Leaky weirs success through drought

Leaky weirs success through drought

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Luke Peel crossing a leaky weir at The Mulloon Institute. Photo: The Mulloon Institute

Luke Peel crossing a leaky weir at The Mulloon Institute. Photo: The Mulloon Institute

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Great example of floodplain kept alive during the drought

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Drought across the Southern Highlands since mid-2016 has exasperated pastoral conditions for landholders reliant on pasture growth and clear water supplies to maintain stock numbers.

So, how have the leaky weirs established at The Mulloon Institute (TMI) fared during that extended dry period?

The initial pilot project at Mulloon Creek Natural Farms included the installation of 17 structures along three kilometers of the creek which slowed the flood event in June 2016 and spread water across the floodplain.

TMI research coordinator, Luke Peel, pointed out that widespread flooding recharged the floodplain groundwater storage which was slowly released back into the creek as water levels in the creek fell.

"With the severe drought that followed and with little or no flow from upstream, that section of the creek continued to have water flow due to the floodplain storage," Mr Peel said.

"The flow of water to downstream landholders continued through until November 2019."

At that time, the creek was reduced to a chain of ponds and that series of connected waterholes and in turn kept stock, flora and native fauna alive during the drought.

"It was amazing to see the amount of water even a small floodplain can store for dry times that kept the creek flowing," Mr Peel enthused.

Those structures formed the original phase of The Mulloon Institute's flagship Mulloon Rehydration Initiative (MRI) which demonstrates how landscapes can become more resilient to climate extremes through rebuilding the landscape and ecosystem functions with leaky weirs, re-vegetation and managed grazing of livestock.

TMI science officer, Nolani McColl, noted the works along the Mulloon Creek have re-established a reliable stream flow while also enhancing agricultural productivity and improving water available to down-stream users over longer timeframes during times of drought.

"The structures have stabilised the creek bed from further erosion by slowing the flow of water and have moderated the flow of water from rainfall events in February and March this year," Ms McColl said.

"With only minor rainfall since March, the structures have had the desired effect of providing a consistent water flow of 15 megalitres a day - the equivalent of 15 olympic swimming pools."

Ms McColl pointed out that flow was sufficient for the relatively small creek to provide water to eventually flow into the Tallowa Dam which supplies part of Sydney's drinking water.

The structures are designed to slow water flow, filter the water through planted native riparian vegetation and to reconnect the stream with the adjacent floodplain.

"The results have included the recreation of wetlands and habitat along the creek for native flora and fauna, improved water quality and more sustained water flow, even in dry times," Ms McColl said.

The deposit of silt and other nutrients across the floodplain was further noted by Mr Peel who said in so many ways landholders along the Mulloon Creek gained from the loss of topsoil and nutrients from the bushfire-affected areas upstream following the March rain event.

"Stream gauges along the Mulloon Creek measured high turbidity for the first three days and then subsided back to good levels," he said.

"That reduced the impact of water quality downstream."

The introduced structures also meant water levels had since been maintained even though little rain had fallen since March.

"The structures have moderated the flood pulses and that has had the desired effect of providing the consistent water flow of 15ML/day," Mr Peel said.

"The structures are not stopping water flow, but they do slow the flow enough to provide many benefits for immediate landholders and all users downstream.

"Imagine if many creeks were rehabilitated in this way and were protected during times of floods and drought, and if those areas were retaining more water and green vegetation, they could also help provide barriers to wildfires."

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