The Wentworth Group of scientists has condemned the draft plan to save the Murray-Darling river system as using ''indefensible assumptions'' and misleading the public and Parliament on the matter of groundwater.
The Murray-Darling Basin Authority's draft plan recommends returning 2750 billion litres of surface water to the river system each year, mostly through cuts to irrigation. But it also recommends allowing an extra 2600 billion litres a year to be extracted from underground aquifers in the basin.
The Wentworth Group, which includes top water and climate scientists, concluded that the authority had ignored the long-term connection between surface and groundwater.
''Important issues have been neglected and indefensible assumptions have been made in the analysis of groundwater sustainability to the extent that the public, and ultimately Parliament, are being misled about the sustainability of groundwater use in the Basin,'' its report, released yesterday, says.
About half the extra groundwater eligible for extraction was good quality water, much of which came from fractured rock aquifers in the eastern highlands that provide part of the base flow for virtually all the creeks and rivers in eastern and central basin.
''Surface water users and the environment will feel major effects as a result of this potential increase in groundwater extractions,'' they state.
A spokeswoman for the authority said it had received the Wentworth report and was ''looking at it carefully''.
Yesterday the CSIRO released a report on the basin draft plan that attempts to put a dollar figure on the intangible benefits of improving the health of the river system. It concluded that improvements to habitat ecosystems were worth between $3 billion and $8 billion and the value of carbon sequestration by protecting river red gum and black box woodlands was worth $120 million to $1 billion.
Both irrigators and environmentalists seized on the report to support their cases. While the report offered individual figures on economic benefits from returning 2800 billion litres to the rivers – the figure previously indicated by the authority – it acknowledged it could not put an overall dollar value on fixing the system.
Other improvements included greater ''aesthetic appreciation'' worth more than $330 million, reduced costs of $30 million in supplying fresh water, and tourism benefits worth up to $160 million a year. The CSIRO stressed there was ''a paucity of information about the value to society'' of changes to ecosystems.
The government previously estimated the loss of agricultural production from cuts to irrigation water at $542 million a year. The head of the National Irrigators Council, Tom Chesson, said the authors were at least being honest in acknowledging no one truly knew the dollar value on the basin plan.