Matthias Kemter, Melanie Fischer, Lisa Luna, Elisabeth Schönfeldt, Johannes Joscha Vogel, Anissia Banerjee, Oliver Korup, Kirsten Thonicke

• Following an unprecedented drought, Australia's 2019/2020 "Black Summer" fire season caused severe damage, gravely impacting both humans and ecosystems, and increasing susceptibility to other hazards. Heavy precipitation in early 2020 led to flooding and runoff that entrained ash and soil in burned areas, increasing sediment concentration in rivers, and reducing water quality. We exemplify this hazard cascade in a catchment in New South Wales by mapping burn severity, flood, and rainfall recurrence; estimating changes in soil erosion; and comparing them with river turbidity data. We show that following the extreme drought and wildfires, even moderate rain and floods led to undue increases in soil erosion and reductions in water quality. While natural risk analysis and planning commonly focuses on a single hazard, we emphasize the need to consider the entire hazard cascade, and highlight the impacts of ongoing climate change beyond its direct effect on wildfires. Plain Language Summary In 2019/2020, a chain of natural hazardsFollowing an unprecedented drought, Australia's 2019/2020 "Black Summer" fire season caused severe damage, gravely impacting both humans and ecosystems, and increasing susceptibility to other hazards. Heavy precipitation in early 2020 led to flooding and runoff that entrained ash and soil in burned areas, increasing sediment concentration in rivers, and reducing water quality. We exemplify this hazard cascade in a catchment in New South Wales by mapping burn severity, flood, and rainfall recurrence; estimating changes in soil erosion; and comparing them with river turbidity data. We show that following the extreme drought and wildfires, even moderate rain and floods led to undue increases in soil erosion and reductions in water quality. While natural risk analysis and planning commonly focuses on a single hazard, we emphasize the need to consider the entire hazard cascade, and highlight the impacts of ongoing climate change beyond its direct effect on wildfires. Plain Language Summary In 2019/2020, a chain of natural hazards impacted Australia's East Coast. Following the severest drought since weather records began, record-breaking wildfires known as the "Black Summer" ravaged the region for months. In early 2020, the rainfall that extinguished the last of these fires caused further damage, as the burned soils repelled much of the rain. Water took the exposed soil and charred vegetation with it on its way to the rivers, flooding streets and polluting drinking water. We show an example of this cascade of hazards in a single river catchment. We found that after the wildfires, even moderate rainfall caused floods, increased soil erosion, and reduced water quality drastically. Natural risk analyses mostly focus on single types of events in isolation. However, this hazard cascade shows that, especially in the face of ongoing climate change, scientists and decision makers need to consider events not just by themselves, but connected with each other.…