Falling moisture levels and rising mercury point to risk of major fires in NSW

Falling moisture levels point to the prospect of an early and active fire season for many forests in NSW. Photo: Wolter Peeters
Falling moisture levels point to the prospect of an early and active fire season for many forests in NSW. Photo: Wolter Peeters

Forest regions of NSW, including areas around Wollongong, are rapidly drying out, raising the risk of early and significant bushfire activity, new mapping techniques indicate.

Vegetation in some regions is approaching or exceeding critical moisture levels associated with all major blazes in the state since 2000, including the large fire that destroyed more than 200 homes around Winmalee in the lower Blue Mountains in October 2013.

Fire authorities have a narrow window to conduct hazard-reduction burns before conditions become more risky. Photo: Wolter Peeters

Fire authorities have a narrow window to conduct hazard-reduction burns before conditions become more risky. Photo: Wolter Peeters

"Unless something changes dramatically … we're looking at conditions similar to 2013," said Ross Bradstock, director of the Centre for Environmental Risk Management of Bushfires at the University of Wollongong.

"I'd be most worried about September," Professor Bradstock said. "That's when things can heat up – we can get days in the 30 degrees, in front of a cold front."

Temperatures are certainly picking up. Wollongong's top reached almost 26 degrees on Friday - eight degrees above normal - and the Bureau of Meteorology is forecasting a string of days in the mid-20s during the middle of the coming week. Odds also favour the dry winter across most of NSW lasting well into spring.

Professor Bradstock's team worked with Western Sydney University to develop new methods to pinpoint the dryness of fuel on the forest floor, such as leaves and branches. They can also detect the rate live plants are drying out, approachs that could help authorities prioritise fire prevention efforts.

Using remote sensing and highly localised climate data on vapour pressure deficits – effectively the level of atmospheric moisture in a region – researchers can "rapidly predict fuel dryness even during the day as humidity changes", Professor Bradstock said.

"It's sort of been a holy grail for [moisture content] estimation for litter on the ground."

Professor Bradstock stresses that a major rain event – such as 50-100 millimetres – could rapidly send fire risks back to more normal levels. Any blaze would also need a cause of ignition to get started.

Even so, data points to low moisture levels for both dead plant material and in live vegetation for parts of the Far South Coast, the Blue Mountains and  Wollemi. (See charts below.)

As an historical guide, live fuel moisture content levels that fall below the 101.5 per cent level correspond to a big increase in fire activity.

All major events examined in the 2000-2013 period in NSW, the ACT and Victoria occurred below 80 per cent, according to research published by Professor Bradstock last year.

"We've found very good relations between cut-off points and threshold and area burnt," Professor Bradstock said. "As soon as you hit these thresholds, the area burned jumps up hugely."  

Apart from generating up-to-date moisture readings, the methods also reveal how areas are changing over time.

The Sydney and Illawarra regions have rapidly dried out since the end of March, with areas around Wollongong, Katoomba in the Blue Mountains and Richmond home to dry clusters. (See charts below for live fuel moisture content.)

Given the prospect of rising temperatures and often windy conditions, the window for hazard reduction burns in dry regions is fast closing, Professor Bradstock said.

Ben Shepherd, an inspector with the NSW Rural Fire Service, said the moisture maps were "starting to line up" with their own surveys.

"What we are seeing is the potential for an increased risk for forest areas for this season," he said, adding the dry spell may mean a reduced risk from grassfires with less of that material around.

"Hopefully, we'll get a nice dump of rain," Mr Shepherd said. "We don't want to alarm people but they need to start to think about the fire season."

Actions residents can start to take include moving firewood piles – still possibly in use given the cool nights – away from homes, mowing lawns and "dusting off" response plans so that households are prepared in case of a fire, he said.

Marta Yebra, a research fellow at Australian National University who reviewed Professor Bradstock's paper, said authorities could use such work to provide a "near-real time monitoring system".

Dr Yebra noted that along with specific weather conditions being favourable, a large fire would also depend on the amount of fuel load, not just its dryness.

The structure of the forest, such as whether there are major natural or manmade breaks, would also play a role.

Dr Yebra has also worked on fire predictive models using land-based and aerial LiDAR (Laser altimetry or Light Detection and Ranging) equipment, results of which were published this week.

Part of the work is already being used to assess historical fire threats at a national level. The use of lasers to assess fuel loads could complement moisture content methods to provide a clearer picture of immediate fire risks.

"We are heading in that direction – to include more dynamic variability to help firies," Dr Yebra said.