The Role of Wildfire in Soil Hydrology
Stefan H. Doerr
University of Wales Swansea, Department of Geography, School of Environment and Society, UK,
s.doerr@swan.ac.uk, Tel. +44 1792 295147, Fax. +44 1792 295955
Keywords: soil hydrology, wildfire, catchment, land management, climate change
During the last few decades, there has been an increasing awareness that wild- and prescribed
fires can profoundly affect soil hydrological responses. Both the direct effects of fire and also the
overall changes to the ecosystem encountered in a post-fire situation can lead to short-, mediumand long-term changes to the soil system. Locally, the presence of wettable ash and the
destruction of surface water repellency in areas of excessive soil heating may increase infiltration
rate and water storage capacity locally. Typically, however, fire-induced changes to soil
properties such as aggregate stability, pore size distribution and water repellency, together with
other effects including vegetation and litter removal, and clogging of macropores, have been
reported to cause increased runoff responses. Such changes are often of concern to land
managers as for example, post-fire floods and sediment redistribution can cause considerable
damage within the burnt area and downstream, affecting the ‘natural’ environment as well as
infrastructure and property. Furthermore, water supply catchments are often located in forested
areas and changes to catchment hydrological behaviour following fire need to be considered in
water supply management. Many studies have investigated these increased post-fire runoff from
soils at small scales, however, hydrological response to fire at the catchment scale have received
less attention largely because of the greater difficulties of installing and maintaining instruments,
the long recovery period or ‘relaxation time’ at this scale, the greater spatial heterogeneity of
environmental factors such as geology, topography, vegetation and soils, as well as fire extent,
severity and history over large catchments. As a consequence, our ability to predict hydrological
responses following forest fires has remained limited.
Much of our understanding is based on studies carried out in the USA, South Africa,
Australia and, increasingly so in recent years, also Europe. These studies have demonstrated
that the post-fire hydrological responses can be increased by up to two orders of magnitude, with
increases in total flow being often considerably less than those in peak discharge. These
increases tend to be most marked in humid ecosystems with high pre-fire evapotranspiration,
whereas in drier regions, for example, water use of recovering vegetation can lead to reduced
runoff compared to pre-fire conditions. Amongst the key parameters known to determine post-fire
soil hydrological response at various scales are vegetation characteristics, fire severity and the
timing and magnitude of rainfall events. As more studies from other environments become
available, our understanding of how these variables interact and what we can expect as typical
post-fire soil hydrological responses, as well as subsequent response recovery behaviour and
timescales, is likely to improve further. These advances, however, are likely to be challenged by
the increased occurrence of unprecedented pre- and post-fire environmental conditions
associated with changes in land management and global climate. This contribution aims to
provide a brief overview of the current knowledge of fire effects on soil hydrology and examines
the prospects of addressing key research gaps.