Deadly and destructive debris flows often follow wildfire, but understanding of changes in the hazard potential with time since fire is poor. 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 thresholding protocol to a test case in the San Gabriel Mountains (California, USA), the results are consistent with existing regional empirical thresholds and rainstorms that caused runoff- and infiltration-generated debris flows soon after and three years following a wildfire, respectively. We find 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.

中文翻译：

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野火后土壤水力恢复和残骸流危害的持续存在

致命和破坏性的泥石流通常在野火之后发生，但对火灾造成的潜在危害随时间的变化的了解很差。我们开发了一个基于模拟的框架，以量化泥石流的水文触发条件随着野火后渗透特性随时间推移而发生的变化。我们的方法通过基于物理的水文模拟为径流和渗透产生的泥石流生成随时间变化的降雨强度-持续时间阈值，这些模拟使用广泛可用的水文气候、植被反射率和土壤质地数据进行参数化。当我们将阈值协议应用于圣盖博山脉（美国加利福尼亚州）的测试用例时，结果与现有的区域经验阈值和暴雨一致，分别在野火后不久和三年后导致径流和渗透产生的泥石流。我们发现两种观察到的泥石流类型的水文触发机制与火灾对土壤饱和导水率的影响相结合。具体而言，通过径流产生泥石流所需的降雨强度在野火后随时间增加，而通过地下孔隙水压力产生泥石流所需的降雨持续时间减少。我们还发现，土壤水分、降雨气候、中值粒度和根系加固的变化可能会影响野火后泥石流的年中值概率。我们得出的结论是，用于计算降雨阈值的基于模拟的方法是一种易于处理的方法，可在野火后的几年内提高泥石流灾害的态势感知能力。我们框架的进一步发展对于量化可变气候、植被类型和火灾状况下的野火后危害水平非常重要。