Understanding the impact of compound bushfire-rainfall extremes on river runoff in Southeast Australia

Bushfires of unprecedented scales ravaged Australia in the 2019-2020 season, scorching more than 25 million acres and with some parts of the country burning for as long as eight months. Torrential rains helped contain the fires in early 2020 but soaked the relatively populated southeastern cities resulting in flash floods. The extreme societal impacts raise the question of how and how frequently compound bushfire/extreme rainfall-runoff events occur in this region. Although many studies have evaluated post-fire runoff responses, impacts are usually studied at small spatiotemporal scales and most often in North American climate zones. We use Global Fire Assimilation System (GFAS, 2003-Present) satellite-derived daily fire radiative power observations, high-resolution daily Climate Hazards Group InfraRed Precipitation with Station (CHIRPS, 1981-Present) data, and daily river discharge values (Australian Bureau of Meteorology) to study the relations between local climatology/rainfall regime, watershed properties, fire occurrence, and post-fire runoff in the river basins of southeast Australia at subseasonal timescales over the 2003-2020 time-period. We explore statistical linkages using different fire properties and show that runoff responses are affected more by fire extent (burned basin area fraction, BAF) than intensity. Testing if river discharge from extreme rainfall is greater after fire and is BAF-dependent, we find that river basins with above-median BAF exhibit ~35% greater increase in discharge than below-median BAF basins for a doubling of rainfall. We also detect and highlight a strong in post-fire runoff responses such that the runoff-rainfall ratio is dramatically higher for small to moderate rainfall events. This might be explained by decreased soil infiltration capacity after fire, but which is delicate and more readily degraded or eroded by high rainfall and overland flow. This trend of decreasing post-fire runoff ratio with increasing event size also suggests that the most damaging discharges may occur for moderate rainfall amounts. These outcomes imply that understanding the structure and causes of variability in the rainfall-runoff relationship is essential for reliable flood projections and for reinforcing adaptation systems to manage potential ecological disturbances.