The effect of drought and temperature extremes on burned area in Southeastern Australia

Natural hazards often result from interacting physical processes across a wide range of spatial and temporal scales. Namely, the occurrence of heatwaves and droughts increases the risk of wildfires, with recurrent extensive human, ecological and economic losses being reported throughout the world. In Australia, several extreme bushfires have occurred following severe droughts and heatwaves, namely the Black Saturday bushfires in 2009 and the extreme bushfire season of 2019-2020.

In this work, we analyze the relation between fire occurrence, drought conditions, and temperature extremes in southeastern Australia for the period 1982-2018 and considering the months between December and February. Monthly burned area (BA) was retrieved from the FireCCILT11 dataset, with a spatial resolution of 0.25˚. The drought index SPEI was used to assess the drought conditions and was computed using monthly precipitation and temperature data from the CRU TS 4.05 database. The occurrence of temperature extremes was assessed using the index Number of Hot Days (NHD), which was computed using daily maximum temperature obtained from the ERA5 dataset.

The study area comprises pixels that have burned at least 25 times on these months. A correlation analysis was performed between BA and SPEI at time scales of 1, 3, and 6 months, and between BA and NHD. The influence of current and previous conditions on BA was assessed, by correlating BA with SPEI and NHD at the current month, and in the previous 1 to 3 months. The joint probability of BA, drought, and temperature extremes was also assessed, using copula functions.

The results show a negative correlation between BA and SPEI, and a positive correlation between BA and NHD. For previous months, the correlation is stronger between BA and SPEI, than for BA and NHD, pointing to an effect of the drought conditions on previous months, whereas the effect of temperature on BA is seen instantaneously. The probabilistic analysis shows a clear increase in the probability of BA exceeding the 80th percentile, given drought conditions, compared to non-drought conditions. A similar result was obtained for the case of extreme temperature.

Acknowledgements: This study was partially supported by national funds through FCT (Fundação para a Ciência e a Tecnologia, Portugal) under projects FIRECAST (PCIF/GRF/0204/2017) and Floresta Limpa (PCIF/MOG/0161/2019); and the 2021 FirEUrisk project, funded by the European Union’s Horizon 2020 research and innovation programme under the Grant Agreement no. 101003890.