewed the same year, "could not be considered fit for purpose" in its present format.
In particular, Mr Hughes said the model used to show sufficient water above environmental flows would generally be available in the Donnelly River and Record Brook tributary, from forest runoff west of Manjimup, to allow the SFIS to take up to 9.3 gigalitres a year during peak flow periods, "over-predicts" actual volumes of water likely to be available.
A 1961-1990 baseline historical data period used for the DWER modelling had proved to be "particularly wet" in the Donnelly River catchment and featured a "significant trend" which created "issues" for future water availability predictions, Mr Hughes' report stated.
"Due to continued reductions in rainfall across the catchment over the period since the end of the baseline historical period, some of the future climates (DWER modelling essentially presented three climate option scenarios out to 2050) have higher mean rainfall than recent observed rainfall," his report stated.
"Given that all future climates were intended to represent a drier future, these future climate time-series are not appropriate and should not be used," it stated.
The particular model used was also "very likely to over-predict runoff" in conditions drier than during the historic data period, Mr Hughes said.
The phenomenon of drastic reduction in runoff with declining rainfall - even in wetter years - due to increasing depth of groundwater in the south west of Western Australia has been "acknowledged in academic publications" for more than 10 years, he pointed out.
Mr Hughes' report vindicated assertions made last year by hydrologist and former Department of Water director general Kim Taylor.
Mr Taylor's claim "simple" modelling techniques employed by DWER appeared not to take account of declining groundwater levels and subsequent reductions in run off in South West catchments, were reported in Farm Weekly last year.
Mr Hughes recommended a later and more complex model capable of predicting reliably in conditions of declining runoff, be used.
He also recommended the latest climate information and a baseline data period updated to as current as possible be used with the more complex model.
In the report Mr Hughes was critical of the lack of relevant streamflow data, with only one gauge upstream of where the SFIS proposes to take water, monitoring Donnelly River flows constantly from 1952 through to present.
Two others had only measured flows since 2010 and other gauges in the catchment area had been abandoned in the 1990s.
He recommended a review of available stream flow data be conducted to act as "a reference" with which to "judge what might be expected in various future climate scenarios".
Any estimated streamflow from future climate scenarios should be analysed "with reference" to the recommended review of streamflow data, he said.
Mr Hughes also recommended where possible historic streamflow and groundwater monitoring sites be reinstated.
In a joint statement last week, Agriculture and Food Minister Alannah MacTiernan and Water Minister Dave Kelly said the review found that while the Donnelly River model used for the SFIS was based on water modelling methodologies that were commonly used at the time, the scientific understanding of climate change and its impacts has since continued to evolve.
DWER is consulting with CSIRO and is reviewing the report, with a preliminary assessment of its implications to be provided in coming months, they said.
Ms MacTiernan and Mr Kelly said the State government - which has previously promised $18m for the SFIS, with the Federal government promising $39m - has committed to work with the Southern Forests Irrigation Co-operative (SFIC) and the local community to assess the impact of reduced water availability on the future of the SFIS proposal.
"Climate change is having a major impact on WA's South West and, as the review notes, this makes water modelling extremely complex," Ms MacTiernan said.
"This independent review provides an important path forward to determine the future viability of the proposed Southern Forests Irrigation Scheme.
"Clearly any significant reduction in the water available to the scheme will affect the scheme's business case.
"Our ultimate aim is to deliver reliable water so the farmers can continue to feed our people and we need to deliver a viable, credible project to do that."
The SFIC has accepted refundable two per cent deposits from about 70 local landowners who have signed 90 contracts with it for a total allocation of just under 7.8gL of scheme water.
The contracts require a further 8pc payment when a formal decision is taken to proceed with the SFIS and the balance of the contracted amount when water is first delivered to properties.
The SFIC is also considering how best to provide the remaining 1.7gL for maximum benefit.
It received expressions of interest totalling 3.885gL.
As previously reported, under the SFIS proposal, up to 9.3gL annually would be pumped from the Donnelly River to a 15gL storage dam built across Record Brook.
From there, the water would be pumped to two ridge-top header dams then distributed to contracted customers along two proposed underground pipelines servicing farms and horticulture properties up to 40 kilometres away.
Under the "medium" climate change impact scenario selected from the DWER modelling, there is supposed to be a 95pc probability of the required volume of water being available by 2030, but local farmers and members of Manjimup Water Security Group (MWSG), who have been measuring stream flows themselves, claim this level of probability is not borne out by current measurements.
Ms MacTiernan and Mr Kelly confirmed the CSIRO review of water modelling and methods used was initiated in response to community concerns.
"For the past three years we've been telling them (State government and SFIC) that the water DWER said was in the Donnelly River for the irrigation scheme, was not actually there,'' said MWSG chairman John Kilrain, reacting to the public release of the review results and recommendations.
"Now the CSIRO review has confirmed what we've been saying," Mr Kilrain said.
"When there's only three millimetres of water in Record Brook (the stream the SFIS proposes to dam to create a 15gL storage for water pumped from Donnelly River during winter peak flows), you're not going to catch 3gL.
"Kim (Taylor) put his name out there by stating he did not think the modelling used was accurate enough to determine water would be there for the SFIS to take with any certainty into the future and now the CSIRO has found his assessment was right.
"I think he deserves an apology from some people who were critical of his assessment.
"We've invited both ministers (Ms MacTiernan and Mr Kelly) to come and see for themselves, but they're not interested.
"I was up there (Donnelly River west of Manjimup) at the weekend and, in the middle of winter, there's 20mm of water in Record Brook flowing through the pipe (under the access road) and the upper Donnelly was bone dry."
Mr Kilrain said he and members of the MWSG met with Ms MacTiernan on Saturday after the CSIRO report was made public, to discuss what would happen now.
He said he was told the CSIRO would work with DWER staff over 10 weeks to improve knowledge of water modelling and interpreting data.
"She (Ms MacTiernan) is pretty keen to come up with an alternative (to the current proposal based on discredited modelling)," Mr Kilrain said.
"The CSIRO will try to come up with some idea of how much water is likely to be there (in the Donnelly River) into the future.
"We (MWSG) would prefer to have the CSIRO take over from DWER in working on an alternative model.
"Until DWER gets its house in order and has the necessary competency, we don't think they should be involved.
"We'd hate to see water taken from farmers for this scheme - that wasn't supposed to happen," he said.
