The Effects of Vegetation, Structure Density, and Wind on Structure Loss Rates in Recent Northern California Wildfires

Analysis: Loss rates for single family homes threatened by wildfires in nine Northern California fires in the 2015-2021 time period are analyzed. 26,915 homes are included in the study. 21,504 of those homes were destroyed (79.8%). Vegetation cover is estimated for the area within 25 meters and within 500 meters of each structure point by reclassifying a Normalized Difference Vegetation Index (NDVI) image derived from high resolution infrared aerial imagery. Weather parameters for the maximum loss day on each fire are taken from the nearest Remote Automatic Weather Station (RAWS). Logistic regression is used to estimate the probability of structure loss, considering weather variables, vegetation cover, and structure patterns. Results: Structure density, vegetation cover within 25 meters and within 500 meters of a structure point, maximum wind levels and maximum temperature levels are all positively related to structure loss. Homes in high structure density areas (i.e., more than 400 structures per km2 within a 200-meter distance) have a predicted loss rate 20% higher than homes in low structure density areas, most likely due to increases in structure-to-structure spread. 33% of the losses in high structure density areas can be attributed to structure density compared to 13% of losses in the low structure density areas. Conclusions: Although reductions in vegetation cover do decrease structure loss rates, even very large changes in vegetation cover would not be sufficient to reduce loss rates to low levels. With a reduction of 50% in the average vegetation cover levels in both the 25-meter and 500-meter zones, predicted loss rates for these fires would still be over 50% for low structure density areas and nearly 80% for high structure density areas.