The impact of bushfires on water yield from south-east Australia's ash forests

Widespread disturbance within forested catchments typically increases runoff. However, following widespread fire in 1939 throughout south‐east Australia, Kuczera (1987) reported persistent reductions in runoff that were attributed to increased evapotranspiration from regenerating “ash” forests. Kuczera projected ongoing reductions of water yield for ∼150 years. In 2003, widespread fire in the headwaters of the Murray‐Darling Basin (MDB) again stimulated extensive regeneration of ash forests, raising the prospect of subsequent water yield reductions. To understand the potential impact of the 2003 bushfires, we re‐evaluated yield reductions from three of the catchments originally studied by Kuczera using the same calibration period. We also used an expanded prefire calibration period (1908–1938) based on data not originally available to Kuczera. The trend of postfire water yield that we observed in 1939‐affected catchments is qualitatively consistent with Kuczera's projections, but the quantitative details were, as expected, sensitive to the prefire calibration period used. We then used a simplified method to examine a further five ash‐dominated catchments affected by the 2003 fires. We report relative reductions in mean annual stream flow in all five catchments and a statistically significant (α = 0.05) postfire reduction in one of five catchments. Postfire yield reductions during the austral summer (October to April) were greater in relative magnitude in all five catchments and were statistically significant (α = 0.05) in three of five catchments. We conclude that a postbushfire Kuczera‐type response may be widespread in regenerating ash forests. On that basis, we anticipate postfire yield reductions in ash forests elsewhere and conclude that further reductions in stream flow are likely in the MDB for at least another decade. Key Points Observed trends in post‐fire flow are consistent with previous projections We report significant reductions in flow from catchments affected by 2003 fires Our findings indicate yield reduction from ash forests may be widespread