Forest hydrology is a complex science but, put simply, water yield or stream flows from forested catchments depends on rainfall patterns, droughts, soil type, soil depth, forest type and forest age.
Decades of pioneering research by the MMBW have shown that wet mountain ash forests consume nearly 70% of the rainfall leaving the remainder available for streamflow.
Mountain ash forests are very susceptible to bushfire and are generally killed, but they regenerate rapidly from seed.
The younger and more vigorous regrowth, with thousands of stems per hectare, intercept and slurp up much more water compared to older forests with large and widely spaced trees.
This graph published nearly 40 years ago by George Kuczera illustrates the dramatic response in water yield from mountain ash forests as they age.
The important thing to note is that water yield drops sharply immediately after bushfires when the trees are killed, and vigorous regrowth forest becomes established. Streamflow remains low for 20-30 years and then slowly recovers.
A major bushfire in Melbourne’s forested water catchments would have serious consequences for the city’s water supplies. It’s been calculated that a major fire could reduce stream flows by as much as 60% and that it would take about 150 years for the catchments to return to pre-fire levels.
Victoria’s Forests & Bushfire Heritage 