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Ebel, Brian A. ¹

¹ Cooperative Institute for Research in Environmental Sciences (CIRES)

Wildfire is one of the most prevalent disturbance events in the disturbance regime of mountainous terrain and can substantially impact hydrologic processes. Recent evidence suggests wildfire incidence, susceptibility, and synchrony are increasing in some regions. The interactions between wildfire disturbance and pre-existing landscape-scale controls on hydrology such as hillslope aspect are not well quantified, but are important for understanding the long term impacts of wildfire on ecological and geomorphic processes. We monitored subsurface hydrologic response to rainfall at the plot-scale for north- and south-facing hillslope aspects in burned and unburned conditions within the area impacted by the 2010 Fourmile Canyon Fire near Boulder, Colorado, USA. Our observations documented that the combustion of the litter/duff and forest canopy had the largest hydrologic impact on north-facing hillslopes, resulting in the loss of the “hydrologic buffering” capacity present in the unburned state. In contrast, unburned south-facing hillslopes did not have a robust pre-fire vegetation canopy or litter/duff layer and post-fire changes in hydrologic response were primarily the result of decreases in soil-water retention resulting from soil organic matter reduction. Overall, subsurface hydrologic response had greater variability and more rapid dynamics in wildfire-impacted soils. Furthermore, wildfire homogenized pre-fire hillslope aspect-driven differences in hydrologic response thus “clearing the slate” of some pre-fire landscape-scale controls on subsurface hydrologic conditions. The timescale of altered hydrologic and accompanying ecologic and geomorphologic processes likely depends on re-establishment of vegetation communities and soil recovery. Quantifying this timescale is an important direction for future research.