Wildfire ignitions in California: spatial and temporal patterns and drivers

Abstract:

As a key component of wildfire, ignition is regulated by complex interactions among human activities, climate, fuel, and topography. Many studies have been made to advance our knowledge about the dynamics and drivers of total area burned, but much less is known about wildfire ignitions. In order to better design fire prevention strategies, it is critical to understand the patterns and predict the probability of wildfire ignitions. We here analyzed and modeled fire ignition frequency and probability across the whole state and sub-ecoregions of California, random forest models were developed to integrate the time-series of historical ignition data since 1992, anthropogenic and biophysical variables. The models captured the spatial patterns and seasonal and inter-annual dynamics of wildfire ignitions. Anthropogenic ignitions dominated the areas closer to human settlement, driven mostly by population and human modification and secondly by vegetation conditions; in contrast, ignitions in more remote and higher elevation areas were mainly caused by lightning, e.g., in Sierra Nevada and North Interior, where elevation had a bell-shaped relationship with ignition probability while precipitation enhanced ignition. Both the observation record and model simulation showed that the ignitions in California advanced significantly earlier, by 0.8 day/year, since 1990’s. This trend was dominated by human-caused ignitions. Our findings will help reconstructing historical ignitions across the state, improve fire risk assessment, and provide guidance for fire prevention and land management to limit future fire hazard.