While natural disasters can be caused by a single anomalous event, such as heavy rainfall, windstorms, heatwaves, droughts and bushfires, high-impact natural disasters are often the result of a complex interaction between several events. In these situations, natural disasters may occur in either singular or separate regions, concurrently or in sequence. This combination of hazards is referred to as multi-hazard 'compound' events. Compound events that occur in a single region amplify the impact on the community, whereas independent events that occur in separate regions have increased impact due to the extra load placed on emergency response resources. Typically, risk assessment only considers one hazard and one event at a time. However, an area can be affected by a number of these hazards (or one multiple times), over a short time, putting pressure on emergency services and exacerbating the impact. 

An example of when a single meteorological parameter can produce compound events at the same location is rainfall. Flooding can occur on days where the rainfall is not particularly extreme but the precipitation falls on an already saturated catchment. This is because the infiltration capacity of the soil is greatly reduced. Capturing the antecedent conditions of the catchment is vital for correctly estimating the rainfall to run-off process and in turn, flood risk. We explore a suite of methods to accurately represent antecedent conditions in Tasmanian catchments.

Here we present results on how Tasmanian compound flooding events are changing over time using four climatologically diverse river catchments representing regional variability in climate. We highlight that flooding can also occur on relatively low rainfall days if, due to antecedent conditions, the catchments are already saturated. We introduce a number of novel statistical methods to identify these compound events and quantify the relative intensities of such events.