Imagine if we could solve Australia's water security problems by building a series of giant underground storage tanks.

Tanks so gigantic they would stretch for thousands and thousands of kilometres and cross beneath state borders.

In times of drought they could provide the back-up needed to keep communities alive and help mitigate against environmental damage.

The savings in terms of water loss alone would be enormous.

The Australian National University's Andrew Ross estimates up to a third of the water stored in large above-ground reservoirs is lost each year to evaporation. With farm dams, it's as much as half.

The bad news is that the cost of constructing such subterranean mega-structures would be prohibitive.

The good news is that we don't have to. They already exist — they're called aquifers, and they're huge.

'An amazing resource', hidden away

The Great Artesian Basin — the largest aquifer in the world — stretches more than 1,700,000 square kilometres and contains more than 65 million gigalitres.

That's roughly 130,000 times more water than the size of Sydney Harbour, according to Queensland Government statistics.

A Great Artesian Basin-fed bore flows freely in Queensland. ( ABC Rural: Virginia Tapp )

"We have this amazing resource which is hidden away," says Cardiff University's Mark Cuthbert, who has made an extensive study of sub-surface water supplies.

He estimates global reserves are at least 30 times larger than the total water volume found in lakes, dams and rivers.

"That gives us an amazing tool for resilience during drought, and under climate change," he says.

But he and other water scientists warn there's an urgent need for more sophisticated and coordinated management of our water resources, both below and above ground.

One solution could be a little known, but already proven technique — "water banking".

Saving for a dry, not a rainy day

Water banking, or Managed Aquifer Recharge (MAR), involves a simple idea: deliberately injecting water into natural subterranean basins in times of plenty, when water is abundant in various parts of the country.

That might be during the north Australian wet season, for instance, or when there's extensive rainfall or even flooding.

Rains from northern Australia's wet season could be a reliable source of water if banked. ( Getty: Auscape/Universal Images Group )

These extra reserves can then be drawn upon in a measured and managed way without the danger of depleting natural groundwater levels.

"It's very convenient," says Professor Ross, who's also a member of the National Centre for Groundwater Research and Training.

"Your infiltration points don't take up a lot of land, but you can take water out in quite a distributed area. It's a very stable and reliable source."

There are various techniques used for water banking.

Water can be injected into an aquifer via a well or directed to flow into purpose-built channels.

Another way used to recharge an aquifer is through the building of special ponds with porous bottoms.

Water banking is currently undertaken in various countries, but in Australia — the world's driest inhabited continent — it's yet to catch on in any substantial way.

In the two decades Professor Ross has championed the idea, he says interest has waxed and waned.

Politicians have been interested during times of extended drought, but quickly lose interest when the rain starts again.

Australia might not have an extensive water banking system already in place, but that's not to say we haven't already been drawing substantial quantities of sub-surface water.

Underground basins are a major source of water for agriculture.

And bore water is relied on for domestic use in many country towns and even metropolitan areas.

Mark Cuthbert says climate change makes the need to water bank more pressing. ( Supplied )

Back in 2015, the CSIRO estimated groundwater made up approximately 85 per cent of water consumption for the greater Perth area. That's dropped significantly since then, but it's still calculated at about 46 per cent.

Dr Cuthbert's research suggests many parts of the world are drawing on groundwater reserves in potentially unsustainable ways.

He says while the global rate of natural groundwater replenishment is higher than the rate of extraction, distribution is uneven and there are "hot spots" where environmental problems are likely to develop.

One such hot spot is southern California, where extended drought has seen huge amounts of groundwater extracted for farming.

Californian farmers have dealt with years of droughts. ( Getty: Al Seib/Los Angeles Times )

The hope is that managed aquifer recharge could form part of a suite of measures aimed at better stewardship of our global groundwater supplies.

But scientists warn we must stop thinking of aquifers as a bottomless resource.

A looming environmental timebomb

Complicating matters is the fact that climate change is already altering the traditional rates of natural recharge in many geographic areas, according to Dr Cuthbert.

And that's likely to limit the amount of subterranean water we'll be able to draw in future unless countries adopt a water banking approach.

"Groundwater systems generally respond really slowly, compared to surface water which responds very quickly to changes in rainfall," he says.

"Only around 25 per cent of the groundwater on Earth responds in a human time scale of around 100 years."

So, as some areas of the planet become drier, the amount and speed of replenishment is also decreasing.

"We have to think long-term about this resource," Dr Cuthbert says.

"We have short-term political cycles, or management cycles, and groundwater is moving much more slowly than that."

Getting it right and scaling it up

Water quality is an issue when it comes to managed replenishment, but Professor Ross says some systems already use recycled stormwater runoff, treated sewage and even desalinated water.

And there's another significant risk.

"You have to avoid a situation where the storage of water underground will cause rising groundwater, which may bring salts to the surface and cause problems for rural production," he says.

Increased salt content is harmful to the soil and can affect farmers' land and how they farm. ( Getty: Universal Images Group )

Another difficulty revolves around engagement, ensuring the main commercial users of groundwater get onboard, not just government. In the Australian context that means farming interests.

Agriculture accounts for more than 70 per cent of all water usage in general.

To encourage farmers to take part, Professor Ross says governments need to look at establishing some form of recovery entitlement scheme to give investors security.

"That means, for example, that in dry years the take from aquifers could be limited to the highest priority users, and the argument is that people who recharge water into an aquifer should be in that highest priority category," he says.

"At the moment that doesn't exist, so water that is recharged into an aquifer just becomes part of the general kind of consumptive pool."

MAR trials and projects have been run in all Australian states. Some date back decades, but the contribution they make to the nation's water management remains small.

Western Australia runs a full-scale groundwater replenishment scheme.

It's located in the northern suburbs of Perth and, according to the WA Water Corporation, has the capacity to recharge more than 28 billion litres of water each year.

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That may sound like a lot, but even the corporation describes it as "a drop in the aquifer".

Their goal is to increase that recharge amount to 115 billion litres by 2060.

Professor Ross cautiously hopes the dry, fire-prone conditions of the past 12 months will inspire Australian authorities to invest more energy and funds into such schemes.

"It's important to seize the time, to make use now of the increased interest in this concept and really get it going," he says.

The independent government advisory body Infrastructure Australia has just released its annual Infrastructure Priority List.

It's urged the Australian Government to create a National Water Strategy and to put a greater emphasis on improving water security for Australia's towns and cities.

The organisation's key theme this year was resilience — an ideal that would seem to fit perfectly with the water banking philosophy and approach.