South-east Queensland's largest dam has been releasing water for a week to help increase its capacity for the upcoming wet season.
Water Minister Glenn Butcher ordered Wivenhoe Dam's drinking water storage capacity be reduced to 80 per cent.
Premier Annastacia Palaszczuk said it was an early move to increase flood mitigation capacity at the dam to prepare for the "worst-case scenario".
"It is being done safely and in a controlled manner, and it is being done to ensure we are on the front foot after the crucial BOM [Bureau of Meteorology] advice," she said.
Wivenhoe Dam is located on the Brisbane River in the Somerset Regional Council area, and along with Lake Somerset, it supplies 70 per cent of south-east Queensland's water supply.
So, what capacity is the dam at now?
After releases began last Saturday, Wivenhoe Dam's capacity sits at 83.7 per cent.
Before its spillway was opened, the dam's water supply was 90 per cent full.
A Seqwater spokesperson said the Wivenhoe Dam was previously operating at 90 per cent due to the "dam improvement program".
How are the Wivenhoe Dam releases going?
Wivenhoe Dam's spillway is expected to remain open for another week.
But a Seqwater spokesperson said release plans might change if wet weather occurs.
"Minor gated releases from Wivenhoe Dam have remained steady since Saturday and will be maintained until into next week, weather permitting," the spokesperson said.
Seqwater said the temporary supply level of 80 per cent will remain in place until February 28 "to provide additional flood mitigation benefits".
"Any water that goes in additional to that 80 per cent will be released under the flood manual," the water minister said.
Engineers, hydrologists and dam operators are rostered 24/7 to monitor conditions and safely manage water releases and dam operations.
Why 80 per cent?
Mr Butcher said the decision to reduce the dam's drinking supply to 80 per cent was based on expert advice.
"I've taken advice and information from meteorologists, scientists, dam engineers, the dam safety regulator and environmental experts as we plan appropriate steps for the wet season ahead," he told parliament.
Mr Butcher said it is a careful balance managing the dam levels.
"Last November we were down to 55 per cent and we were looking at going into water restrictions," he said.
He said the experts are confident that the 80 per cent capacity level will make the dam manageable with the forecast wet season ahead.
Where does the water go when it is released from Wivenhoe Dam?
Wivenhoe Dam's water releases flow into the Brisbane River.
Seqwater said river rises downstream of Wivenhoe are anticipated.
But it said in these releases, river flows will remain well within the riverbanks.
How much rain would it take for Wivenhoe Dam to reach 100 per cent again?
For Wivenhoe Dam to drop from 90 per cent to 80 per cent capacity, more than 116,000 megalitres of water need to be released.
To reach 100 per cent capacity again, Wivenhoe Dam would need a downpour of more than 232,000 megalitres of water.
But a Seqwater spokesperson said Wivenhoe Dam's level would continue to remain at 80 per cent under the direction from the Minister for Water's temporary full supply level order.
"Seqwater will commence flood releases from the interconnected Wivenhoe and Somerset dams if storage levels exceed the amended full supply levels as a result of rainfall in the dam catchments."
Is the water usage by residents helping to keep the dam levels down?
Mr Butcher said Seqwater would drain the 116,000 megalitres of water over two weeks.
But south-east Queensland residents are also being encouraged to help bring the dam levels down to 80 per cent with an incentive of $55 off water bills.
The discount will be automatically applied to households connected to the SEQ water grid in the Brisbane, Redlands, Logan, the Gold and Sunshine coasts, Noosa, Ipswich, the Lockyer Valley and the Somerset, Scenic Rim and Moreton Bay regions.
"It allows for the use of 13,200 litres of free water for each of those homes and families," Ms Palaszczuk said last week.
"So I encourage families to make the most of this rare opportunity and Gerni the driveway, to wash down the house and to clean out your gutters."
What capacity did Wivenhoe Dam reach during the 2011 and 2022 floods?
In just three days during the this year's flood event, Wivenhoe Dam capacity rose from 58.7 per cent on February 24 to 183.9 per cent by February 27.
During the 2011 flood, Wivenhoe Dam reached 190.3 per cent capacity.
Why can Wivenhoe Dam's capacity reach more than 100 per cent?
Seqwater chief operating officer Stuart Cassie said when a dam is at 100 per cent capacity, that means its drinking water supply volume is full.
Full capacity does not include the entire flood storage compartments.
Wivenhoe Dam is a dual-purpose dam that provides 1,165,000 megalitres of drinking water storage as well as 1,967,000 megalitres of temporary flood mitigation space.
It can hold a grand total of 3.132 million megalitres.
