Effect of frequent bushfire on water supply reliability in Thomson Catchment, Victoria, Australia

Melbourne’s water supply is dependent on forested catchments; consequently, frequent occurrence of fire events in these catchments will have detrimental effects on water supply reliability of Melbourne. This study analyses the effect of frequent bushfire events to estimate the reduction in the water yield after a major bushfire event in the Thomson reservoir, which is the largest reservoir supplying water to Melbourne. The Forest Fire Danger Index (FFDI) which is a measure of fire behaviour was used to carry out fire frequency analysis to estimate the frequency of occurrence of a bushfire with certain severity in the catchment area. Seven hypothetical scenarios were adopted to estimate the total water yield in 2010 after the 1939 bushfires considering the presence of manifold species in different parts of the Thomson catchment; subsequently, reduction in water yield was calculated for the catchment if a certain percentage of the catchment was burnt once every 10 years after 2010. If 10% of the Ash is burnt every 10 years, the percentage reduction in total water yield varies from 12.2 to 13.9% in 2050 for all seven scenarios considered in the analysis; similarly, the percentage reductions in water yield only from Ash species varies from 21.2 to 23.1% in 2050, if 10% of the Ash is burnt every 10 years. It is expected that improved cognizance of the effect of extreme bushfire events on water yield of Thomson catchment due to frequent bushfire events based on the findings from the existing study will facilitate the adequate preparedness for risks associated with frequent bushfire events in Victoria’s water supply catchments.