No Fire Days of Rest: Relating California Wildfire Ignitions to Days of the Week

Abstract:

In California, the majority of wildfires are started by humans, rather than by natural ignitions (i.e. lightning). Given this importance of human activity, it is logical that patterns of ignition should be related to social, as well as ecological factors. In this paper, we explore the relationship of ignition to human behavior at a fine temporal scale: days of the week. Because more people typically enter both wildlands and the wildland-urban interface on weekends, we hypothesized that more wildfires would start on Saturdays and Sundays than on other days of the week.

We used data from California’s Fire and Resource Assessment Program (FRAP) database for human-caused fires occurring from 1950 through 2009, and omitted those that would not reasonably relate to day of the week (e.g. “Power Line”). These criteria yielded a dataset of 8644 fires over the 60-year period.

Overall, 15.5% of the fires began on Saturdays, and 16.4% began on Sundays. This compares to the 14.3% that would be expected if ignitions were evenly distributed through the week. However, different patterns of weekend ignition emerge depending on time period, specific fire cause, and the type of administrative unit in which the fires occurred.

The proportion of fires beginning on the weekend declined from a high in the 1950s (17.3% Saturdays, 18.5% Sundays) to a low in the 1990s (13.5% Saturdays, 14.4% Sundays), before rebounding in the 2000s to close to the long-term average (15.3% Saturdays, 16.8% Sundays). “Campfire” was the ignition source most concentrated on weekends, with 20.6% of the fires attributed to this cause starting on Saturdays, and 20.2% on Sundays. By contrast, “Equipment” was under-represented on the weekend (13.5% Saturdays, 13.9% Sundays). Fires in California State Parks clearly show a weekend bias, with 19.4% initiated on Saturdays, and 16.7% on Sundays. On National Forests, 15.5% of fires began on Saturdays, and 18.4% on Sundays. Many fires were on private land, much of which is inaccessible to weekend recreationists; here weekend ignitions were still overrepresented, but by smaller margins (15.0% Saturdays, 15.8% Sundays).

Of note, the proportion of weekend ignitions was much higher among fires that eventually became quite large. 142 human-caused fires exceeded 10,000 ha in size; of these 26.1% began on Saturdays, and 19.7 began on Sundays.