Critical Times

In three years, the continent has suffered a net loss of 46 cubic kilometres of fresh water — enough to fill Port Phillip Bay twice.

Initial results of an extraordinary international satellite project provide yet another indication that Australia is drying out.

Based on current consumption patterns of about 1.5 billion litres a day, the water lost could have quenched Sydney’s thirst for more than 80 years.

The discovery has been made with two American and German satellites designed to map all the world’s water stocks, a task never before possible.

Launched by a Russian rocket in 2002, GRACE, the Gravity Recovery and Climate Experiment, involves two identical craft circling 220 kilometres apart, 485 kilometres up.

By repeatedly plotting variations in the tug of Earth’s gravity, GRACE can estimate changes in the mass of all the water below — “even water in aquifers”, said Jay Famiglietti, a hydrologist at the University of California, Irvine.

It also measures water in river basins and reservoirs.

While the main findings will not be published until next year, Professor Famiglietti calculated the overall decline observed in Australia’s fresh water.

Australia lost the 46 cubic kilometres between February 2003 and January this year.

“Of course, some regions are gaining, while others are losing,” the professor said.

He warned that it would take another five years of mapping before his team could say if Australia’s drying was a short-term phenomenon created by drought or a long-term trend triggered by climate change.

Associate Professor Jeffrey Walker, from the University of Melbourne, and former PhD student Kevin Ellett, from the US Geological Survey, have been working in the Murrumbidgee catchment comparing their ground measurements with GRACE’s observations of the entire Murray-Darling basin.

They hope their work will lead to computer models that will allow water managers around the world to interpret the information from space. Mr Ellett said the 46 cubic kilometres of water Australia has lost over the three years “equates to 92 Sydney Harbours”.

The GRACE data also showed that the Murray-Darling region lost “roughly 6000 gigalitres over the three years, or 12 Sydney Harbours”during the same period.

Working on the principle that water is heavy, GRACE maps the world’s water every month.

When one satellite passes over water, the increased gravitational pull makes the probe above speed up, altering the distance between it and its orbiting twin.

“The distance between the GRACE satellites is a couple of hundred kilometres … we can track the difference to within the thickness of a red blood cell,” Professor Famiglietti said. “It blows me away.”