Soil was alight on Black Saturday

 Soil was alight on Black Saturday

22 April 2009

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Australia — The climatic conditions leading to the Black Saturday bushfires were so extreme that fuel reduction would not have made any difference, a leading scientist has said.

David Karoly, from the School of Earth Sciences at Melbourne University, told a seminar last night the climatic conditions experienced in Victoria on February 7 were unprecedented, with temperatures so high the soil caught on fire.

Professor Karoly said the devastated area northeast of Melbourne had experienced a 12-year drought before the fires, which had already reduced the fuel load. “But fuel reduction burning would have made no difference. The fires would have been uncontrollable with minimal amounts of fuel.”

He said the fire was so intense that bare soil burnt in some places, and there were reports of the humus in ploughed ground burning.

“We had record high temperatures, a record heatwave two weeks earlier and record low rainfall. We also had record low humidity,” he said.

The previous three years had been so dry the region had effectively missed one year’s rain. The area was also experiencing an unprecedented sequence of days without rain.

“The preceding heatwave from the 28th to the 30th of January, when Melbourne had three days above 43C, was also unprecedented,” Professor Karoly said. “That heatwave would have kiln-dried everything.”

On the McArthur fire danger index, Black Friday 1939 was rated 100. Ash Wednesday in 1983 was rated 120, but in southern Victoria on February 7 this year there were unprecedented ratings of between 120 and 190.

The unprecedented climatic conditions of February 7 also showed up the shortcomings of bushfire modelling.

Kevin Tolhurst, from the School of Land and Environment at the University of Melbourne, said: “One of the things that has been shown to us with the February 7 case is our understanding of the propagation of wildfire is not well-described.”

He told the seminar that while they predicted the extent of the fire “pretty well” using their models, “it is the mechanism of how they get there that is not well understood”.

He said that information was imperative for everything from planning the design of houses to the decisions people make on whether to leave or stay and fight a bushfire.

Dr Tolhurst said fire modelling had been two-dimensional, but fire was a three-dimensional process. “The way the fire interacts with the atmospheric conditions isn’t currently accounted for in an adequate way in our fire behaviour models.”

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