Professor Ross Thompson
Director, Centre for Applied Water Science
University of Canberra
Fire and water don’t mix: Time for a conversation on Australia’s Water Catchment
The recent major fires across south eastern Australia have been exceptional in their scale and intensity, burning more than 46 million acres, an area a third the size of France. It is difficult to comprehend the scale of these fires and their impacts on water catchments. Warragamba Dam, which supplies 80% of Sydney’s water, had more than a third of its 905,000 hectare catchment burnt. Across the south east of Australia fires have led to millions of tonnes of burnt debris, ash and sediment entering waterways. The 2003 fires in and around Canberra provide a degree of insight into the impacts of fire on water catchments over decadal time scales. The Cotter catchment, Canberra’s main water supply was extensively burned during those fires. Heavy rainfall shortly afterwards resulted in large scale erosion of the catchment due to the loss of ground cover by plants and destabilisation of banks by fallen trees. Downstream in the Murrumbidgee River some pools were filled by over 5m of newly deposited sediment, with substantive loss of habitat for numerous species including the endangered Macquarie Perch. In the time since the 2019/2020 fires there has been active interventions across multiple parts of the fireground to manage erosion risks and prevent sediment entering water supplies.
The reality that emerges from the most recent fires is that the climate in south eastern Australia is changing. There is profound evidence for warmer average temperatures, but also more frequent, intense and prolonged heat waves. Rainfall has dramatically reduced, but timing of rainfall has also shifted, with repeated failure of spring and autumn rains, and increases in extreme summer rainstorms. These conditions generate high fuel loads and reduce the opportunity to manage these through hazard reduction burning. Even with fuel management fires are impacting more regularly and to a greater extent on water catchments. There is no doubt that with respect to water supplies we will need to diversify sources, both through the use of desalinisation and the inevitable shift to stormwater and wastewater harvesting and treatment for potable use. From a biodiversity perspective, there will need to be urgent efforts to manage non-fire stressors such as weeds and invasive animals to take pressure off recovering catchments and increase their resilience to future fire.