Fire behaviour analysts are increasingly finding previously used measures, such as the Keetch-Byram Drought Index (KBDI) and Drought Factor (DF), used for the last 60 years, are not providing the nuance in the dryness of forest fuels across south-east Australia to determine bushfire potential.
CFA Predictive Services Specialist Andy Ackland saw an opportunity to investigate the potential to use gridded soil moisture as a more accurate alternative, similar to work done in the United States.
“In recent years, there have been vast improvements in research and data sources in relation to landscape water balance and soil moisture, with national grids of soil moisture now published daily and historically by the Australian Bureau of Meteorology (BOM),” Andy said.
“We know drought is bad for bushfires and BOM soil moisture products helped us flag heightened risks ahead of the 2019-20 bushfire season, but a key science gap is understanding when and how forests dry out and transition to a condition that can support bushfires.”
“To start off, I extracted every fire since 1972 in Victoria up until 2024 and retrieved the corresponding soil moisture values for every one of those 37,000 fires and their vegetation classes. This revealed fascinating patterns of fire occurrence in relation to soil moisture.”
“I discovered that there is quite a sharp threshold once you’re getting below 40 percent root zone soil moisture and below 4 percent in upper soil moisture. That’s where fires suddenly started to appear.”
“When values were much higher than this, there was generally low occurrence of bushfires, even if bushfire weather conditions were elevated.”
Andy will be speaking at the AFAC Conference in Melbourne today, 18 August 2026, alongside colleagues Dr Sarah Harris, Rachel Bessel, Kristy Butler, Nick McCarthy, Brigid Ennis, Georgina Hill, Liv Duffey and Tanya Lumley.
Image: CFA Media
