Surface Fuels are ground level combustible materials. Modeling surface fuels is critical in wildfire simulations because they directly affect wildfire behavior, including spread rate and intensity.
Technosylva provides surface fuel maps using the latest burn scars and disturbances, dynamically estimating changes in fuel loading for rapidly changing landscapes.
ML indicates the fuel model category was created or refined using Technosylva’s machine-learning-based fuels classification, rather than being only a standard Scott & Burgan fuel model.
Surface Fuels Model Appendix
Grasses
This class includes nearly pure grass and/or forbs.
Grasses are fine, herbaceous plants with narrow leaves and flexible stems. They usually dry out quickly and can ignite easily, especially during hot, dry, or windy conditions.
In fire behavior, grasses often act as fast-burning surface fuels. They can carry fire quickly across open areas, but because they are usually light fuels, they may burn out faster than heavier shrubs or timber fuels.
Examples include annual grasses, perennial grasses, pasture grasses, and grassland vegetation.
Forbs are non-woody, herbaceous plants that are not grasses. They are typically broadleaf plants, including many wildflowers and low-growing flowering plants.
In fuels modeling, forbs are often considered part of the herbaceous fuel layer alongside grasses. Their influence on fire behavior depends on their moisture content, density, and seasonal condition. Green forbs may slow fire spread, while cured or dried forbs can contribute to fine surface fuel.
Examples include clover, lupine, dandelion-type plants, and many wildflowers.
|
Symbol |
Fuel Model |
Description |
|---|---|---|
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GR1(101) |
Sparse grass, though small amounts of fine dead fuel may be present. The grass in GR1 is generally short, either naturally or by grazing, and may be sparse or discontinuous. |
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GR2(102) |
Grass, though small amounts of fine dead fuel may be present. Load is greater than GR1, and the fuelbed may be more continuous. Shrubs, if present, do not affect fire behavior. |
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GR3(103) |
Continuous, coarse, humid-climate grass. Grass and herb fuel load is relatively light; fuelbed depth is about 2 feet. Shrubs are not present in significant quantity to affect fire behavior. |
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GR4(104) |
Continuous, dry-climate grass. Load and depth are greater than GR2; fuelbed depth is about 2 feet. |
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GR5(105) |
Humid-climate grass. Load is greater than GR3 but depth is lower, about 1 to 2 feet. |
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GR6(106) |
Continuous humid-climate grass. Load is greater than GR5 but depth is about the same. Grass is less coarse than GR5. |
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GR7(107) |
Continuous dry-climate grass. Load and depth are greater than GR4. Grass is about 3 feet tall. |
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GR8(108) |
Continuous, very coarse, humid-climate grass. Load and depth are greater than GR6. Spread rate and flame length can be extreme if grass is fully cured. |
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GR9(109) |
Dense, tall, humid-climate grass. Load and depth are greater than GR8, about 6 feet tall. Spread rate and flame length can be extreme if grass is fully or mostly cured. |
Grass-Shrub Mixtures
These classes are a mixture of grass and shrub with up to 50 percent shrub coverage.
Shrubs are woody plants that are usually shorter than trees and often have multiple stems near the ground. They can include chaparral, sagebrush, manzanita, scrub oak, and other brush-type vegetation.
Shrubs can be important wildfire fuels because they often contain both fine twigs and woody material. Depending on moisture, density, and structure, shrubs can produce higher flame lengths and more intense fire behavior than grasses or forbs. Dense shrublands can also help fire transition from surface fuels toward taller vegetation.
|
Symbol |
Fuel Model |
Description |
|
|
GS1(121) |
Grass and shrubs combined. Shrubs are about 1 foot high, grass load is low. Spread rate is moderate; flame length low. Moisture of extinction is low. |
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GS2(122) |
Grass and shrubs combined. Shrubs are about 1 to 3 feet high, grass load is moderate. Spread rate is high; flame length moderate. |
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GS3(123) |
Grass and shrubs combined. Moderate grass/shrub load, average grass/shrub depth less than 2 feet. Spread rate is high; flame length moderate. Moisture of extinction is high. |
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GS4(124) |
Grass and shrubs combined. Heavy grass/shrub load, depth greater than 2 feet. Spread rate is high; flame length very high. |
Shrubs
In these classes, shrubs cover at least 50 percent of the site, grass is sparse to nonexistent.
|
Symbol |
Fuel Model |
Description |
|
|
SH1(141) |
Woody shrubs and shrub litter. Low shrub fuel load, fuelbed depth about 1 foot; some grass may be present. Spread rate is very low; flame length very low. |
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SH2(142) |
Woody shrubs and shrub litter. Moderate fuel load (higher than SH1), depth about 1 foot, no grass fuel present. Spread rate is low; flame length low. |
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SH3(143) |
Woody shrubs and shrub litter. Moderate shrub load, possibly with pine overstory or herbaceous fuel, fuelbed depth 2 to 3 feet. Spread rate is low; flame length low. |
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SH4(144) |
Woody shrubs and shrub litter. Low to moderate shrub and litter load, possibly with pine overstory, fuelbed depth about 3 feet. Spread rate is high; flame length moderate. |
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SH5(145) |
Woody shrubs and shrub litter. Heavy shrub load, depth 4 to 6 feet. Spread rate is very high; flame length very high. Moisture of extinction is high. |
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SH6(146) |
Woody shrubs and shrub litter. Dense shrubs, little or no herbaceous fuel, fuelbed depth about 2 feet. Spread rate is high; flame length high. |
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SH7(147) |
Woody shrubs and shrub litter. Very heavy shrub load, depth 4 to 6 feet. Spread rate lower than SH5, but flame length similar. Spread rate is high; flame length very high. |
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SH8(148) |
Woody shrubs and shrub litter. Dense shrubs, little or no herbaceous fuel, fuelbed depth about 3 feet. Spread rate is high; flame length high. |
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SH9(149) |
Woody shrubs and shrub litter. Dense, finely branched shrubs with significant fine dead fuel, about 4 to 6 feet tall; some herbaceous fuel may be present. Spread rate is high; flame length very high. |
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SH42(1442) |
High load, transitional-climate shrub. |
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SH4D(154) |
Shrub Dynamic ML 4 (TSYL 2025). |
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SH5D(155) |
Shrub Dynamic ML 5 (TSYL 2025). |
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SH7D(157) |
Shrub Dynamic ML 7 (TSYL 2025). |
Timber Understory
Timber understory refers to vegetation growing beneath a tree canopy. This can include grasses, forbs, shrubs, small trees, and accumulated woody material.
In fuels modeling, timber understory matters because it can carry fire through forested areas beneath the canopy. If the understory is dense or dry enough, it can increase surface fire intensity and contribute to torching or crown fire potential.
|
Symbol |
Fuel Model |
Description |
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TU1(161) |
Low load of grass and/or shrub with litter. Spread rate is low; flame length low. |
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TU2(162) |
Moderate litter load with shrub component. High extinction moisture. Spread rate is moderate; flame length low. |
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TU3(163) |
Moderate forest litter with grass and shrub components. Extinction moisture is high. Spread rate is high; flame length moderate. |
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TU4(164) |
Short conifer trees with grass or moss understory. Spread rate is moderate; flame length moderate. |
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TU5(165) |
Heavy forest litter with a shrub or small-tree understory. Spread rate is moderate; flame length moderate. |
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TUML1(171) |
Timber Understory Dynamic ML (TSYL 2022). |
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TUML2(172) |
Low-to-moderate fuel load timber understory, Dynamic ML (TSYL 2022). |
Timber Litter
Timber litter is the layer of dead plant material on the forest floor, such as fallen leaves, needles, small twigs, bark, and other organic debris.
This fuel type often supports surface fire in forested environments. It may not spread as rapidly as grass under the same conditions, but it can sustain combustion and contribute to fire intensity, especially when litter is deep, dry, or mixed with downed woody material.
|
Symbol |
Fuel Model |
Description |
|
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TL1(181) |
Compact forest litter. Light to moderate load, fuels 1 to 2 inches deep. May be used to represent a recently burned forest. Spread rate is very low; flame length very low. |
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TL2(182) |
Broadleaf (hardwood) litter. Low load, compact broadleaf litter. Spread rate is very low; flame length very low. |
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TL3(183) |
Moderate load conifer litter, light load of coarse fuels. Spread rate is very low; flame length low. |
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TL4(184) |
Moderate load of fine litter and coarse fuels. Includes small-diameter downed logs. Spread rate is low; flame length low. |
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TL5(185) |
High load conifer litter; light slash or mortality fuel. Spread rate is low; flame length low. |
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TL6(186) |
Moderate load broadleaf litter, less compact than TL2. Spread rate is moderate; flame length low. |
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TL7(187) |
Heavy load forest litter, includes larger-diameter downed logs. Spread rate is low; flame length low. |
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TL8(188) |
Moderate load long-needle pine litter, may include a small amount of herbaceous load. Spread rate is moderate; flame length low. |
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TL9(189) |
Very high load, fluffy broadleaf litter. TL9 can also represent heavy needle-drape. Spread rate is moderate; flame length moderate. |
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TLML(191) |
Timber Litter ML (TSYL 2022). |
Slash / Blowdown
Slash is leftover woody debris from logging, thinning, pruning, or vegetation management. Blowdown refers to trees or branches knocked down by wind or storms.
These fuels can create heavier surface fuel loads. When dry, they may increase fire intensity, flame length, and residence time because larger woody fuels burn longer than fine grasses or forbs.
|
Symbol |
Fuel Model |
Description |
|
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SB1(201) |
Light dead and down activity fuel. Fine fuel load is 10 to 20 t/ac, weighted toward the 1-to-3-inch diameter class, depth less than 1 foot. Spread rate is moderate; flame length low. |
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SB2(202) |
Moderate dead and down activity fuel or light blowdown. Fine fuel load is 7 to 12 t/ac, evenly distributed across the 0-0.25, 0.25-1, and 1-3 inch diameter classes, depth about 1 foot. Blowdown is scattered, with many trees still standing. Spread rate is moderate; flame length moderate. |
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SB3(203) |
Heavy dead and down activity fuel or moderate blowdown. Fine fuel load is 7 to 12 t/ac, weighted toward the 0-0.25-inch diameter class, depth more than 1 foot. Blowdown is moderate; trees compacted to near the ground. Spread rate is high; flame length high. |
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SB4(204) |
Heavy blowdown fuel. Blowdown is total, fuelbed not compacted, most foliage and fine fuel still attached. Spread rate is very high; flame length very high. |
Wildland-Urban Interface (WUI) Fuels
Fire encroachment into the Wildland-Urban Interface (WUI) is a critical component of Technosylva’s modeling, as it determines impacts to buildings, infrastructure, critical facilities, and exposed populations. To improve encroachment predictions, Technosylva developed custom WUI fuel models that extend beyond the standard Scott and Burgan (2005) wildland fuel classification system.
These enhanced fuel models incorporate building density parameters, characteristics of adjacent burnable vegetation, and WUI typology classification (isolated buildings, intermix, interface, urban core), along with roads and infrastructure elements. This comprehensive WUI characterization enables significantly more accurate calculation of potential fire impacts within developed areas.
In 2026, Technosylva added 12 new WUI fuel types to more accurately represent the range of conditions found in WUI environments. These enhanced fuel types help the model better understand how fire may spread through or around developed areas. Urban fuels are classified into two primary categories:
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Intermix and road zones: Areas where structures are surrounded by or mixed with vegetation, including roads and lower-density development patterns. These areas include the following fuel types: UIL, USL, UCL, UIH, USH, UCH, RNL, RNH, RML and RMH.
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Urban core zones: More densely developed areas within larger population centers, where structures and built infrastructure predominate. These areas include the following fuel types: UNB and RNB.
By distinguishing between these urban environments, the model can more accurately account for differences in fire spread potential, WUI exposure, and impacts to buildings and populated areas.
Road Classes
These classes are based on road category and surrounding fuels.
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Symbol |
Fuel Model |
Description |
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RNL(941) |
Minor roads surrounded by low-fire behavior fuel. |
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RNH(942) |
Minor roads surrounded by high-fire behavior fuel. |
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RML(943) |
Major roads surrounded by low-fire behavior fuel. |
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RMH(944) |
Major roads surrounded by high-fire behavior fuel. |
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RNB(949) |
Roads surrounded by non-burnable fuels. |
Urban Classes
Based on WUI density and the fire behavior of the surrounding fuel load type (originally NB91 Urban).
|
Symbol |
Fuel Model |
Description |
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UIL(911) |
Isolated urban surrounded by low-FB fuel. |
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USL(912) |
Scattered urban surrounded by low-FB fuel. |
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UCL(913) |
Urban core surrounded by low-FB fuel. |
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UIH(914) |
Isolated urban surrounded by high-FB fuel. |
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USH(915) |
Scattered urban surrounded by high-FB fuel. |
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UCH(916) |
Urban core surrounded by high-FB fuel. |
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UNB(919) |
Non-burnable urban areas. |
Agricultural Classes
Technosylva custom agricultural fuel classes are based on seasonal changes and fuel load (originally NB93, Agricultural Non-Burnable).
|
Symbol |
Fuel Model |
Description |
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ASL(931) |
Agricultural low-load fuels, with seasonal changes to their burnable condition. |
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ASH(932) |
Agricultural high-load fuels, with seasonal changes to their burnable condition. |
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AGC(938) |
Golf courses — non-burnable (no encroachment). |
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ANB(939) |
Agricultural fields maintained in a non-burnable condition. |
Water
Technosylva custom water fuels classes are based on water body type and surrounding fuels (originally NB8(98)).
|
Symbol |
Fuel Model |
Description |
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WNL(981) |
Minor water streams surrounded by low-load fuel (moderate encroachment). |
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WNH(982) |
Minor water streams surrounded by high-load fuel (high encroachment). |
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WML(983) |
Major water streams surrounded by low-load fuel (moderate encroachment). |
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WMH(984) |
Major water streams surrounded by high-load fuel (high encroachment). |
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WBD(989) |
Water bodies. |
Non-Burnable Fuel Type Models
These classes contain insufficient wildland fuel to carry a wildland fire under any condition. Excluding NB9(99) (bare ground), Technosylva uses custom fuels.
|
Symbol |
Fuel Model |
Description |
|
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NB1(91) |
Urban. |
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NB2(92) |
Snow / ice. |
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NB3(93) |
Agricultural. |
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NB8(98) |
Open water. |
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NB9(99) |
Bare ground. |
Customer-Specific Fuels
British Columbia (BC) Only
The following fuels apply to British Columbia (BC) customers only. BC fuels are custom fuels by Technosylva.
|
Symbol |
Fuel Model |
Description |
|
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BCML2(372) |
Moderate fuel load timber understory, Dynamic ML (TSYL 2023). |
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BCML4(374) |
High fuel load timber understory, Dynamic ML (TSYL 2023). |
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BCML6(376) |
Very high fuel load timber understory, Dynamic ML (TSYL 2023). |
Texas (TX) Only
The following fuels apply to Texas (TX) customers only. Texas fuel models were created by Joe Scott for the Texas A&M Forest Service and are not Technosylva custom fuels.
|
Symbol |
Fuel Model |
Description |
|---|---|---|
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9PPL(83) |
Texas — Timber 9PPL. |
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HWD(84) |
Texas — Timber HWD. |