More than 7 million hectares of forest have been burned in eastern Australia so far this fire season, larger than the entire area of Tasmania. Understandably, the sight of such vast burned areas, blackened trees, and ash invokes fear of large-scale and catastrophic impacts on our catchments. With such an enormous area of burned forest, much of the commentary in recent months has discussed the potential for major consequences for our water resources by changes to the way water moves through our catchments. The aftermath from past bushfires support these fears so anticipating and responding to the threats bushfires pose is important. However, during emergency response and recovery, there is a real risk that the efforts to make a difference will be misguided, poorly targeted, and founded on perceived fears rather than scientific knowledge. As resources for bushfire recovery are often limited, the response must be developed by carefully considering goals and management options, using the best-available evidence and robust risk assessment and economic frameworks. Resources must be allocated where they are needed the most and where they are most likely to make a difference.
Figure 1. Surface runoff can increase dramatically after bushfire. Hillslope near Flowerdale after Black Saturday Wildfires.
So, what do we know about bushfire and its impact on rivers and water supply?
There are two key mechanisms by which catchment hydrology is impacted by bushfire.
First, there is the immediate post-fire increase in surface runoff where reduced vegetation cover and water repellent soils result in much more overland flow during rainstorms. More overland flow in the psot-fire period means more erosion, debris flows and dangerous flash-flooding. Both hillslopes and drainage networks in steep highlands are at risk of increased erosion.
In the event of intense rainfall, high concentrations of sediment and nutrients enter streams, rivers and reservoirs. Ash from the fires contain high concentrations of nutrients and other contaminants, which further impacts on water quality. Bushfire-effects on surface runoff and erosion in Eucalypt forest typically last for 2-4 years.
Figure 2. Post-fire debris flows deliver huge volumes of sediment (including fine clay and silts) to rivers. Photo from Wellington River near Licola, 2007 (Adrian Murphy, Melbourne Water).
Second, over longer timeframes, there is a change in the catchment water cycle as vegetation recovers and new plants grow to replace those damaged or destroyed by fire. Increased water use by plants results in reduced runoff reaching rivers. Extent of change in plant-water use and streamflow depend on forest type, fire severity and fire extent. Using Melbourne as an example, for expansive, stand-replacing fires in mountain ash forest (which regenerate from seed after fire and make up the bulk of Melbourne’s water supply catchment), the reduction in streamflow can be major (up to 50% reduction) lasting for several decades. In mixed-species eucalyptus forests the effects are much less dramatic with reduction in streamflow being of relatively low magnitude and short-lasting . However, it’s noteworthy that even small changes in plant-water use can have large implications for water yield at catchment-scales when the area affected by bushfire is several million hectares.
How will these hydrological changes unfold across the 7 million of hectares of burned areas in eastern Australia?
This is complex. Geology, terrain, soils, vegetation type, fire severity, and climate will all play a role in how individual catchments respond to these bushfires. There are some regions (e.g. Eastern Uplands of Victoria, Nattai Catchment in Sydney and the Namadgi National Park, ACT) where the science on bushfire hydrology is well developed. Elsewhere the knowledge base is relatively poor, and the science is incomplete. Planning for changes in water yield and quality and making informed decisions about management interventions is therefore challenging.
A high-level priority must be to consolidate the current science, stemming from intensive and localised research programmes, to provide general insights and conceptual models that can be used to guide management of burned catchments across the large fire-affected areas.
Figure 3. Sediment control structures (debris racks, small settling basin and risers) installed in the Thomson Catchment after wildfire in 2019 to protect road infrastructure and reduce post-fire sediment delivery into the Thomson Reservoir.
Risk assessment can help guide recovery efforts and planning
In the absence of complete knowledge, for local municipalities, water utilities and catchment managers, the first step towards designing an effective response is to follow some guidelines for risk assessment:
- Identify relevant assets (built and ecological) and clearly articulate the management objectives for these assets
- Determine their vulnerability to the potential hydrological change due to bushfire.
- Determine the exposure of assets to changes in hydrological processes. What is the likely magnitude of hydrological change?
- What is the potential consequence of this hydrological change for the asset?
- What is the likelihood?
- Combine consequence and likelihood into a risk matrix.
The guidelines are described in more detail here. In the absence of quantitative tools and data on post-fire hydrology, these guidelines for risk assessment can be applied qualitatively, using hydrological principles and expert input, without exact information on processes and magnitude of impact.
Such risk assessments should underpin all management intervention. It provides a basis for prioritisation of resources during recovery efforts and will highlight key knowledge gaps that should be addressed with further research and model development. And most importantly, a robust procedure for risk assessment will help identify the type of management interventions (if any) that may make a difference.
It would be remiss not to refer to our changing climate. The impacts of bushfires are going to be experienced more frequently as higher temperatures increase the likelihood of severe bushfire conditions. In addition, likely increases in the intensity of short-duration rainfall exacerbate water quality and erosion risk in the immediate post-fire period. In general, with the ongoing changes in rainfall and fire regimes, it is likely that forests in southeast Australia will be subject to higher frequency of extreme erosion events. Understanding of risks and how to effectively apply management responses (including fire and fuel management) will become even more important to protect the substantial ecosystem services provided by our forested catchments into the future.