Past and future trends in Australian floods: what are the causes?

The possibility of changing flood hazards due to climate change is an issue of concern for engineers, governments and emergency services. However it is not yet clear how flood hazard will change and how these changes may vary across Australia. Here we approach these questions by considering the complex interactions between flood-causing variables. This is achieved by synthesizing existing research on the trends and likely changes in the causes of floods. These causes include the possible changes to meteorological variables, in particular extreme rainfalls, as well as impacts of changing catchment conditions including catchment wetness and land use modifications due to vegetation changes and/or urbanization. There has been research into trends in daily rainfall extremes around the world and for Australia but less work on sub-daily rainfall. Australian daily rainfall extremes show limited evidence of trends when the annual maximum series is considered although there is significant variability in these time series and changes in seasonality are not accounted for in this type of analysis. Sub daily rainfalls show clearer evidence of generally increasing trends. The interaction between these generally increasing trends and changes to catchment wetness which are more likely to be linked to annual rainfall and evaporation trends is a question of particular interest. An issue of concern is how to separate trends from other cycles of variability introduced through large scale climate drivers. Improved statistical methods are required to separate these interactions both in rainfall and more importantly in the flood hazards. Promising work from detection and attribution studies on the likelihood of individual extreme events needs to be translated into methods that can assess the likelihood of trends and the possibility of detecting these trends. Here the focus is on how the trends in each of these aspects can be combined together to better understand the observed changes in flood risk. Using this improved knowledge, projections of changes in flood- producing mechanisms can be combined to establish likely future flood risk. Open research questions have been identified that could be addressed by the Australian research community over the next 5 to 10 years include: • The need for robust future projections of extreme precipitation over daily and longer durations from a large ensemble of climate projections at resolutions (~3 – 10km) that can capture important phenomena such as east coast lows, orographic effects, bands on thunderstorms etc. • Work in extending the findings from convection-permitting climate models to inform engineering practices that depend on sub-hourly and sub-daily extremes. • Common frameworks for undertaking catchment scale assessments of future flood risk to allow findings to be translated and regionalized.