What lies beneath: tapping into Australia’s hidden waters
It runs from our taps, keeps crops alive and rivers flowing – yet no one knows exactly how much groundwater we have, or how often it’s renewed. UNSW researchers are venturing underground to find out.
Much of Australia’s water is sourced from under our feet. Towns, farms, industries and ecosystems across the country rely on the groundwater that forms when water fills the spaces between sand, soil and rock.
Groundwater accounts for more than 30% of our total water use – but little is known about how its reserves are refilled, which could have major consequences as climate change intensifies droughts and changes rainfall patterns across much of the continent.
“We’re tackling this big question of ‘how does water get underground – and how much rainfall is needed to replenish it?’,” says Professor Andy Baker, a hydrologist from UNSW Sydney.
“That’s not only poorly understood, it’s also hard to measure.”
Between a rock and a hard place
Australia is a land of many environments, which means each groundwater system is different. Depending on the local geology of the region, rocks can be porous, with water in the pores, or fractured, with water in the cracks and crevices. Water reaches these areas after heavy rain, when soils on the surface are saturated.
Rivers, lakes and wetlands can also connect to a groundwater system. In some cases, these surface water systems are fed by groundwater, and in other cases they are recharging it.
The challenge of not knowing how much groundwater there is and what it takes to replenish it is what drives a national collaboration between UNSW Sydney, the CSIRO and other research partners, with backing from the Australian Research Council.
Their project – the National Groundwater Recharge Observing System – is a world-first network of sensors installed in caves, tunnels and mines across almost every state and territory in Australia.
Instead of investigating groundwater directly, Prof. Baker says the team took a few steps back.
“We said, ‘let’s avoid the whole problem of going straight to the groundwater’,” he says.
“We instead went to the area between the surface and the groundwater – the point where water percolates down through fractures.
“From here, we can work out when water movement increases and what sort of weather and rainfall patterns are related to that.”
The system’s sensors detect individual water drips from the ceiling of the cave, tunnel or mine. This is direct evidence of how rainfall becomes groundwater.
By comparing that data with weather records, the researchers can determine how much rain – and what kind of rainfall event – is needed to trigger recharge across different landscapes.
It’s a complex process but one that can help Australia plan for its future.
A natural laboratory
One of the entrances to the caves in Wellington, NSW.
One of the entrances to the caves in Wellington, NSW.
At Wellington in central New South Wales, Prof. Baker and his colleagues created one of the most complete groundwater research sites in the world.
“The UNSW Wellington Research Station and the Wellington Caves are about 10 kilometres apart,” he says.
“The research station has around 45 boreholes across the landscape. We can pump the water out, do experiments – and our engineering students learn all the fundamentals of groundwater.”
The Wellington research station provides a real-life lab for UNSW students studying groundwater.
The Wellington research station provides a real-life lab for UNSW students studying groundwater.
Here, UNSW environmental engineer Associate Professor Martin Andersen and his colleagues also established one of Australia’s five Critical Zone Observatories.
“This is a network observing hydrologic and ecologic processes at the Earth’s surface and subsurface – where water, air, soil and rock meet,” says Prof. Andersen.
“The system traces the path of water from rain falling on the tops of trees to its final trickle through the rocks to the groundwater resource below.”
Having caves close by provides a rare opportunity to directly monitor rainwater as it makes its journey underground.
“This is the only place – maybe in the world, but definitely in the country – where we’re monitoring all parts of the groundwater recharge process using a combination of boreholes, a Critical Zone Observatory and caves as observatories of groundwater recharge,” Prof. Baker says.
Hidden time capsules
Dr Ellen Corrick is a palaeoclimatologist.
Dr Ellen Corrick is a palaeoclimatologist.
UNSW’s Dr Ellen Corrick says caves also preserve clues about the past, through chemical markers in cave stalagmites.
“Cave stalagmites form when calcite minerals precipitate out of drip water falling from the cave ceiling,” Dr Corrick says. “So, they only grow when the groundwater is recharging.”
Stalagmites form very slowly, layer by layer.
“The chemical markers in each layer record information about the climate and environment above the cave at the time,” Dr Corrick says.
This is a stalactite, which hangs from a cave ceiling and forms from dripping water. When droplets fall to the cave floor, they can build up over time to form stalagmites, which grow upward from the ground.
This is a stalactite, which hangs from a cave ceiling and forms from dripping water. When droplets fall to the cave floor, they can build up over time to form stalagmites, which grow upward from the ground.
By studying the chemical markers in stalagmites across the sites, the team are now working to reconstruct the pattern of groundwater recharge and rainfall over the last few hundred years.
“This longer-term context provided by the stalagmites offers a unique opportunity to travel back in time,” says Dr Corrick.
From this the team can develop a more complete understanding of the relationship between groundwater recharge and climate drivers such as the El Niño Southern Oscillation – the climate pattern that alternates between El Niño and La Niña.
Stalagmites rise upwards from the floor of a cave, like this one in Cathedral Cave, Wellington, NSW.
Stalagmites rise upwards from the floor of a cave, like this one in Cathedral Cave, Wellington, NSW.
Seeing water clearly
While groundwater underpins Australia’s water security, it remains largely invisible.
When rivers and other surface waters run dry, we can see it. But groundwater offers few visual cues when under stress.
UNSW Associate Professor Marilu Melo Zurita says this partial invisibility has shaped how groundwater is governed.
“When groundwater is out of sight, it’s often out of mind. That makes it easier to overuse, and harder to protect,” A/Prof. Melo Zurita says.
By making groundwater recharge more visible through drip sensors, caves and long-term monitoring, researchers are generating new data while also reshaping how groundwater is understood.
“This work can help people – decision makers, managers and, importantly, community – see that groundwater is not an endless backup,” A/Prof. Melo Zurita says.
“It is a system that depends on specific rainfall events, land use and long-term care.”
From rain to recharge
Prof. Baker says the next frontier is understanding how extreme weather and long-term climate patterns influence groundwater recharge.
“We now know how much rainfall is needed to generate recharge. It’s a lot, at least 20 millimetres in a single rainfall event in Wellington,” he says. “So, in the top 10% of all rain events.
“And it is not just at Wellington – we find this at all our monitoring sites. These are not your typical rain events. The weather for floods is what you want for recharge.”
However, he says high-intensity rainfall alone may not always be enough for recharge to occur.
Water ‘lost’ through evaporation, plant transpiration and soil saturation can all influence whether recharge actually occurs at all.
Back underground, the subsurface structure itself adds yet another layer of complexity. Understanding water movement through the soil layer and the epikarst – the highly fractured layer of rock found directly beneath soil – also has an effect.
Then there are other environmental factors, such as bushfires. A recent case study showed that after a severe fire, less rainfall was needed to replenish groundwater because burnt vegetation and damaged soil no longer absorbed as much moisture.
“It’s quite interesting,” Prof. Baker says. “How the complication of other processes like fires and land-use change can affect recharge.”
Keeping the taps running
Prof. Baker says this observational work from the team is only the beginning.
Early results from the first year of monitoring showed recharge can occur in as little as 24 hours after rainfall – with the amount of rain needed changing from site to site.
At the Naracoorte Caves in South Australia, two different rainfall events in one month had thresholds between 15 mm and 31 mm – showing a more complex, non-linear process at play, even at single sites.
“It’s very hard to quantify whether you’re using that water sustainably or not,” Prof. Baker says.
The problem, he says, is that “if you put a borehole into some groundwater, you just don’t know where that groundwater has come from, or when it was last replenished.
“As this project continues, this data can reveal how recharge thresholds vary with season, geology and climate drivers such as El Niño and La Niña.
“As the network grows, we can hopefully link an international system that helps safeguard Earth’s groundwater for the future.”
The National Groundwater Recharge Observing System was funded by the Australian Research Council, led by UNSW investigators Andy Baker, Marilu Melo Zurita and Martin Andersen; Wendy Timms (Deakin University) and Margaret Shanafield (Flinders University). The current team includes Stacey Priestley (CSIRO), Danyang Sun and Akhilesh Kumar (both UNSW) and Nane Weber (TU Dresden). Research at Wellington Caves is supported by Dubbo Regional Council.
Credits
Story: Melissa Lyne
Photos: Richard Freeman, Aleksandr Wynne
Video: Lee Henderson, Marty Jamieson
Media enquiries
Please contact Melissa Lyne, News & Content Co-ordinator, by phone on + 61 415 514 328 or email at m.lyne@unsw.edu.au
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