Australia — A thick blanket of dry vegetation is becoming a growing danger, writes Nicky Phillips.
It’s said that bad luck comes in threes.
After a drought that lasted a decade and crippled much of the state, the rains came, flooding vast expanses of eastern Australia.
And while the record rainfall this year brought many regions back to life it has also encouraged a blanket of grass to advance across the state, transforming large parts of NSW, and Australia, into a significant fuel source for bushfires.
Advertisement: Story continues below Forest fires in Katoomba this week signalled the bushfire season is upon us. Yet it is the tall, thick band of fine fuel which extends from Queensland to Victoria; and west of the Great Dividing Range to South Australia that poses the real danger.
Despite the Bureau of Meteorology’s forecast for above average rainfall through spring and summer, fire experts say it takes only a week of hot, rain-free weather for grass to ”cure” or dry out, turning much of the state into a fire hazard.
The bureau predicts above average summer temperatures for southern parts of the state, while northern regions should experience temperatures slightly below average.
The Commissioner of the NSW Rural Fire Service, Shane Fitzsimmons, says more than 2000 grass fires have ignited since July.
Despite the forecast of a weaker La Nina this year, parts of the state are unlikely to get away unscathed.
”Across NSW we’ve got grassland fuel loads the likes of which we have not seen for 30 or 40 years,” he says.
The last time the state had similar levels of grass growth was in the summer of 1984-85, when six people died and 40,000 head of stock were destroyed by fires that raged across more than 4.5 million hectares, he says.
Any rain over spring and summer would have no effect on the grass that has already grown and cured over the past 18 months, Fitzsimmons says.
”It’s all dry, it’s all dead,” he says. Rain that fell on partially cured grass would extend the drying process.
”We’re happy to see the La Nina outlook, but we need to make sure that doesn’t put into place a false sense of security, complacency or apathy,” Fitzsimmons says.
Melbourne University fire ecologist Dr Kevin Tolhurst says the fine fuel nature of grass made it easy to ignite, often by lightning strikes.
But the main danger of grass fires is their speed, he says.
”Typically a grass fire over an hour will average about 20 to 22 kilometres, where as a forest fire going between two and five kilometres an hour is [considered] a pretty fast forest fire,” Tolhurst says.
Though he adds that grass fires are usually not as intense.
”The heat output is much less and the time taken for a fire to burn out is much quicker,” Tolhurst says.
The behaviour of a grass fire will depend on whether there are trees in the landscape.
”If trees are present, then spotting, the process of numerous new fires starting ahead of the main fire front, is much more likely.”
In pure grassland fires, spotting is limited which makes them more controllable and easier to predict, he says.
While their speed can catch people unawares, historically forest fires cause the most damage to houses and properties.
”In a forest fire you might be burning something between 10 and 50 tonne of fuel per hectare. In a grassland area you are probably burning between two and six tonnes per hectare,” Tolhurst says.
It is easier for people or their property to become trapped in a forest fire rather than be overrun by it, he says.
To predict grassland fire danger, the NSW Rural Fire Service combines weather data from the Bureau of Meteorology with satellite data and field reports on the amount of grass, measured in tonnes per hectare, and the degree it is cured or dried.
The manager of community planning at the service, Dr Simon Heemstra, says satellite images provided by the bureau, capture the colour or ”greenness” of the vegetation, which can be used to estimate the amount of moisture in the fuel.
”The colour from green to straw to light brown gives us an estimate of the percentage cured,” Heemstra says. But the images can give false low readings if there is too much green tree canopy.
”Once we get over 70 per cent cured, we are starting to get into a situation where you could have bad fires,” he says.
The amount of fuel per hectare is measured by field reports.
”At the moment it is continuous across most of NSW,” he says .
”In some cases we are getting it up to people’s armpits.”
Once fuel conditions have been calculated they are combined with the daily temperature, relative humidity and wind speed to give what is called a Grassland Fire Danger Index.
This index gives a scale of how difficult a fire under those specific conditions will be to control.
Dr Andrew Sullivan, a senior research scientist at CSIRO, says historically the index was a ranking between 0 and 100.
However advances in weather and fuel data means that the scale is now open-ended.
”Once you get an FDI above 50, it is very difficult to do anything about it,” Sullivan says.
”You are now into the realm of we’ll do the best we can, but the conditions are pretty bad.”
Once a grass land fire has ignited, fire managers use a model developed by CSIRO to predict the rate of spread and intensity of the fire.
Sullivan, who helped develop the model, says it takes into account the weather as well as the type of pasture such as natural ungrazed grass land, mown fields or paddocks eaten out by stock.
”If you know where a fire started you can use the model to determine what its average speed will be, and then for a given length of time how far it will travel,” Sullivan says.
It can also predict the fire’s shape and the height of the flames, although its overall accuracy depends on the reliability of the input values such as weather data and fuel conditions, he says.
After a busy start to the bushfire season, Fitzsimmons says landowners need to prepare their properties for bushfire season, with appropriate firebreaks in grassy regions.
”The real key is everyone should have a bushfire survival plan.”