It might feel like the depths of winter, but Australian fire services are preparing for an early start to the bushfire season. Sydney has been covered with smoke from hazard reduction burns, and the New South Wales Rural Fire Service has forecast a “horrific” season.
Predicting the severity of a bushfire season isn’t easy, and – much like the near-annual announcements of the “worst flu season on record” – repeated warnings can diminish their urgency.
However, new modelling that combines Bureau of Meteorology data with NASA satellite imaging has found that record-setting July warmth and low rainfall have created conditions very similar to 2013, when highly destructive bushfires burned across NSW and Victoria.
Crucially, this research has found we’re approaching a crucial dryness threshold, past which fires are historically far more dangerous.
How to measure bushfire fuel
On September 10, 2013 several bushfires in Sydney’s West caused havoc well before the official start of the bushfire season. These were a precursor to fires that destroyed more than 200 properties a month later. Warm, dry winter weather had dried out the fuels in Sydney’s forests and bush reserves beyond “normal” levels for the time of year.
The timing and severity of those preseason fires were a reminder that the region’s forests are flammable all year round; they can burn whenever the fuel they contain dries out past a certain threshold.
In most forests, there is an abundance of fuel in the form of leaf litter, dead twigs, branches and logs, lower vegetation such as shrubs and grasses, as well as higher foliage and branches.
The flammability of all these different kinds of fuel depends largely on their moisture content. Leaf litter and fine dead branches on the soil surface can dry out in a matter of days, whereas logs may take weeks or months to lose their moisture. The moisture content of shrubs and tree canopies varies depending on the amount of water in the soil, so they reflect the overall rainfall and temperatures across a whole season.
The flammability of an entire forest is therefore a complex calculation of all these different kinds of fuel (both alive and dead) and their different moisture levels.
Mapping Sydney’s forests
In a recent collaborative study, we combined data from a Bureau of Meteorology project that maps water availability levels across Australia with satellite imagery to develop new tools for mapping and monitoring moisture levels of different fuels in forests and woodlands.
We checked these tools by modelling fuel moisture levels during fires in NSW, Victoria and the ACT between 2000 and 2014, and comparing our predictions to historical bushfires.
Our research has identified critical dryness thresholds associated with significant increases in fire area. Rather than a gradual increase in flammability as forests dry out, when dead fuel moisture drops below 15% subsequent bushfires are larger. Another jump occurs when dead fuel moisture levels fall below 10%. We found similar thresholds in growing plants, although their moisture content is measured differently.
These dryness thresholds are pivotal, because they may represent the breakdown of moist natural barriers in landscapes that prevent fires from spreading. Understanding these mechanisms makes it possible to predict fire risk much more accurately.
As part of this project we compared the fuel moisture in Sydney Basin’s forested areas in 2013 and 2017. As shown in the chart below, currently the live fuel moisture level is tracking well below the 2013 values, and is approaching a crucial threshold (indicated by the dotted line).
The moisture content of dead fuel has been more variable, but it has also dipped below the 2013 curve and, if warm dry weather continues, could reach critical levels before the end of August.
In another worrying sign, mapping shows critically dry live fuel is much more abundant in 2017 than it was in 2013.
It’s clear that much of the Sydney Basin is dangerously primed for major bushfires, at least until it receives major rainfall. Forecasts for windy but largely dry weather in coming weeks may exacerbate this problem.
These new insights into landscape-scale fuel dryness provide a powerful indicator of what might be expected. They also build our capacity for week by week monitoring of fire potential.
Preparation by both fire management authorities and exposed homeowners is now an immediate priority, to cope with the strong likelihood of an early and severe fire season.
Matthias Boer, Associate Professor, Hawkesbury Institute for the Environment, Western Sydney University; Rachael Helene Nolan, Postdoctoral research fellow, University of Technology Sydney, and Ross Bradstock, Professor, Centre for Environmental Risk Management of Bushfires, University of Wollongong
When you live in a bushfire-prone area you can’t ignore the danger. Most individuals and families address this necessity by preparing a bushfire survival plan. The best way to survive a bushfire is not to be there when it arrives.
For most Australian fire agencies the “leave early” policy has largely replaced the previous “stay and defend or leave early” one. This
reflects an emphasis on preserving human life during a bushfire event – an emphasis that has strengthened since the 2009 Black Saturday bushfires.
Even when planning to leave early, unexpected events can occur. Not being able to find a child or family pet may delay departure until it’s no longer safe to travel. Taking refuge in your home then becomes a last resort, a worst-case scenario. But this contingency is worth considering as part of your bushfire survival plan.
If you do need to take refuge inside your home during a bushfire, which parts are likely to be the safest? As part of my PhD research, I asked 252 residents living in bushfire-prone areas which parts of their houses they would shelter in during a bushfire, which parts they would avoid, and why. I then analysed the features of these locations against the known places where people died in their home during bushfires in Australia from 1901 to 2011.
Determining the safer places to shelter is further complicated as all houses are not the same. There are many different types, with large variations in design, construction materials, location and surrounding vegetation. It is therefore not possible to give absolute answers on where people should take shelter in their homes during a bushfire, but some general guidelines can be given.
Where are the safer spaces to shelter?
Upstairs is generally a more dangerous space to seek shelter during a bushfire. Upstairs levels are more difficult to escape from. Often they have large windows and sliding glass doors which are designed to capture views, but due to radiant heat and strong winds can crack and implode. Upper levels are often constructed of lightweight materials that are more flammable and vulnerable to direct flame contact from burning trees.
The ground floor is generally a safer space to shelter. The ground level usually has more external doors from which the occupant can escape. On a sloping block, however, the easiest level from which to exit may be the first floor. The ground level often has smaller windows (except those leading to entertainment areas). From the ground floor it is easier to get to the driveway and closer to an external water source such as a water tank.
People often suggest the bathroom as a good place to shelter during a bushfire. However, the bathroom can also be dangerous. During a bushfire, mains water is often cut or the pressure is reduced to a trickle. Despite having tiled walls, non-combustible fittings and a water supply, bathrooms like other rooms are vulnerable to the collapse of a burning ceiling when embers have ignited in the roof cavity.
Most bathrooms do not have an external door that residents can use to exit the house. In a bathroom it can be difficult to see the progress of a fire. And as bathrooms are small enclosed spaces they may be more vulnerable to carbon monoxide poisoning.
My advice is to look at all the external ground floor doors (while remembering that glass doors can be dangerous because of their vulnerability to radiant heat), and determine which of them provide access to adjoining outside paved, gravel, concrete or other non-combustible areas. You should also see if there is a small window from which you can observe the progress of the bushfire, and if there is a sink close by to store water. Where possible consider installing a fire alarm that has a carbon monoxide sensor with audible and visual alerts.
When you have identified the most suitable place in the house to actively shelter during a bushfire, follow the bushfire preparation activities provided by fire authorities. Some of these will include looking out of a window to follow the progress of the fire and being aware of current bushfire updates on the radio and via mobile phone. There is no such thing as passive sheltering.
Being inside your home as the fire passes offers more protection than being outside. But it should be seen as a last resort, with leaving early the preferred action. Fire agencies work hard to inform residents of days when bushfires are likely, and to provide updates on fires that do break out. Residents in bushfire-prone areas should take these warnings and updates seriously and leave their properties when advised to do so, especially when catastrophic fires are expected.
The advice given in this article is general and may not suit every circumstance.
The number of house severely damaged or destroyed now stands at 26 confirmed, plus other buildings. The number may yet rise.
Thirteen homes have been destroyed in the worst bushfire in South Australia since 1983.
That number is expected to rise to more than 30 homes as specialist crews of firefighters assess the damage in the Adelaide Hills and try to control the blaze, which has been burning since Friday, the Australian Broadcasting Corporation (ABC) reports.
But it is a race against time as weather conditions are expected to hamper efforts on Wednesday, with high temperatures and winds expected.
More than 1,100 properties lie in the affected area and authorities say they will have a better idea of the damage as the day goes on.
Fire crews are working to protect pockets of homes from the blaze and will increase their efforts to control the fire from the air.