Some Dethridge wheels are delivering nearly 18 percent more water than neighbouring meters but the extra water is not recorded against the farmer's usage according to a research report released today by Goulburn-Murray Water (G-MW). The report, In-situ REVS Testing of Large Dethridge Meter Outlets in the GMID authored by Hydro Environmental, reviews the results from more than 200 tests undertaken at 43 sites (meters) across G-MW's six irrigation areas during the 2007/08 season.
G-MW Managing Director David Stewart said the tests were undertaken by G-MW and Theiss Services and build on G-MW's 2007 pilot meter test program. G-MW intends to build the data set to enable development of a mathematical model to more accurately estimate meter error by site.
"If every meter over delivered by the same amount we could simply recalibrate the meters. This research suggests that while one farmer might get 100 ML through his wheel, his neighbour could receive almost 118 ML each and every year. That's around $40,000 of permanent entitlement - highlighting the inequity that can arise from inaccurate measurement," said Mr Stewart.
Test program improves modernisation planning and water savings estimates
G-MW has already made the decision not to install new Dethridge meters due to their non-compliance with the new national metering standards and their sensitivity to the local operating environment.
"Our meter testing program is aimed at better understanding and estimating the impact of local conditions on meter accuracy. This knowledge will improve estimates of water savings from modernisation and rationalisation, and farmers can better anticipate the impact of more accurate outlets on their on-farm operations," said Mr Stewart.
Unlike past test programs that involve moving the wheel into a laboratory and attempting to replicate field conditions, G-MW's in-situ testing involves up to seven separate tests at each field site. The initial test is undertaken without adjusting the meter, and subsequent tests measure the impact of adjustments such as improved clearance or new bearings.
"The program is a significant investment that will improve the relevance and quality of information available to G-MW, our customers and organisations investing in our irrigation network," said Mr Stewart.
Total distribution losses unchanged
Mr Stewart explained that the test results should be good news for irrigators.
"We measure the volume of water released into an irrigation area and we know how much is recorded through our customers' outlets. The difference is distribution losses and the challenge is to determine exactly where the losses occur so we can target our modernisation works to capture them," said Mr Stewart.
"This result does not change the total loss - it indicates that less of the total loss is attributed to meter error. According to these results farmers will ‘lose' less by upgrading their meter, and gain through targeted modernisation works such as channel automation and on-farm reconfiguration," said Mr Stewart.
Project to develop meter-error model
A key outcome of this year's program is G-MW's commitment to test at least 100 sites. G-MW, in partnership with the University of Melbourne, will use the expanded data set to develop a model that can accurately account for factors such as bottom clearance, flow rate and effective supply depth to generate an estimate of meter error for a specific meter in its local conditions.
"The model will provide a cost effective, rapid and accurate estimate of meter error for a particular site. Combined with other local factors such as the soil that the channel runs through, we can more accurately determine potential water savings for a particular section of the network," said Mr Stewart.
Average meter error less than 10 percent
The 2008 Report indicates an average meter error of 7.5 percent which is less than historical estimates. Research published for at least the past five decades generally point to Dethridge wheels delivering around 10 percent more water, on average, than is recorded by the meter.
Click here for a copy of the report, In-situ REVS Testing of Large Dethridge Meter Outlets in the GMID. The report includes individual site test results for each of the 53 sites from the 2007 and 2007/08 test programs.
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