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Scientists crack water table puzzle -

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ELEANOR HALL: Groundwater is becoming increasingly important for agriculture and will be even more critical as weather patterns become more unpredictable in the future.

But it's hard to measure just how much groundwater there is, so it's at risk of being depleted too quickly.

Today research findings have been released which define the depth at which groundwater extraction starts to kill or harm trees, as Tom Nightingale reports.

TOM NIGHTINGALE: Australian scientists haven't been able to figure out how much groundwater can be used, without risking depleting an aquifer. But they know it's crucial in everyday life.

DEREK EAMUS: It's highly critical to both industry for watering cattle and for irrigating crops, and for direct drinking by humans. It's also critical to certain aspects of the ecology of Australian landscapes.

TOM NIGHTINGALE: Professor Derek Eamus is from the National Centre for Groundwater Research and Training.

DEREK EAMUS: We didn't know how much you could drop the ground water table before we would see a response in the forests, and so one of the things we were interested in was answering that question - how low can you drop the water table before you see a response in the woodlands and forests?

And our data suggests you can't go beneath about nine, eight or nine metres.

TOM NIGHTINGALE: Could there be other explanations though for difference in tree health?

DEREK EAMUS: We don't think so because we did look at a whole range of other variables such as the slope of the land, the pH of the soil. These were not found to be causing the variation that we're seeing in the attributes. And in the analysis you can do, you can show that it is in fact depth to ground water is a key reason for the changes that we're seeing.

TOM NIGHTINGALE: Professor Eamus and others used the girth of trees and the size of leaves among other measurements to define the depth.

DEREK EAMUS: Well we think it's very significant because it's going to tell us the sorts of things that we should be measuring in a woodland or a forest when we are extracting ground water and that's important because that means we've got some handle on a measure of ecosystem health and we'll be able to monitor a certain number of factors, a number of variables and show whether the ground water extraction is having an impact.

TOM NIGHTINGALE: A key strategy in decreasing carbon emissions is planting trees. But Professor Eamus says he doubts whether the ground water use is worth it.

DEREK EAMUS: Given vast acreage of trees that you would need in order to absorb a significant amount of carbon, I do ask the question - do we really have the water available to do that strategy in terms of reducing Co2 emissions for Australia.

My back of envelope calculation suggests it's not going to be at all easy.

TOM NIGHTINGALE: And he says the research could help lead to satellite images of trees being used to tell when too much ground water is being extracted. That concept is likened to speed cameras for water management.

Professor Eamus says an early version of the technology is nearly ready for use, although it wouldn't be put to that purpose.

DEREK EAMUS: I'm sure it'll be done within the next year, we'll be able to use satellites to identify where the groundwater dependant ecosystems are, and that's a really important step for protecting those ecosystems.

TOM NIGHTINGALE: The results are being outlined for the first time today, and are now being written up for publication in scientific journals from mid-next year.

ELEANOR HALL: Tom Nightingale reporting.