KEY water savings estimates underpinning the State Government’s plans to pipe 75 gigalitres of water a year from the Goulburn River to Melbourne will have to be revised after a new report on Dethridge wheels found metering losses have been significantly overstated.
Roughly 50GL of the 200GL savings expected from rationalizing or replacing 18,000 dethridge wheels across the system will have to be found elsewhere after it emerged they under recorded by a third less than the 10pc previously thought.
Tests of 43 large dethridge wheels conducted by Goulburn Murray Water and Thiess Services found an average meter error of 6.9pc in favour of the irrigator, instead of the 10pc derived from tests last year of a tiny sample of 10 meters. The average error across all 53 tests is 7.5pc.
The results in part confirm doubts raised by irrigators, anti pipe lobby groups and the Auditor General about the reliability of savings estimates underpinning the $2.2bn modernization of the Goulburn Murray irrigation district.
Plug the Pipe chairman Andrew Leahy said the latest figures were still based on only a tiny number of wheels and did not justify the “horrendous” cost of replacing wheels.
“A further set of tests could see the wheels accuracy fall within the national standard of +-5pc avoiding their replacement,” he said.
He said Plug the Pipe did not regard reducing metering error as a valid saving as water was in productive use.
Eliminating metering losses made up 81GL of the forecast 225GL savings from the first stage of the $2.2bn upgrade of the Goulburn Murray irrigation district, from which the State Government plans to take 75GL for Melbourne in return for $900m funding.
They were also arguably the easiest to deliver. Modernisation is forecast to eliminate 100pc of metering losses, with 20pc of the system’s wheels slated for rationalization and the rest for replacement with magflow meters and flume gates with accuracy within the +/-5pc required by national standards.
Some 1600 wheels will be replaced this season under the Central Goulburn channel 1,2,3 & 4 modernisation and the food bowl project.
A small bright spot for farmers is that the results indicate the impact on farm of meter upgrades will be less than expected as they have been getting less ‘free’ water than authorities claimed.
Goulburn Murray Water’s executive manager of modernization, Alex Marshall, said the latest figures suggested metering losses accounted for a quarter rather than a third of annual losses.
Based on last year’s record low losses of 380GL - around half the long term losses on which the project’s savings are premised – that left losses of around 28GL unaccounted for.
“We thought meter error was 10pc, and metering losses accounted for a third of
losses or around 115GL,” he said. “At 7.5pc the difference is about 28GL.”
But Mr Marshall said the fact less of the losses were attributable to meter error did not reduce the overall system losses or the savings that could be achieved, just meant they occurred elsewhere.
“It is still a loss,” he said. “My gut feeling is that it is in leakage and seepage.”
Although the latest figure is more robust than last year’s, Mr Marshall said it was still not definitive due to the large number of variables that could affect meter accuracy including bottom clearance, tailwater depth and upstream water level.
The authority wants to bring the number of tests up to 100, from which it plans to develop a model in conjunction with the University of Melbourne to reliably estimate meter error under a range of local conditions.
But he said the range of error – some meters under recorded by up to 18pc – confirmed the authority’s view that there was no future for the dethridge wheel against a national standard that required metering accuracy of +/-5pc.
He said the widespread view wheels could be maintained to bring them within that range was wrong.
“We are confident we can’t do that,” he said. “Some of those that fell outside the 5pc error had been maintained in the last 12 months. There are just too many parameters we can’t control.”
