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.

Key Words:

• Non-stationarity
• flood hydrology
• streamflow
• climate change
• climate variability
• El Niño Southern
• Oscillation
• Indian Ocean dipole
• southern annular mode
• downscaling

Additional information

Notes on contributors

S Westra

Seth Westra is a hydroclimatologist with five years experience in flood hydrology, seasonal forecasting of rainfall and streamflow, and climate impacts assessments. Seth holds a PhD from the University of New South Wales, which focused on the identification of sources of climate variability on water resource availability and hydrologic extremes, and the inclusion of this information in statistical models that provide seasonal forecasts with up to a one-year lead time. Current research interests include continuous rainfall simulation, development of statistical downscaling approaches to estimate future rainfall extremes at the sub-daily timescale, evaluation of the capabilities of global climate models to simulate the low-frequency variability in hydrological time series, and seasonal rainfall and streamflow forecasting.

I Varley

Ian Varley works in the Sydney office of Sinclair Knight Merz and has 25 years experience in hydrology, embracing environmental flow studies, water supply option studies, flood studies and spillway adequacy studies. Ian has worked on projects throughout Australia, Asia and Africa.

P Jordan

Dr Phillip Jordan is a senior hydrologist with consulting firm Sinclair Knight Merz, who has 15 years of experience in hydrology and water resources engineering. He holds a BE(Hons) from the University of Queensland and a PhD from Monash University. Phillip is the product project leader for the eWater CRC’s Catchment and Climates Project, which is developing the Watercast integrated water quantity and quality model. Phillip has authored or co-authored six papers that have appeared in Australian and international journals, and 24 conference papers. His research and consulting interests include flood hydrology, stochastic modelling and applications of meteorological radar in hydrology.

R Nathan

Dr Rory Nathan is the Principal Hydrologist with Sinclair Knight Merz. He holds degrees in different aspects of engineering hydrology from the universities of Melbourne and London, and has around 30 years experience in academic and consulting positions. Rory’s technical interests are in the modelling of hydrological processes, environmental hydrology, and the analysis and simulation of extreme events.

A Ladson

Tony Ladson has more than 20 years experience in hydrology and river management, and has worked on projects throughout Australia, and in the US and Taiwan. He has a PhD from the University of Melbourne, a Master of Science from the University of Minnesota and also studied at Uppsala University in Sweden. He retains associate status of both Monash University and the University of Melbourne. One of Tony’s main interests is the application of hydrologic principles to improve the environmental condition of Australia’s rivers. He was a key developer of stream condition assessment methods that have been applied in Victoria, South Australia and Tasmania, and has advised on river health assessment in the Lake Eyre Basin. He has worked extensively on environmental flows, and has contributed to strategy plans and policies to improve river health. In 2000 he was awarded a Victorian Fellowship to undertake international research into adaptive management of environmental flows, and in 2005 he received the GN Alexander Medal from Engineers Australia for work on evaluation of stream rehabilitation projects. Tony has more than 70 refereed publications and recently completed a book on Australian hydrology for Oxford University Press.

A Sharma

Ashish Sharma is an Associate Professor in the School of Civil and Environmental Engineering, the University of New South Wales. His main research interests are related to the use of statistical methods for addressing water engineering problems.

P Hill

Peter Hill works in the Melbourne office of Sinclair Knight Merz and has more than 15 years experience in flood hydrology. After undertaking a Master of Engineering Science by research on flood estimation, he worked as Project Leader within the Cooperative Research Centre for Catchment Hydrology on loss modelling for flood estimation. His research interests include design flood estimation, the impact of land use change on catchment hydrology and dam safety risk assessment. He is a member of Engineers Australia’s National Committee on Water Engineering.