How ash controls runoff after bushfires

Posted on July 1, 2013

Bushfires can lead to significant hydrological changes, including big increases in stormwater runoff and sediment transport.  They can also reduce water quality by increasing concentrations of heavy metals and nutrients.  For these reasons there’s interest in understanding how fires affect soil structure and runoff.    In the aftermath of a wildfire in the area of Fourmile Canyon, Colorado in September 2010, which was the costliest fire in the state’s history, a monitoring program was set up to compare soil conditions at burned and unburned sites.   Field sampling showed that as a result of the fire, organic litter was replaced by a layer of ash up to 80 mm thick.  In response to two subsequent rainstorms, the ash layer absorbed water quickly, but burned soil showed a delayed (1-2 day) increase in water content, presumably because the surface ash had a buffering effect and acted as a slow-release reservoir for the underlying, relatively impermeable soil.  Runoff generation was critically dependent on the ash layer, which stored 97-99% of rainfall.   Although the amount of rain that fell in the first storm was double that of the second storm, there were no differences between the storms in terms of discharge or the duration of run-off.  This seemed to be due to the fact that during the second storm, rainfall intensity exceeded the hydraulic conductivity of the ash for a very short (12 minute) period.  As a result, runoff was controlled by excess saturation at the ash-soil boundary during storm 1, but by infiltration capacity at the ash surface during storm 2.  Hydrological modellers therefore need to take into account the modifying effects of ash on runoff in the months following wild fire. 

Reference:   Ebel, B.A., Moody, J.A. & Martin, D.A.  2012.   Hydrologic conditions controlling runoff generation immediately after wildfire.  Water Resources Research 48, W03529, doi:10.1029/2011WR011470