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Surface Fuel Model Descriptions


Carrier Fuel Types

Non-burnable (NB) Fuels: The nonburnable “fuel models” are included on the next five pages to provide consistency in how the nonburnable portions of the landscape are displayed on a fuel model map. In all NB fuel models, there is no fuel load—wildland fire will not spread.

  • NB1 (091) –URBAN/SUBURBAN; Fuel model NB1 consists of land covered by urban and suburban development. To be called NB1, the area under consideration must not support wildland fire spread. In some cases, areas mapped as NB1 may experience structural fire losses during a wildland fire incident; however, structure ignition in those cases is either house-to-house or by firebrands, neither of which is directly modeled using fire behavior fuel models. If sufficient inflammable vegetation surrounds structures such that wildland fire spread is possible, then choose a fuel model appropriate for the wildland vegetation rather than NB1. 
  • NB2 (092) – SNOW/ICE; Land covered by permanent snow or ice is included in NB2. Areas covered by seasonal snow can be mapped to two different fuel models: NB2 for use when snow-covered and another for use in the fire season.
  • NB3 (093) – AGRICULTURAL FIELD; Fuel model NB3 is agricultural land maintained in a non-burnable condition; examples include irrigated annual crops, mowed or tilled orchards, and so forth. However, there are many agricultural areas that are not kept in a non-burnable condition. For example, grass is often allowed to grow beneath vines or orchard trees, and wheat or similar crops are allowed to cure before harvest; in those cases, use a fuel model other than NB3.
  • NB8 (098) – OPEN WATER; Land covered by open bodies of water such as lakes, rivers and oceans.
  • NB9 (099) – BARE GROUND; Land devoid of enough fuel to support wildland fire spread is covered by fuel model NB9. Such areas may include gravel pits, arid deserts with little vegetation, sand dunes, rock outcroppings, beaches, and so forth.

Grass (GR) Fuels: The primary carrier of fire in the GR fuel models is grass. Grass fuels can vary from heavily grazed grass stubble or sparse natural grass to dense grass more than 6 feet tall. Fire behavior varies from moderate spread rate and low flame length in the sparse grass to extreme spread rate and flame length in the tall grass models. While the FB fuel models are static, all of the GR fuel models are dynamic, meaning that their live herbaceous fuel load shifts from live to dead as a function of live herbaceous moisture content. The effect of live herbaceous moisture content on spread rate and intensity is very strong.

Grass/Shrub (GS) Fuels: The primary carrier of fire in the GS fuel models is grass and shrubs combined; both components are important in determining fire behavior. All GS fuel models are dynamic, meaning that their live herbaceous fuel load shifts from live to dead as a function of live herbaceous moisture content. The effect of live herbaceous moisture content on spread rate and intensity is strong, and depends on the relative amount of grass and shrub load in the fuel model.

Shrub (SH) Fuels: The primary carrier of fire in the shrub fuel models is live and dead shrub twigs and foliage in combination with dead and down shrub litter. Fuel models SH1 and SH9 are dynamic, due to a small amount of herbaceous fuel loading in them. The effect of live herbaceous load transfer to dead fine fuel on spread rate and flame length can be significant in those two dynamic SH models.

Timber Understory (TU) Fuels: The primary carrier of fire in the TU fuel models is forest litter in combination with herbaceous or shrub fuels. TU1 and TU3 contain live herbaceous load and are dynamic, meaning that their live herbaceous fuel load is allocated between live and dead as a function of live herbaceous moisture content. The effect of live herbaceous moisture content on spread rate and intensity is strong, and depends on the relative amount of grass and shrub load in the fuel model.

Timber Litter (TL) Fuels: The primary carrier of fire in the TL fuel models is dead and down woody fuel. Live fuel, if present, has little effect on fire behavior.

Slash/Blow down (SB) Fuels: The primary carrier of fire in the SB fuel models is activity fuel or blow down. Forested areas with heavy mortality may be modeled with SB fuel models.

Grass & Grass Shrub Fuel Model Descriptions

(fuels in shaded rows: dynamic transfer of herb fuel load from live to dead)


Dry Climate Grass & Grass –Shrub Fuel Descriptions

FB1 (01): Fire spread is governed by the fine herbaceous fuels that are cured or nearly cured. Fires move rapidly through cured grass & associated material. Very little shrub or timber is present, generally less than one-third of the area. Grasslands & savanna are represented along with stubble, grass tundra, & grass-shrub combinations that meet the above area constraint. Annual & perennial grasses are included fuels.

FB2 (02): Fire spread is primarily through fine herbaceous fuels, either curing or dead. These are surface fires where the herbaceous material, besides litter and dead-down stem wood from the open shrub or timber overstory, contribute to the fire intensity. Open shrub lands and pine stands or scrub oak stands that cover one-third or two thirds of the area may generally fit this model, but may include clumps of fuels that generate higher intensities and may produce firebrands. Some pinyon-juniper may be in this model.

GR1 (101): This model uses dynamic transfer of herb fuel load from live to dead. The primary carrier of fire is sparse grass, though small amounts of fine dead fuel may be present. The grass in GR1 is generally short, either naturally or by heavy grazing, and may be sparse or discontinuous. Moisture of extinction of GR1 is indicative of dry climate fuelbeds, but may also be applied in high-extinction moisture fuelbeds, because in both cases predicted spread rate and flame length are low compared to other GR models

GR2 (102): Uses dynamic transfer of herb fuel load from live to dead. Primary carrier of fire is grass, though small amounts of fine dead fuel may be present. Load is greater than GR1. Fuelbed may be more continuous. Shrubs do not affect fire behavior.

GR4 (104): This model uses dynamic transfer of herb fuel load from live to dead. The primary carrier of fire is continuous, dry-climate grass. Load and depth are greater than GR2; fuelbed depth is about 2 feet.

GR7 (107): Uses dynamic transfer of herb fuel load from live to dead. Primary carrier is continuous dry-climate grass. Load & depth greater than GR4. Grass about 3 feet tall.

GS1 (121): This model uses dynamic transfer of herb fuel load from live to dead. The primary carrier of fire is 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.

GS2 (122): Primary carrier is grass & shrubs combined. Shrubs are 1-3 feet high, grass load is moderate. Spread rate is high; flame length moderate. Moisture of extinction low.

Humid Climate Grass & Grass Shrub Fuel Descriptions

FB3 (03): Fires in this fuel are the most intense of the grass group and display high rates of spread under the influence of wind. The fire may be driven into the upper heights of the grass stand by the wind and cross standing water. Stands are tall, averaging about 3 ft., but may vary considerably. Approximately one-third or more of the stand is considered dead or cured and maintains the fire. Wild or cultivated grains that have not been harvested can be considered similar to tall prairie and marshland grasses.

GR3 (103): This model uses dynamic transfer of herb fuel load from live to dead. The primary carrier of fire is 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.

GR5 (105): This model uses dynamic transfer of herb fuel load from live to dead. The primary carrier of fire is humid-climate grass. Load is greater than GR3 but depth is lower, about 1-2 feet.

GR6 (106): This model uses dynamic transfer of herb fuel load from live to dead. The primary carrier of fire is continuous humid-climate grass. Load is greater than GR5 but depth is about the same. Grass is less coarse than GR5.

GR8 (108): This model uses dynamic transfer of herb fuel load from live to dead. The primary carrier of fire is 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.

GR9 (109): This model uses dynamic transfer of herb fuel load from live to dead. The primary carrier of fire is 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.

GS3 (123): This model uses dynamic transfer of herb fuel load from live to dead. The primary carrier of fire is 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.

GS4 (124): The primary carrier of fire is grass and shrubs combined. Heavy grass/shrub load, depth greater than 2 feet. Spread rate high; flame length very high. Moisture of extinction is high.

Shrub and Timber Understory Fuel Model Descriptions

(fuels in shaded rows: dynamic transfer of herb fuel load from live to dead)


Dry Climate Shrub & Timber Understory Fuel Descriptions

FB4 (04): Fire intensity and fast-spreading fires involve the foliage and live and dead fine woody material in the shrub layer. Besides flammable foliage, there is dead woody material that significantly contributes to fire intensity. Deep litter layer may also confound suppression efforts.

FB5 (05): Primary carrier is litter cast by the shrubs, and the grasses or forbs in the understory. Shrubs are generally not tall, but have nearly total coverage of the area. Young, green stands with no deadwood.

FB6 (06): Fire carries through the shrub layer, requiring at least moderate winds. Fire will drop to the ground at low windspeeds or openings in the stand. The shrubs are older. A broad range of shrub conditions is included here.

SH1 (141):This model uses dynamic transfer of herb fuel load from live to dead. The primary carrier of fire in SH1 is 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.

SH2 (142): The primary carrier of fire in SH2 is woody shrubs and shrub litter. Moderate fuel load (higher than SH1), depth about 1 foot, and no grass fuel present. Spread rate is low; flame length low.

SH5 (145): The primary carrier of fire in GS4 is grass and shrubs combined. Heavy grass/shrub load, depth greater than 2 feet. Spread rate very high; flame length very high. Moisture of extinction is high.

SH7 (147): The primary carrier of fire is woody shrubs and shrub litter. Very heavy shrub load, depth 4-6 feet. Spread rate lower than SH5, but flame length similar. Spread rate is high; flame length very high.

FB10 (10): Dead down fuels include greater quantities of 3-inch or larger limbwood resulting from overmaturity or natural events that create a large load of dead material. Crown fire and spotting is more frequent in this fuel situation.

TU1 (161): This model uses dynamic transfer of herb fuel load from live to dead. The primary carrier of fire in is low load of grass and/or shrub with litter. Spread rate is low; flame length low.

TU4 (164): The primary carrier of fire is grass, lichen or moss understory plants. If live woody moisture content is set to 100 percent, this fuel model mimics the behavior of Norum’s (1982) empirical calibration for Alaska Black Spruce. Spread rate is moderate; flame length moderate.

TU5 (165): The primary carrier of fire in TU5 is heavy forest litter with a shrub or small tree understory. Spread rate is moderate; flame length moderate.

Humid Climate Shrub & Timber Understory Fuel Descriptions

FB7 (07): Fires burn through the surface and shrub strata with equal ease and can occur at higher dead fuel moisture contents because of the flammable nature of live foliage and other live material. Stands of shrubs are generally between 2 and 6 ft. high. Palmetto-gallberry understory within pine overstory sites are typical and low pocosins may be represented. Black spruce-shrub combinations in Alaska may also be represented.

SH3 (143): The primary carrier of fire in SH3 is woody shrubs and shrub litter. Moderate shrub load, possibly with pine overstory or herbaceous fuel, fuel bed depth 2-3 feet. Spread rate is low; flame length low.

SH4 (144): The primary carrier of fire in SH4 is woody shrubs and shrub litter. Low to moderate shrub and litter load, possibly with pine overstory, fuel bed depth about 3 feet. Spread rate is high; flame length moderate.

SH6 (146): The primary carrier of fire in SH6 is woody shrubs and shrub litter. Dense shrubs, little or no herbaceous fuel, fuelbed depth about 2 feet. Spread rate is high; flame length high.

SH8 (148): The primary carrier of fire in SH8 is woody shrubs and shrub litter. Dense shrubs, little or no herbaceous fuel, fuelbed depth about 3 feet. Spread rate is high; flame length high.

SH9 (149): This model uses dynamic transfer of herb fuel load from live to dead. The primary carrier of fire in SH9 is woody shrubs and shrub litter. Dense, finely branched shrubs with significant fine dead fuel, about 4-6 feet tall; some herbaceous fuel may be present. Spread rate is high, flame length very high.

TU2 (162): The primary carrier of fire in TU2 is moderate litter load with shrub component. High extinction moisture. Spread rate is moderate; flame length low.

TU3 (163):This model uses dynamic transfer of herb fuel load from live to dead. The primary carrier of fire in TU3 is moderate forest litter with grass and shrub components. Extinction moisture is high. Spread rate is high; flame length moderate.

Timber Litter and Slash/Blowdown Fuel Model Descriptions


Timber Litter Fuel Descriptions

FB8 (08): Slow-burning ground fires with low flame heights are the rule, although the fire may encounter an occasional "jackpot" or heavy fuel concentration that can flare up. Only under severe weather conditions involving high temperatures, low humidities, and high winds do the fuels pose fire hazards. This layer is mainly needles, leaves, and some twigs since little undergrowth is present in the stand.

FB9 (09): Fire runs through the surface litter faster than FB8 and have higher flame height. Both long-needle conifer & hardwood stands, especially the oak-hickory types, are typical. Fall fires in hardwoods are representative, but spotting by rolling and blowing leaves in high winds will cause higher rates of spread than predicted. Concentrations of dead-down woody material will contribute to torching & spotting.

TL1 (181): The primary carrier of fire is compact forest litter. Light to moderate load, fuels 1-2 inches deep. May be used to represent a recently burned forest. Spread rate is very low; flame length very low.

TL2 (182): The primary carrier of fire is broadleaf (hardwood) litter. Low load, compact litter. Spread rate is very low; flame length very low.

TL3 (183): The primary carrier of fire is moderate load conifer litter, light load of coarse fuels. Spread rate is very low; flame length low.

TL4 (184): The primary carrier of fire is moderate load of fine litter and coarse fuels. Includes small diameter downed logs. Spread rate is low; flame length low.

TL5 (185): The primary carrier of fire is High load conifer litter; light slash or mortality fuel. Spread rate is low; flame length low.

TL6 (186): The primary carrier of fire is moderate load broadleaf litter, less compact than TL2. Spread rate is moderate; flame length low.

TL7 (187): The primary carrier of fire is heavy load forest litter, includes larger diameter downed logs. Spread rate low; flame length low.

TL8 (188): The primary carrier of fire in is moderate load long-needle pine litter, may include small amount of herbaceous load. Spread rate is moderate; flame length low.

TL9 (189): The primary carrier of fire is very high load, fluffy broadleaf litter. Can also be used to represent heavy needle-drape. Spread rate is moderate; flame length moderate.

Slash Blowdown Fuel Descriptions

FB11 (11): Fires are fairly active in the slash and intermixed herbaceous material. The spacing of the rather light fuel load, shading from overstory, or the aging of the fine fuels can contribute to limiting the fire potential. The less-than-3-inch material load is less than 12 tons per acre. The greater-than-3-inch material is represented by not more than 10 pieces, 4 inches in diameter, along a 50-ft transect.

FB12 (12): Rapidly spreading fires with high intensities capable of generating firebrands can occur. When fire starts, it is generally sustained until a fuel break or change in fuels is encountered. The visual impression is dominated by slash, most of it less than 3 inches in diameter. Fuels total less than 35 tons per acre & seem well distributed.

FB13 (13): Fire is generally carried across the area by a continuous layer of slash. Large quantities of greater-than-3-inch material are present. Active flaming is sustained for long periods and firebrands of various sizes may be generated. These contribute to spotting problems. Situations where the slash still has "red" needles attached but the total load is lighter, more like model 12, can be represented because of the earlier high intensity and quicker area involvement.

SB1 (201): Primary carrier of fire is light dead & down activity fuel. Fine fuel load is 10 to 20 t/ac, weighted toward fuels 1-3 in diameter class, depth is less than 1 foot. Spread rate is moderate; flame length low.

SB2 (202): The primary carrier of fire is moderate dead and down activity fuel or light blowdown. Fine fuel load is 7 to 12 t/ac, evenly distributed across 0-0.25, 0.25-1, and 1-3 inch diameter classes, depth is about 1 foot. Blowdown is scattered, with many trees still standing. Spread rate is moderate; flame length moderate.

SB3 (203): The primary carrier of fire is heavy dead and down activity fuel or moderate blowdown. Fine fuel load is 7 to 12 t/ac, weighted toward 0-0.25 inch diameter class, depth is more than 1 foot. Blowdown is moderate; trees compacted to near the ground. Spread rate is high; flame length high.

SB4 (204): The primary carrier of fire is heavy blowdown fuel. Blowdown is total, fuelbed not compacted, most foliage and fine fuel still attached to blowdown. Spread rate very high; flame length very high.



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