Simulating the combined effects of climate and wildfire on streamflow

In February of 2009, after prolonged periods of hot and dry weather, wildfires spread across part of the state of Victoria, affecting more than 363 km2 of forests in the Melbourne water supply catchments. This has potential to alter long term water use (>100 years) of the forests and subsequent water yield from these catchments. Climate can have an important role in modifying the effects of fire on streamflow. One of the few ways to examine these is through a process-based modelling study. It is important that the combined effects of climate and wildfire on long term streamflow are included in simulations used by water managers for planning and decision making. We used a spatially explicit process-based model to explore relationships between climate and wildfire, and examine the combined influence of climate and wildfire on the post-fire streamflow response. Catchments were disaggregated into spatial units at which energy and water balances are simulated. Changes in land cover were expressed as changes in leaf area index (LAI) for the fire sensitive ash-type eucalypt species, and for the more fire-tolerant mixed eucalypt species forests. Catchment water balances were simulated for 100 years after wildfire to include most of the expected changes. We assumed post-fire mortality of vegetation from recent field studies, along with several climate scenarios to examine the response of post-fire catchment streamflow. Effects of climate variability were removed by creating a synthetic climate, with no inter-annual variability, and using this as an input to model. Under wetter than average conditions, change in post-fire water yield was largely explained by changes in average age of the forest, where ET is largely determined by the conductance and interception of the forest canopy. Under drier conditions, evapotranspiration is largely under the control of soil water content, and so differences in vegetation cover and canopy conductance across catchments are likely to have less effect on evaporative fluxes and consequently on streamflow. Under lower than average rainfall conditions, when water becomes limiting, annual rainfall was the best predictor of post-fire change in water yield. This can result in little or no statistically significant change in post-fire streamflow, even when large areas of a catchment are affected by fire (Bart and Hope, 2010). Under conditions of low rainfall and low soil water content that are conducive to larger wildfires, any initial increase in post-fire streamflow due to reduced canopy cover may not occur or be detected because a substantial soil water deficit must first be removed before appreciable changes in streamflow will occur. We conclude, therefore, that the likelihood of detecting changes in streamflow after severe wildfire is lower when rainfall is low.