Influence of climate, fire severity and forest mortality on predictions of long term streamflow: Potential effect of the 2009 wildfire on Melbourne's water supply catchments

In February 2009, wildfire affected nine catchments, or approximately 28% of forested catchment area that supplies water to the city of Melbourne, Australia. This has potential to significantly affect the long term water use of these Eucalyptus forests and the consequential water yield because of the ecohydrologic response of some eucalypt species. Approximately 11% of the catchment area was severely burnt by intense fire, where vegetation mortality is higher. Catchment scale models using a physically-based approach were developed for the fire-affected water supply catchments. Different inputs of climate and forest mortality after fire were used to examine the relative contributions of rainfall, fire severity, forest type and forest age on post-fire streamflow. Simulations show the effect of fire on long term streamflow is likely to depend on a number of factors, the relative influence of which changes as rainfall becomes more limiting. Under average rainfall conditions, total reduction in post-fire streamflow after 100 years estimated to be between 1.4% (similar to 12 GL year(-1)) and 2.8% (similar to 24 GL year(-1)) are an order of magnitude lower than reductions in total catchment inflow during the period of low rainfall between 1997 and 2009, in which reservoir inflow was reduced by nearly 37%. The main reasons for the lower than expected changes in water yield are that a lower proportion of the catchments were affected by severe fire, and so mortality within the fire area was relatively low, and that the average age of the forest canopy (93 years) is younger than what is generally considered old growth forest. This means that the baseline (no-fire) streamflow used for reference is lower than would be expected with older, mature forest. The greatest post-fire affect on total water yield was predicted for the O'Shannassy catchment. This is due to the average forest age, which is the oldest of any of the catchments, that it has the highest average rainfall (1680 mm year(-1)), and that it contains the largest proportion of ash-type forest severely burnt (38.7%). Under wetter than average conditions, change in post-fire water yield is largely explained by changes in average age of the forest. The rates of ET are largely determined by the conductance and interception of the forest canopy. Under lower than average rainfall conditions, when water becomes limiting, annual rainfall is the best predictor of post-fire change in water yield. 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. This partly explains the lack of increase in initial post-fire streamflow reported after wildfire compared to an increase in streamflow following forest harvesting experiments during wetter periods. (C) 2013 Elsevier B.V. All rights reserved.