POSTWILDFIRE SOIL-HYDRAULIC RECOVERY AND THE PERSISTENCE OF DEBRIS FLOW HAZARDS

Deadly and destructive debris flows often follow wildfire but quantitative constraints on changes in hazard potential with time are lacking. We develop a simulation-based framework to quantify changes in the hydrologic triggering conditions for debris flows as postwildfire infiltration properties evolve through time. Our approach produces time-varying rainfall intensity-duration thresholds for runoff- and infiltration-generated debris flows with physics-based hydrologic simulations that are parameterized with widely available hydroclimatic, vegetation reflectance, and soil texture data. When we apply our simulation framework to a test case in the San Gabriel Mountains (California, USA), the resultant thresholds are consistent with existing regional empirical thresholds and rainfall conditions that caused runoff- and infiltration-generated debris flows soon after and three years following a wildfire, respectively. We demonstrate that the hydrologic triggering mechanisms for the two observed debris flow types are coupled with the effects of fire on the soil saturated hydraulic conductivity. Specifically, the rainfall intensity needed to generate debris flows via runoff increases with time following wildfire while the rainfall duration needed to produce debris flows via subsurface pore-water pressures decreases. We also find that variations in soil moisture, rainfall climatology, median grain size, and root reinforcement could impact the median annual probability of postwildfire debris flows. We conclude that a simulation-based method for calculating rainfall thresholds is a tractable approach to improve situational awareness of debris flow hazard in the years following wildfire. Further development of our framework will be important to quantify postwildfire hazard levels in variable climates, vegetation types, and fire regimes.