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 Ni o 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.