Near-full water storages and the likely return of dry weather patterns are two factors influencing River Murray System management strategies for the year ahead.
The Murray-Darling Basin Authority released its 2023-24 annual operating outlook (AOO) last week.
MDBA executive director of river management Andrew Reynolds said the outlook explained how the river would be run based on a range of potential climatic and rainfall scenarios.
“We have looked at a range of scenarios in the outlook and at this stage we expect to meet demands for water across the River Murray system this water year,” Mr Reynolds said.
“River Murray System storage volume was high at the start of the water year with active storage levels currently at more than 90 per cent.
“Reasonable inflows are continuing from tributaries downstream of Hume adding to unregulated flows between Hume and the South Australian border. Some areas in the mid-Murray are still experiencing minor flooding.
“Menindee Lakes are currently at 83 per cent capacity and Lake Victoria (Tar-Ru) is currently 71 per cent capacity and will likely fill over the coming months.
“However, we know the Bureau of Meteorology climate outlook suggests a very high chance of an El Niño and positive Indian Ocean Dipole (IOD) forming from now into spring.
“A positive IOD typically decreases winter/spring rainfall for much of Australia and can exacerbate the drying influence from El Niño.”
Mr Reynolds said if the Bureau of Meteorology’s prediction of dry conditions ensue, higher irrigation demands are expected.
“We will need to release water from Hume Dam and call on Inter-Valley Trade from the Goulburn and Murrumbidgee valleys to meet demands upstream of Wentworth Weir,” he said.
“We also expect that as conditions dry off transfers from Hume Dam will be required to support Lake Victoria (Tar-Ru) storage levels and the supply of water to South Australia across the peak demand period and to meet the end-May minimum reserve in Lake Victoria (Tar-Ru).”
He said flooding over the past few years had mobilised more sand into the Barmah-Millewa reach of the river, a known constraint point downstream of Yarrawonga Weir, although this was not expected to significantly increase delivery risks this water year.
“Flow measurements undertaken after last year’s floods have informed the planning assumption in the AOO of a regulated channel capacity downstream of Yarrawonga Weir of 8,300 megalitres a day,” he said.
“We will continue to monitor capacity in 2023-24 to confirm our initial findings and the longer-term impact on river operations.”
Mr Reynolds said under the Murray Darling Basin Agreement, water from Menindee Lakes will be released to meet system demand as levels are above 480 gigalitres and therefore part of the shared water resource.
“Transfers from Menindee to the Murray may start in October at the earliest and as late as January 2024,” he said.
“Transfers will only be made once demands in the Murray system need to be supplied from water storages.
“Releases from Menindee Lakes will be actively managed in conjunction with WaterNSW to preferentially conserve water stored in Lakes Wetherell and Pamamaroo.”
He said full South Australian entitlement had been secured under normal water-sharing arrangements and will be delivered in all scenarios.
“Additional dilution flow to South Australia will be triggered in September increasing entitlement by three gigalitres a day, and this will continue under agreed operating rules.”
Mr Reynolds explained the AOO was a key part of the broader ongoing river operations planning process that the MDBA undertakes.
“This is not set and forget. Actual operations will respond to observed conditions, regularly updated forecasts, and any constraints at the time.”
MDBA investigated six scenarios where assumptions around demands and losses were developed and approaches identified to operate the system efficiently within system constraints and operating rules.
You can read the full annual operating outlook in the puiblications section of the ‘publications and data’ tab at www.mdba.gov.au.