Addressing climatic non-stationarity in the assessment of flood risk

Abstract

Present-day flood estimation practise is underpinned by the assumption that flood risk in a future climate will reflect historical flood risk as represented by the instrumental record. This assumption, which is commonly referred to as the assumption of stationarity, recently has been questioned as a result of both an increased appreciation of the natural variability in our hydroclimate at temporal scales beyond that of the instrumental record, as well as the projected intensification of the hydrologic cycle due to anthropogenic climate change. These developments have led some authors to suggest that the stationarity assumption should henceforth be considered invalid, thereby calling into question all the methods that are underpinned by it, including flood frequency analysis using observed streamflow records, and rainfall-runoff modelling informed by instrumental precipitation and streamflow records. In this paper we review a wide range of possible sources of non-stationarity in the Australian climate record, and highlight that the primary sources of non-stationarity relevant for flood risk assessments include natural climate modes that vary at timescales similar to the length of the instrumental record, as well as long-term trends and step changes that are attributable to anthropogenic climate change. Although prescriptive guidelines that describe how to address this non-stationarity are currently unavailable in Australia, this review nonetheless highlights the importance of using long records for flood analysis, possibly by extending records using nearby stations. Furthermore, it will become increasingly necessary to develop plausible estimates of how the climate will evolve, and we describe some climate modelling tools that allow for the development of future climate scenarios. Finally, we emphasise that removing the assumption of stationarity will inevitably result in an increase in the uncertainty associated with future flood estimates, and suggest that this may require new methods to conceptualise and manage future flood risk.