Modeling topographic influences on fuel moisture and fire danger in complex terrain to improve wildland fire management decision support

Abstract

Fire danger rating systems commonly ignore fine scale, topographically-induced weather variations. These variations will likely create heterogeneous, landscape-scale fire danger conditions that have never been examined in detail. We modeled the evolution of fuel moistures and the Energy Release Component (ERC) from the US National Fire Danger Rating System across the 2009 fire season using very high resolution (30 m) surface air temperature, humidity and snow ablation date models developed from a network of inexpensive weather sensors. Snow ablation date occurred as much as 28 days later on North-facing slopes than on South-facing slopes at upper elevations. South-facing slopes were hotter and drier than North-facing slopes but slope position, in addition to aspect, was also important because nocturnal air temperatures were coolest and humidity was highest in valley bottoms. These factors created heterogeneous fuel moistures and fire danger across the study area. In the late season (August and September), nocturnal cold air drainage and high relative humidity fostered fuel moisture recovery in valley bottoms, where fuel moistures and ERC values were 30% and 45% higher and lower, respectively at peak fire danger (September 29th). Dry fuel moistures and relatively high ERC values persisted on low elevation, South-facing slopes. The driest conditions were observed 100–200 m above the valley floor where mid-slope thermal belts frequently developed above areas of cold air pooling. We suggest that a complete understanding of these variations may help improve fire management decision making.