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National fuel availability factor from daily soil moisture grids
Fire behaviour analysts are frequently challenged with predicting bushfire potential in the days and weeks ahead, but are constantly hamstrung by deficiencies in models that can reliably forecast the availability of forest fuels. There remains no adequate characterisation of wetting and drying dynamics in Australian fire behaviour models.
Crude indices, such as the Keetch-Byram Drought Index (KBDI) and Drought Factor (DF) are over 60 years old and have poor empirical basis and numerous issues in adequately capturing Australian forest hydrology and flammability observed in the field.
In recent years, there have been vast improvements in research and data sources in relation to landscape water balance, with national grids of soil moisture now published daily and historically by the Australian Bureau of Meteorology. I saw opportunity to investigate the potential to use gridded soil moisture as a more accurate indicator of forest fuel availability.
Firstly, I explored the relationship between soil moisture and historic bushfire occurrence by extracting historic gridded soil moisture values for every bushfire in Victoria since 1972. I then plotted frequency distributions of historic bushfires against different levels of soil moisture and vegetation types. Most bushfires occurred when upper soil moisture (top 10cm) was below 3% and lower soil moisture (10-100cm) below 40%. When values were much higher than this, there was generally low occurrence of bushfires, even if bushfire weather conditions were elevated. A relatively sharp threshold is evident, below which there is likely reduced limiting effect of moisture in the fuel bed on equilibrium fine fuel moisture content and thus flammability.
I have translated results of this analysis into a prototype daily national fuel availability factor derived from soil moisture grids, with values empirically related to the likelihood of bushfires occurring. This shows significant promise as a potential alternative to DF in Australian fire behaviour models.
Crude indices, such as the Keetch-Byram Drought Index (KBDI) and Drought Factor (DF) are over 60 years old and have poor empirical basis and numerous issues in adequately capturing Australian forest hydrology and flammability observed in the field.
In recent years, there have been vast improvements in research and data sources in relation to landscape water balance, with national grids of soil moisture now published daily and historically by the Australian Bureau of Meteorology. I saw opportunity to investigate the potential to use gridded soil moisture as a more accurate indicator of forest fuel availability.
Firstly, I explored the relationship between soil moisture and historic bushfire occurrence by extracting historic gridded soil moisture values for every bushfire in Victoria since 1972. I then plotted frequency distributions of historic bushfires against different levels of soil moisture and vegetation types. Most bushfires occurred when upper soil moisture (top 10cm) was below 3% and lower soil moisture (10-100cm) below 40%. When values were much higher than this, there was generally low occurrence of bushfires, even if bushfire weather conditions were elevated. A relatively sharp threshold is evident, below which there is likely reduced limiting effect of moisture in the fuel bed on equilibrium fine fuel moisture content and thus flammability.
I have translated results of this analysis into a prototype daily national fuel availability factor derived from soil moisture grids, with values empirically related to the likelihood of bushfires occurring. This shows significant promise as a potential alternative to DF in Australian fire behaviour models.
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