Double-counting of groundwater: we may have less than we think

Posted 2 July 2007

Macleay River, New South Wales

Thanks to a national network of recording stations, Australia can keep a detailed watch on how much rain falls, how much water is stored in dams and even a rough idea of how much water flows in most of its rivers.

But we may have a lot less water than we think because, as a recent national audit found, we literally haven't yet made the connection between the water we see on the surface and water beneath the ground. In short, we've often made the mistake of counting the groundwater as a separate entity.

As a result, the continent may be even drier than previously thought, according to a July 2007 report by the National Water Commission, which has undertaken the most comprehensive study of Australia's water resources to date.

That audit found that in 2005 Australia's total water resource was about 360,000 gigalitres: that was one-fifth less than the previous survey found almost a decade earlier.

Drought played a part in the lower figure, but the rest of the fall revealed a greater accuracy in the assessment. In the earlier assessment, many water managers had mistakenly counted their groundwater and surface water as distinct resources.

"Double counting" of water resources was commonplace and had occurred during the last audit, the report said.

Commissioner Peter Cullen said: "One of the mistakes that we have been making ... has been we've been treating surface and groundwater as though they are separate systems.

"In a number of our systems they are highly connected, so if people take too much surface water then the groundwater drops. And similarly, or more importantly, if we allow uncontrolled extraction of groundwater - and of course that's been happening as an emergency measure during this drought - that has an immediate impact on stream flow."

And although the groundwater was "a buffer" that could be used when surface water was low, it was also dependent on rainfall and therefore affected by drought.

"The groundwater isn't a magic pudding that we can use whenever we have a drought ... it also runs out," Prof Cullen said.

He said about 90% of the average 364 mm of rain that fell across the country in 2004-05 - well below the 457 mm long-term average - either evaporated or was used by plants. Only 10% could be considered a water resource, with about 83 per cent of that becoming run-off to rivers and lakes and about 17% recharging groundwater supplies.

Many water authorities had placed caps on surface water use in response to the drought, such as the Murray-Darling Basin cap, the commission found.

But their failure to acknowledge the link to groundwater and limit its use "may pose a serious threat" to the long-term surface water availability.

National Water Commission chairman Ken Matthews noted that water management in Australia was failing on three main counts, with decisions being made without proper data, without considering groundwater-surface water interaction, and without nationally-consistent methods to assess river and wetland health.

Main news source: AAP

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