Evaluating Fire Indices for Fire Ignition and Spread Rate in the Western United States

Abstract

Wildfires are complex events that are challenging to model. Researchers and land managers often rely on fire indices as a proxy of fire risk in operational forecasts and future projections. However, in the scientific literature, the choice of which fire index to use is often arbitrary and not tied to specific fire behaviors. Here, we provide a comprehensive evaluation of 21 hydrometeorological variables and fire indices based on their ability to represent daily ignition probability and spread rate in the western United States. Hydrometeorological variables like saturation vapor pressure (E_S) and vapor pressure deficit (VPD) perform the best and most consistently at representing fire ignition probability across regions and vegetation types. Water‐balance‐based indices like 1,000‐hr dead fuel moisture (FM1000) and the Keetch‐Byram Drought Index (KBDI) are best at representing ignition probability in forests. In contrast, indices like the Hot‐Dry‐Windy index (HDW) best represent fire spread rates, especially for non‐forest fires, while indices from the National Fire Danger Rating System are best for forest fire spread. Partitioning the skill of variables/indices into contributions from capturing seasonality and anomalies, ignitions are distributed based on the seasonal cycles, while seasonality and anomalies are comparably important for spread rate. We demonstrate that computational complexity does not always result in better fire variables/indices, and that distinct sets of variables/indices are needed to represent different aspects of fire behavior, emphasizing the importance of making careful and justifiable selections of fire variables/indices.