Mega Forest Fires Intensify Flood Magnitudes in Southeast Australia

Abstract

Wildfires are uncontrolled fires that usually occur during severe droughts and/or heatwaves. In contrast to controlled, prescribed fires, wildfires can directly cause billions of dollars in damage to properties and ecosystems, and indirectly affect the carbon cycle and climate (Godfree et al., 2021;Loehman, 2020;Moritz et al., 2014). With changes in vegetation and soils, wildfires can also affect hydrological processes. Over short periods of time, they may reduce forest water use by the large loss of vegetation cover (Collar et al., 2021), and they may reduce infiltration by soil hydrophobicity after fires (Ebel & Moody, 2017). Both processes may cause changes in catchment water yields, impacting the regional water supply (

Upper limits for post-wildfire floods and distinction from debris flows

“…This is consistent with site-based investigations that found that the largest post-wildfire floods (28) were generated by infiltration-excess runoff from high-intensity rainfall. It is also congruent with recent regional analysis in Australia showing that the largest post-wildfire floods are generated by infiltration-excess runoff mechanisms (9). However, our analysis also suggests that relatively large post-wildfire floods may be generated by other streamflow generation mechanisms, such as saturation-excess overland flow and subsurface storm flow, or mixtures of streamflow generation mechanisms, especially in basins >1000 km 2 (Fig.…”

“…4C). This finding is supported by small hillslope studies in burned basins that observed streamflow generation from the saturation-excess mechanism ( 44 , 45 ), regional studies in Australia that indicated post-wildfire saturation-excess runoff generation ( 9 ), and observed increases in shallow and deep subsurface streamflow from groundwater contributions ( 46 ). Modeling frameworks designed to predict the full spectrum of flood magnitudes following wildfire, rather than just the largest floods, may need to incorporate the full spectrum of streamflow generation mechanisms to predict post-wildfire floods, not just the infiltration-excess mechanism.…”

“…It also suggests that further work on post-wildfire intense rainfall and flood climatology ( 27 , 47 ) would be beneficial to understanding flood generation mechanisms that have been shown to be complex in wildfire-prone, mountainous regions such as the western United States ( 48 ). The post-wildfire flood data analyzed here are primarily from the western United States, but the principles governing exceptional flood upper limits, basin area scaling, and flood generation mechanisms transfer readily to other burned regions ( 9 ). The post-wildfire flood envelope curves and conclusions derived from this analysis provide interim guidance for emergency managers until more post-wildfire flood data are available globally.…”

“…The fire-heat impulse into the soil can reduce infiltration (4), porosity (5), and surface roughness (6), thus intensifying surface runoff processes relative to unburned conditions. These post-wildfire process shifts may cause increases in the frequency and magnitude of floods, relative to similar rainstorms in unburned conditions (7)(8)(9)(10). Increases in postwildfire flood magnitudes, expressed as a ratio relative to unburned floods for similar precipitation events at the same basin, have been shown to be 10 to 100, but can be as high as 2300 times greater in the western United States (10).…”

HESS Opinions: The Sword of Damocles of the Impossible Flood

“…Another example of combination of reasons is given by fire-flood interactions. In 2019/2020, wildfires ravaged ecosystems and settlements in Australia after a severe drought, which reduced surface infiltration capacity and intensified flooding and erosion, resulting, among other things, in siltation of Sydney's main water supply reservoirs (Kemter et al, 2021, Xu et al, 2023.…”