The vulnerability of water supply catchments to bushfires: impacts of the January 2003 wildfires on the Australian Capital Territory

Abstract

Eastern Australia has been swept by landscape scale bushfires throughout the Holocene period. In January 2003, major bushfires burnt through the Australian Capital Territory (ACT). They devastated parts of the national capital, Canberra, and almost all the Cotter catchment, a normally pristine source in its upper catchment for ACT drinking water. Intense, local thunderstorms following the fires, estimated to be a 1 in 400 year event, moved large sediment loads from steep, denuded slopes into the supply reservoirs, Corin, Bendora and Cotter dams. Bushfires in Melbourne’s water supply catchments in 1939 produced large decreases in yield that persisted for 50 years as mountain ash forests regrew. The Cotter fires raised concerns over yield decline and short and long term water quality impacts. In this paper, preliminary impacts on water yields and water quality are analysed for Bendora dam and its catchment. Major landscape scale bushfires in the Cotter catchment over the last 150 years have been associated with severe droughts mostly related to positive phases of the Pacific Decadal Oscillation and El Nino events. Our preliminary, non-parametric, yield analysis shows no significant changes in annual upper catchment yield following the fires. Before the 2003 fires, water quality in the storage was excellent, although annual build up in iron and manganese and turbidity occurred at the bottom of the reservoir. The 2003 fires caused unprecedented increases in turbidity, iron and manganese, by up to thirty times previous events in the upper catchment storages. These increases caused disruptions to water supply and resulted in the construction of a major water filtration plant to address turbidity and other water quality problems. While natural revegetation in the upper Cotter has lead to improvements in water quality, the area of former pine plantations in the lower Cotter continues as a major sediment source.

Keywords:

• Bushfires
• drought
• water supply catchments
• water quality
• yield
• vulnerability

Additional information

Notes on contributors

Ian White

Ian White is Professor of Water Resources in the Centre for Resource and Environmental Studies at the Australian National University, his research is in hydrology and sustainable water resource and land management, and in vulnerability and adaptation and he contributes to UNESCO’s International Hydrology Programme.

Alan Wade

Alan Wade is a Visiting Fellow in the Centre for Resource and Environmental Studies at the Australian National University. He was formerly a water quality expert with ActewAGL in Canberra. His research interests are in water quality and catchment management.

Martin Worthy

Martin Worthy is a graduate student in the Centre for Resource and Environmental Studies at the Australian National University. His research lies in sedimentology, geology and geomorphology and he is currently studying paleo sediment flows in the Cotter catchment.

Norm Mueller

Norm Mueller is Managing Engineer Catchments with ECOWISE Environmental in Canberra. He has wide experience in hydrology and hydrological measurement and is currently in charge of rehabilitation of the Cotter catchment.

Trevor Daniell

Trevor Daniell is Associate Professor in the Centre for Applied Modelling in Water Engineering, School of Civil and Environmental Engineering, at the University of Adelaide. He has strong research experience in rainfall-runoff modelling and is active in Engineers Australia and UNESCO’s International Hydrology Programme.

Robert Wasson

Robert Wasson is Deputy Vice Chancellor at Charles Darwin University. He was formerly Director of the Centre for Resource and Environmental Studies at the Australian National University. His research interests are in geomorphology and global change. He contributes to the International Geosphere Biosphere Program.