The use of joint probability analysis to predict water yield for Thomson catchment in Victoria, Australia

Bushfires in water supply catchments can adversely impact the reliability of water supply and thus threaten the wellbeing and prosperity of a city. Frequent fire events are looming to threaten water supply from the forested catchments of Victoria, Australia. Melbourne, the capital of Victoria, has to meet growing water demand due to population increase and economic development. In recent years, the state has confronted with severe drought conditions, resulting in a considerable reduction of the water supply yield of the catchments. These continued multi-year droughts culminated into the latest major bushfire in 2009 (Black Saturday). Thomson catchment, the largest water supply catchment supplying water in Melbourne is fully forested. This catchment is comprised of different Ash-type forests, Mixed species, Alpine vegetation, and Scrubs. The study noted that there is probability of occurrence of high danger fire events at 1 in 20-year return period for Thomson catchment. The objective of this study was to carry out a joint probability analysis for different percentages of catchment burning, if a 20-year fire event occurs at least once between 2010 and the given year, and determine the percent reductions in water yield from years 2030 to 2090. Based on the analysis carried out in the study, if 5% of the Ash-type forest is burnt once since 2010, the combined reduction of the total water yield would be 6.75% (16,968 ML/year) by 2090.