Compound drought and fire effects on runoff generation in southeastern Australia

Natural hazards, including drought and wildfire, can substantially modify how catchments generate streamflow following precipitation events. Yet, how drought and fire events alter such rainfall-runoff relationships remains poorly understood. Here, we use nonlinear deconvolution techniques to reveal how runoff responds to rainfall in 155 Australian catchments, and how these responses are altered by the 2017-2019 Tinderbox Drought and the 2019-2020 Australian wildfires. Our results show the Tinderbox Drought typically halved runoff response (i.e. the streamflow response to a unit of rainfall reduced by half), and subsequent fire impacts appeared paradoxical because post-fire runoff generation was simultaneously altered by climate conditions, fire severity, and drought legacies. During the drought, almost all catchments exhibited declines in per-unit-rainfall runoff responses, with stronger declines in catchments that experienced larger rainfall reductions and had low soil infiltration capacities. Post-fire, these responses increased by up to threefold in some regions, but these increases appear to have been driven by higher rainfall rather than by fire effects. In contrast, catchments in other regions experienced little overall change in runoff response, because the runoff reductions driven by drought legacy effects competed with simultaneous strong increases in runoff response induced by the wildfires. This highlights that compound natural hazard effects on runoff generation may reshape hydrological processes in diverse and initially unintuitive ways.