The Department of Environment, Land, Water and Planning is proposing enabling 130 gigalitres of water to be traded between the Goulburn to the lower Murray River each season.
It's one of the strategies under DELWP's preferred option, resulting from its inter-valley trade review.
DELWP Retail Water Markets and Entitlements senior manager Joe Banks told a recent webinar between 2017-19 there had been a concentration of deliveries from the Goulburn to the Murray River.
"It started to cause some significant environmental consequences," Mr Banks said.
"We are making sure we are getting the trade rules right, particularly from the tributaries, so that traded water doesn't increase delivery risks or damage the environment."
Recent trade volumes between the Goulburn and Murray had been as high as 250GL, during the 2017-18 season, but ranged from 80GL to negative 40GL, prior to that.
The preferred DELWP option includes:
- Maintain the ecological values of the Lower Goulburn (using lower base flows and pulses)
- Restrict tagged use to operate within new Goulburn to Murray IVT rules
- Define the lower Broken Creek as part of the Murray system, while recognising water used in the lower Broken Creek is not delivered through the lower Goulburn.
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- Potential restrictions on Goulburn inter-valley trades over summer, autumn
DELWP has proposed lower variable base flows on the Goulburn River, averaging 1100ML/day.
"On top of that lower variable base flows, there would be three short fortnight pulses of 3000ml a day and dropping back to those low-based flows for extended periods in between," Mr Banks said.
"With pulses around 300ML/day we would expect 130GL in new trade opportunities.
Mr Banks said another key suggestion was moving in channel irrigation pumps on the lower Goulburn to the top of the riverbank to enable pulse flows of up to 6000ML/day.
"Moving the pumps gives us a lot more flexibility as to how we operate and deliver water," Mr Banks said.
This could increase net annual trade to 205GL whilst still maintaining flows within ecological tolerances.
DELWP was also recommending a two-part trade rule, that consisted of a rolling winter-spring limit of 190GL and a fixed limit over summer-autumn that was announced each December.
That would allow IVT account balance to be drawn down, each year.
Tagged use and grandfathered tags would be restricted, in line with allocation trade rules, over time.
Lower Broken Creek customers would be offered an exchange of Goulburn entitlements with Murray water shares and a new system-specific tagging rule would apply.
Mr Banks said delivery risks on the lower Murray were increasing, due to the increase in total deliveries to the region in recent years.
Water availability was subject to the sustainable diversion limit cap, so total demand for trade from Victorian and NSW tributaries, the Goulburn and Murrumbidgee, had increased.
"Delivery risks are real, and they are likely to increase in the future," he said.
DELWP wanted to introduce operating rules that prescribed how water could be delivered, while keeping flows lower over summer and autumn, and closer to patterns of natural variability, rather than sustained, high constant flows.
"We need trade rules that match operating rules, so we only open up trade that we know can be delivered that doesn't damage river or create delivery risks," he said.
The Barmah Choke capacity had reduced by 20 per cent since the mid-1980s, from 11,500 to 9200 megalitres a day.
"That's significant, because when delivering water to the lower reaches of the Murray, much of it comes from the storages, above there, so it is a major constraint on capacity."
DELWP propose to have a transition year in the 2021-22 season, followed by the new arrangements coming into full effect the following year (2022-23).