What are the typical categories of dead fuel fractions used in S-390 calculations?

• A. 1-Hour fuels, 10-Hour fuels, and 100-Hour fuels
• B. 1-Minute fuels, 10-Minute fuels, and 100-Minute fuels
• C. 1-Hour fuels, 50-Hour fuels, and 200-Hour fuels
• D. 0-Hour fuels, 1-Hour fuels, and 10-Hour fuels

Answer: (A)

Dead fuels are grouped by how quickly their moisture content responds to changes in weather, a concept tied to time lag. In S-390 calculations the standard categories are 1-hour fuels, 10-hour fuels, and 100-hour fuels. These names reflect the relative response times: 1-hour fuels are fine fuels like grasses, needles, and light debris that equilibrate with humidity almost as soon as conditions shift; 10-hour fuels are small woody fuels that take longer to respond; 100-hour fuels are larger woody materials that change moisture the slowest. Using these three fractions lets the model account for the full spectrum of dead fuel sizes and their moisture behavior, which in turn influences energy release, spread potential, and fire intensity. The other options don’t fit the established grouping, as they propose nonstandard or nonexistent time-lag categories.