POST-FIRE HILLSLOPE RUNOFF AND EROSION RESPONSES: NUGGETS AND GAPS FROM EMPIRICAL STUDIES

Severe wildfires consume fine organic matter in the canopy, litter layer, and in the near-surface soil horizons. The loss of this organic matter changes soil structure, leaves soil exposed to rainsplash and overland flow, and can create or enhance soil water repellency. The combination of these direct and indirect effects on forest soils with high-intensity rainfall in mountainous areas can lead to orders-of-magnitude increases in surface runoff and soil erosion as compared to pre-fire conditions. Burn severity, time since burning, and spatial scale can also affect the magnitude of post-fire responses, leading to a complex array of possible outcomes depending on site conditions and rainfall inputs. Empirical experiments and case studies have helped us make great progress in understanding mechanisms for these magnified post-fire responses. For example, differences in surface cover, soil water repellency, rainfall intensity, and erodibility explain some of the variability in post-fire responses. Yet we still have much to learn about the specific mechanisms that lead to increases in overland flow and soil detachment. Results from studies in burned forests across the western US will help identify additional research questions regarding primary controls on post-fire overland flow generation and erosion rates.