Dead Woody Fuel Timelag Classes

Dead fuels are categorized into fuel diameter classes named according to the timelag principle (Pyne et al. 1996). This principal is based on the fact that the proportion of a fuel particle exposed to weather is related to its size. Small diameter fuels can change rapidly in response to weather changes, while larger diameter fuels are slower to respond. A timelag is the time required for a fuel particle to reach 63% of the difference between the initial moisture content and the equilibrium moisture content (or equilibrium with changed atmospheric conditions). The categories are named for the “midpoint” of the response time of each fuel category: 1-hour fuels respond in less than 2 hours, 10-hour fuels respond in 2 to 20 hours, 100-hour fuels respond in 20 to 200 hours, and 1,000 hour fuels respond in greater than 200 hours. Below are typical fuels and fire behavior for each of these 4 time lag classes.

The Wildland Fire Assessment System produces daily maps of dead fuel moisture across the U.S. based on time-lag classes: Map of estimated 10- hour fuels, Map of estimated 100-hour fuels, and Map of estimated 1000-hour fuels.

1-hour time lag fuels (< 0.625 cm (0.25 in.) diameter)

1-hour time lag fuels are the most important for carrying surface fires and their moisture content governs fire behavior. One-hour fuels include fallen needle and leaf litter, grassy fuels, lichens, and small twigs. Within this category, response times vary by fuel type. Lichen, grass, and well-cured needles respond to changes faster than freshly fallen needles and hardwood leaves. Due to their high surface area to volume, low moisture content, and location in the combustion zone, they produce little smoke and have low flame residence time. One-hour fuels are consumed by both flaming and smoldering combustion, regularly undergoing complete consumption in most surface fires.

10-hour timelag fuels (0.625 - 2.5 cm (0.25 to 1 in.) diameter)

Common 10-hour fuels include small branches and woody stems. Due to their resistance to drying and greater heat capacity, 10-hour fuels often do not combust in low-intensity surface fires. When moisture is low, however, 10-hour fuels can carry hot fires and help ignite larger (100- and 1000-hour) fuels. Ten-hour fuels are readily consumed when fuel moistures are low.

100-hour timelag fuels (2.5 cm - 7.6 cm (1 - 3 in.) diameter)

Larger downed woody debris is common 100-hour forest fuels. These fuels take longer to dry, deterring their consumption under most conditions. Likewise, 100-hour fuels are slow to gain moisture, so they can combust after prolonged drought, even with recent precipitation. When 100-hour fuels ignite they can burn for hours, in mixtures of flaming and smoldering combustion. Decay of 100-hour fuels can alter their response and makes them combust more readily than intact fuels.

1000-hour timelag fuels (> 7.6 cm (3 in.) diameter)

These fuels, which include large downed branches, logs, and tree stumps, burn only under prolonged dry conditions, or when sufficiently pre-heated by adjacent fuels. Since they do not commonly burn, 1000-hour fuels can act as firebreaks and cause fire shadows. When they do burn, 1000-hour fuels are common smoldering fuels and can burn for days after ignition, creating air quality and re-burn hazards.