EVALUATING THE RELATIONSHIP BETWEEN POST-WILDFIRE DEBRIS-FLOW OCCURRENCE AND VEGETATION RECOVERY USING SATELLITE-DERIVED LEAF AREA INDEX

Debris flows can be a serious threat in steep topography following wildfire. Vegetation and soil-hydraulic recovery are thought to reduce susceptibility to runoff-generated debris flows in a burn area. However, this susceptibility appears to persist for variable durations after fire. In some cases, debris flows have only occurred in the burned area during a few weeks to months after the fire, while in other cases, debris flows occurred over several years. The degree to which vegetation recovery influences the timespan of this susceptible period is poorly understood. To examine the relationship between vegetation recovery and debris-flow occurrence, we compute a recovery metric that compares satellite-derived leaf area index at the time of the debris flow to prefire values for the corresponding time of year. We perform this calculation for a multi-year inventory of more than 500 debris flows from 25 wildfires across the western United States, using leaf area index data sourced from the National Aeronautics and Space Administration (NASA) Moderate Resolution Imaging Spectroradiometer (MODIS) dataset. We find that more than 98% of flows occurred when leaf area index in the burned area was less than two-thirds of typical pre-fire values. Our results indicate that susceptibility to runoff-generated debris flows usually returns to background levels well before complete vegetation recovery, and that the rate of vegetation recovery appears to inversely correlate with the duration of time in which debris flows occurred. We conclude that satellite-derived leaf area index shows promise as a straightforward estimator for post-wildfire debris-flow susceptibility in vegetated areas over time.