United States Department of Agriculture

Forest Service

DEPAR TMENT OF AGRICULT URE

Fuel Characteristic 

Classification System 

Version 3.0: Technical 

Documentation

Susan J. Prichard, David V. Sandberg, Roger D. Ottmar, 

Ellen Eberhardt, Anne Andreu, Paige Eagle, and Kjell Swedin

Pacific Northwest 

Research Station

General Technical Report

PNW-GTR-887

December 2013
Authors

Susan J. Prichard, Anne Andreu, and Paige Eagle are research scientists, and 

Kjell Swedin is a software engineer, School of Environmental and Forest Sciences, 

University of Washington, Box 352100, Seattle, WA 98195; David V. Sandberg is a 

research physical scientist (retired), U.S. Department of Agriculture, Forest Service, 

Pacific Northwest Research Station, Forestry Sciences Laboratory, 3200 SW Jefferson Way, Corvallis, OR 97331; Roger D. Ottmar is a research forester and Ellen 

Eberhardt is a technical information specialist, U.S. Department of Agriculture, 

Forest Service, Pacific Northwest Research Station, Pacific Wildland Fire Sciences 

Laboratory, 400 N 34th Street, Suite 201, Seattle, WA 98103.

Disclaimer

The Fuel Characteristic Classification System (FCCS) is in the public domain 

and the recipient may not assert any proprietary rights thereto or represent them 

to anyone as other than government-produced programs. The FCCS software is 

provided “as is” without warranty of any kind, including but not limited to the 

implied warranties of merchantability and fitness for a particular purpose. The user 

assumes all responsibility for the accuracy and suitability of these programs for a 

specific application.

The use of trade or firm names in this publication is for reader information and 

does not imply endorsement by the U.S. Department of Agriculture of any product 

or service. The views and opinions of authors expressed herein do not necessarily 

state or reflect those of the U.S. government or any agency thereof.

Cover photographs:

Fuelbeds are structurally complex and vary widely in their physical attributes 

and potential fire behavior. These grassland, shrubland, and forest examples 

illustrate a diversity of vegetation and fuels: (1) grassland in southeastern Oregon, 

(2) sagebrush shrubland in southeastern Oregon, (3) young lodgepole pine forest in 

Wyoming, (4) birch and aspen forest in Alaska, (5) longleaf pine forest plantation 

in Florida, and (6) shortleaf and loblolly pine forest in Texas, 1 month after a 

hurricane. All photos by Robert Vihnanek, Fire and Environmental Research 

Applications Team, Pacific Wildland Fire Sciences Laboratory, Pacific Northwest 

Research Station.

1 2

3 4

5 6
Abstract

Prichard, Susan J.; Sandberg, David V.; Ottmar, Roger D.; Eberhardt, Ellen; 

Andreu, Anne; Eagle, Paige; Swedin, Kjell. 2013. Fuel Characteristic 

Classification System version 3.0: technical documentation. Gen. Tech. 

Rep. PNW-GTR-887. Portland, OR: U.S. Department of Agriculture, Forest 

Service, Pacific Northwest Research Station. 79 p. 

The Fuel Characteristic Classification System (FCCS) is a software module that 

records wildland fuel characteristics and calculates potential fire behavior and 

hazard potentials based on input environmental variables. The FCCS 3.0 is housed 

within the Integrated Fuels Treatment Decision Support System (Joint Fire Science 

Program 2012). It can also be run from command line as a stand-alone calculator. 

The flexible design of FCCS allows users to represent the structural complexity 

and diversity of fuels created through natural processes (e.g., forest succession and 

disturbance) and management activities (e.g., forest harvesting and fuels reduction). 

Each fuelbed is organized into six strata, including canopy, shrubs, herbaceous 

vegetation, woody fuels, litter-lichen-moss, and ground fuels. Strata are further 

divided into categories and subcategories. Fuelbeds representing common fuel 

types throughout much of North America are available in the FCCS reference 

library. Users may select an FCCS fuelbed to represent their specific project or customize a fuelbed to reflect actual site conditions. The FCCS reports the following 

results: (1) fuel characteristics by fuelbed, stratum, category, and subcategory; (2) 

surface fire behavior (i.e., reaction intensity, rate of spread, and flame length); and 

(3) FCCS fire potential ratings of surface fire behavior, crown fire behavior, and 

available fuels. With its large fuels data set and ability to represent a wide variety of 

fuel conditions, the FCCS has numerous applications, from small-scale fuel reduction projects to large-scale emissions and carbon assessments. This report provides 

technical documentation of the required inputs and computations in the FCCS.

Keywords: Fuel characteristics, fire behavior, fire modeling, carbon, biomass.
Contents

1 Introduction

4 Inferred Variables

7 Inferred Variables by Species

8 Inferred Variables by Type (i.e., Litter Type)

9 Calculation of Fuelbed Characteristics

11 Weighted Average of Inferred Variables

22 Surface Fire Behavior

26 Reaction Intensity

32 Rate of Spread

37 Flame Length

38 Fuel Model Crosswalks

39 FBPS Fuel Models

39 Standard Fuel Models

43 FCCS Fire Potentials

45 Surface Fire Potentials

46 Crown Fire Potentials

56 Available Fuel Potentials

60 Fuel Consumption and Combustible Carbon

60 Future Model Development and Validation

61 Metric Equivalents

61 Acknowledgments

62 Literature Cited

68 Appendix

70 Glossary
Tables

Table Page

number number Description

1 5 Inferred variables invoked by type used in the canopy, 

 shrub, woody, litter-lichen-moss, and ground fuels strata

2 6 Additional inferred variables by litter, lichen, and moss 

 types and arrangements

3 9 User-input FCCS fuelbed variables used in the calculation 

 of physical characteristics and properties of wildland fuels

4 15 Total aboveground biomass equations by major tree 

 species groups

	 5	 17	 Biomass equations used in Fuel Characteristic Classification

 System to calculate shrub loading

6 21 Inferred live and dead designations

7 21 Proportion of carbon by fuelbed category and subcategory

8 23 Comparison of the original Rothermel (1972) and Fuel 

	 	 	 	 Characteristic Classification System (2007) fire spread

 models

9 24 Comparison of equation forms in Rothermel (1972) and 

	 	 	 	 Fuel Characteristic Classification System (FCCS) (2007)

	 10	 39	 Fuel model crosswalk to the original 13 fire behavior

 fuel models

11 40 Adjective assignment of the percentage difference between 

	 	 	 	 predicted rate of spread and flame length values and

 benchmark values

12 40 Crosswalk matrix from original Fire Behavior Prediction 

 System fuel model assignment to standard fuel models

13 50 Ladder fuel rating by type

14 57 Available fuel potential calculations by fuel loading class

15 59 Combustible carbon equations

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Fuel Characteristic Classification System Version 3.0: Technical Documentation

Introduction

The Fuel Characteristic Classification System (FCCS) provides fire and fuel 

managers with a consistent and effective way to characterize and classify wildland 

fuels. The FCCS can be used to query and record fuelbed information and to generate numerical inputs to fire behavior, fire effects, and dynamic vegetation models. 

Characterizing fuels and assessing their fire hazard has always been important 

to fire and fuel managers. These topics are becoming increasingly important to 

other specialists, including ecologists, air quality and smoke managers, land use 

planners, global change scientists, and carbon modelers. With its large data set 

and ability to represent a wide variety of fuel conditions, the FCCS has numerous 

applications, from small-scale fuel reduction projects to large-scale emissions and 

carbon assessments. Sample applications include:

• Fuel reduction projects—FCCS can be used as a planning tool to target 

which fuels should be removed to effectively reduce fire hazard. It can also 

be used as a reporting tool to summarize pre- and postburn fuel characteristics and fire hazard potential. 

• Emissions inventories—Air quality specialists may run FCCS to calculate 

fuel loading to be used as inputs to emissions and smoke production software packages such as Consume (USDA FS 2012a) or BlueSky (Larkin et 

al. 2009).

• Regional assessments—FCCS fuelbeds and outputs can be used to map 

fuel loading, carbon stores, or fire hazard. Because fuelbeds have no inherent scale, FCCS outputs can be easily mapped in a geographic information 

system (GIS) by assigning polygons or raster cells with a unique fuelbed 

identification number. The FCCS makes point-based calculations of fuel 

characteristics and potential fire behavior and does not simulate fire spread 

across landscapes. 

• Carbon accounting—FCCS can be used to compile carbon stores before 

and after disturbance and to predict carbon flux from fires. 

FCCS defines a fuelbed as the inherent physical characteristics of fuels that 

contribute to fire behavior and effects (Riccardi et al. 2007a). A fuelbed can 

represent any scale considered to be mostly uniform. For example, a 10,000-ac 

western sagebrush ecosystem may be adequately represented by a single fuelbed, 

whereas a 100-ac tropical rain forest may require several fuelbeds to capture 

the variability of vegetation and other fuels. Because FCCS fuelbeds are not 

scale dependent, they are easily mapped, and calculated results can readily be 
2

GENERAL TECHNICAL REPORT PNW-gtr-887

incorporated into geospatial analyses. The FCCS describes fuelbeds in six strata, 

including canopy, shrubs, herbaceous vegetation (herb), woody fuels, litter-lichenmoss (LLM), and ground fuels. Each stratum is further divided into one or more 

categories and subcategories to represent the complexity of wildland and managed 

fuels (fig. 1). 

Figure 1—Fuelbed strata and categories in the Fuel Characteristic Classification System.

FCCS summarizes and calculates fuel characteristics for each fuelbed stratum, 

category, and subcategory. The characteristics include percentage of cover, depth 

(in feet or inches), total fuel loading in tons per acre (tons/ac), live fuel loading 

(tons/ac), dead fuel loading (tons/ac), fuel area index (analogous to leaf area index) 

square foot per square foot (ft2/ft2), packing ratio, tree density (number/ac), tree 

height to live crown (ft), and tree diameter at breast height (dbh). 

Based on fuel characteristics and input environmental variables, FCCS predicts surface fire behavior including reaction intensity in British thermal units 

per square foot per minute (Btu/ft2

/min), rate of spread (ft/min), and flame length 

(ft). The FCCS also calculates a set of FCCS fire potentials including surface fire 
3

Fuel Characteristic Classification System Version 3.0: Technical Documentation

behavior potential, crown fire potential, and available fuel potential. The FCCS 

fire potentials are indexed values, scaled between 0 and 9, and are based on default 

environmental conditions. They provide a flexible means of expressing any fuelbed 

as a function of its potential to produce fire behavior or effects. Their intent is to 

provide an approach to quantifying fuels that is easy to communicate among fuels 

specialists and across a diversity of fuel types.

This report provides an updated and more complete documentation of the 

scientific basis for the FCCS than is available in the previously published literature. 

A series of seven peer-reviewed manuscripts were published in a special issue 

of the Canadian Journal of Forest Research (Berg 2007) and supported the first 

release of FCCS software. Ottmar et al. (2007) provided a history and overview 

of the system. Riccardi et al. (2007a, 2007b) described the development of FCCS 

fuelbeds and calculation of fuelbed characteristics. Sandberg et al. (2007a) detailed 

the conceptual basis and calculation of FCCS fire potentials. A new formulation 

of the Rothermel (1972) surface fire spread model was introduced by Sandberg et 

al. (2007b). Schaaf et al. (2007) introduced a new conceptual approach to crown 

fire potential modeling, building on concept introduced in Sandberg et al. (2007b). 

Finally, McKenzie et al. (2007) showed how FCCS fuelbeds have been mapped at 

large spatial scales across forested landscapes. 

This documentation reflects changes and additions to the capabilities of the 

FCCS software gained through the experience of working with many users in 

training sessions and on-the-ground applications of the system. Because the FCCS 

is a substantial departure from traditional fuel inventory and modeling concepts, 

extensive input was elicited from scientists and resource managers in North 

America during development and testing. User feedback in the past few years has 

guided significant upgrade of the science and software behind FCCS and contributed to further changes detailed in this report.

The FCCS was recently recoded to be better integrated with Consume 

(Prichard et al. 2007) and the Fire Emissions Production Simulator (FEPS) 

(Anderson et al. 2004). A batch version of the FCCS version 3.0 is available for 

download at http://www.fs.fed.us/pnw/fera. FCCS can also be used within the 

Interagency Fuels Treatment Decision Support System (Joint Fire Science Program 

2012) and will be also available for use within an integrated user interface that is 

currently under development for FCCS, Consume and FEPS and will be available 

for download at http://www.fs.fed.us/pnw/fera in early 2014.

The FCCS supports both English and metric inputs and outputs, but actual 

calculations are in the English measurement system. This documentation was 
4

GENERAL TECHNICAL REPORT PNW-gtr-887

developed in part to provide future developers and users of the FCCS with a specification of all calculations within the system. To avoid conversion errors between this 

documentation and the programming code, we retained English units throughout 

this report.

The FCCS scientific documentation is organized into the following sections:

• Inferred variables used in FCCS calculations.

• Calculation of fuelbed characteristics.

• Surface fire behavior calculations, including updates since Sandberg et al. 

(2007b).

• Crosswalk to stylized fuel models (Anderson 1982, Scott and Burgan 2005).

• Crown fire behavior potentials, including updates since Schaaf et al. (2007).

• Available fuel potentials.

Equations are numbered and referenced. For complex equations with several 

component terms, the source equation is listed in parentheses after the component 

term name (e.g., propagating flux ratio in rate of spread, eq. 26). A comprehensive 

list of variable names and associated symbols is included in the appendix. Wherever 

possible, we used common published symbology for variables.

Unless otherwise noted, subscript references in the following equations are 

assigned to fuelbed divisions as follows:

i = species or type, 

j = category or subcategory,

k = stratum, and

m = layer (e.g., surface fuels and low surface fuels). 

Inferred Variables

The FCCS inferred variables are internal data sets used to calculate physical 

characteristics of wildland fuels. Inferred variables are used in association with a 

plant species or type designation (e.g., litter type or duff derivation) (tables 1 and 2). 

They include fuel chemistry, heat content, particle density, and bulk density values 

and currently cannot be modified by users. Nomenclature for many of the inferred 

variables were derived in the development of the Rothermel fire spread model 

(Rothermel 1972, Scott and Burgan 2005) and the Van Wagner crown fire model 

(Scott and Reinhardt 2001, Van Wagner 1977). Their values are based on published 

and unpublished data where available or where a convention has been established 

by widespread use in other models such as BEHAVE (Andrews et. al. 2003). 

Defaults were estimated when no data were available. The FCCS inferred variables 

by species database can be downloaded at USDA FS (2012d). 
5

Fuel Characteristic Classification System Version 3.0: Technical Documentation

Table 1—Inferred variables invoked by type used in the canopy, shrub, woody, litter-lichen-moss, and 

ground fuels strata

Inferred variables

Particle density

Type, of foliage or Low fuel Fuel Surface-area Bulk

Stratum- arrangement, rotten wood heat content area index -to-volume Packing density

category	 or subcategory	 (ρf or ρr

a

)	 (h)	 (FAI)	 ratio (σ)	 ratio (β)	 (ρb)

 British Square Square Pounds

 Pounds per thermal units foot per foot per per

 cubic foot per pound square foot cubic foot cubic foot

Canopy:b

 Arboreal lichen 25.00 8,000 0.26 — 2.00 × 10-6 —

 Dead branches 18.72a 8,000 0.06 — 3.00 × 10-4 —

 Climbing ferns/ 25.00 8,000 0.18 — 2.00 × 10-6 —

 other epiphytes

 Stringy or fuzzy bark 18.72a 8,000 0.12 — 6.00 × 10-6 —

 Vines-lianas 25.00 8,000 0.18 — 1.40 × 10-5 —

 Leaning snags 18.72a 8,000 0.07 — 1.83 × 10-4 —

 Tree regeneration 25.00 8,000 13.8 — 1.33 × 10-4 —

Shrub:c 25.00 8,000 — 2,500 — —

Woody:d

 0 to 0.25 inches — — — 500 — —

 >0.25 to 1 inch — — — 109 — —

 >1 to 3 inches — — — 30 — —

 >3 to 9 inches — — — 8 — —

 >9 to 20 inches — — — 4 — —

 >20 inches — — — 2 — —

Litter-lichen-moss:

Litter—

 Short needle pine 25.00 8,000 — 2000 — 1.65

 Long needle pine Normal 25.00 8,000 — 1500 — 1.65

 Fluffy 25.00 8,000 — 1500 — 1.65

 Perched 25.00 8,000 — 1500 — 1.65

 Other conifer 25.00 8,000 — 2000 — 1.65

 Deciduous hardwood Normal 25.00 8,000 — 2500 — 0.83

 Fluffy 25.00 8,000 — 2500 — 0.83

 Perched 25.00 8,000 — 2500 — 0.83

 Evergreen hardwood Normal 25.00 8,000 — 2500 — 0.83

 Fluffy 25.00 8,000 — 2500 — 0.83

 Perched 25.00 8,000 — 2500 — 0.83

 Palm frond 25.00 8,000 — 2500 — 0.17

 Grass 25.00 8,000 — 3500 — 0.28

Lichen 25.00 6,000 1500 0.28

Moss Sphagnum 25.00 6,000 — 1000 — 0.83

 Other moss 25.00 6,000 — 2000 — 0.83

Ground fuels:

Upper duff— Dead litter and moss 25.00 8,000 — 2000 — 4.41

 Fibric peat 25.00 8,000 — 2000 — 4.41

Lower duff— Humus or muck 25.00 8,000 — 500 — 9.92

 Humic peat 25.00 8,000 — 500 — 12.12
6

GENERAL TECHNICAL REPORT PNW-gtr-887

Table 1—Inferred variables invoked by type used in the canopy, shrub, woody, litter-lichen-moss, and 

ground fuels strata (continued)

Inferred variables

Particle density

Type, of foliage or Low fuel Fuel Surface-area Bulk

Stratum- arrangement, rotten wood heat content area index -to-volume Packing density

category	 or subcategory	 (ρf or ρr

a

)	 (h)	 (FAI)	 ratio (σ)	 ratio (β)	 (ρb)

 British Square Square Pounds Pounds per thermal units foot per foot per per cubic foot per pound square foot cubic foot cubic foot

Squirrel middens 25.00 8,000 — 100 — 2.75

Basal accumulations Bark slough 25.00 8,000 — 500 — 2.75

 Branches 25.00 8,000 — 2,000 — 2.75

 Broadleaf deciduous 25.00 8,000 — 1,500 — 1.71

 Broadleaf evergreen 25.00 8,000 — 1,500 — 1.71

 Grass 25.00 8,000 — 1,500 — 1.27

 Needle litter 25.00 8,000 — 1,500 — 1.65

 Palm fronds 25.00 8,000 — 1,500 — 1.27

a Fluffy is defined as fresh uncompacted litter, and perched is a litter layer dominated by litter perched on herbaceous or woody fuels. b

 Including needle drape.

c

 Including ladder fuels.

d

 Sound and rotten.

Table 2—Additional inferred variables by litter, lichen, and moss types and arrangements

Maximum loading that would Maximum fuel Relative Reaction

consume in the flaming stage of	 area index value	 packing	 efficiency

Type	 Arrangement	 combustion (MaxFlameWL)	 (maxFAI)	 ratio (β')	 (ηβ')

Square foot per

Pounds per acre square foot

Litter type—

Short needle pine 2,900 4.44 8.03 0.18

Long needle pine Normal 5,800 6.66 6.35 .27

 Fluffy 6,000 6.89 4.38 .49

 Perched 7,500 6.89 1.64 .95

Other conifer 2,900 4.44 8.03 .18

Deciduous hardwood Normal 3,100 5.93 10.31 .11

 Fluffy 4,000 7.65 6.65 .30

 Perched 5,000 7.65 1.66 .95

Evergreen hardwood Normal 3,100 5.93 10.31 .11

 Fluffy 4,000 7.65 6.65 .30

 Perched 5,000 7.65 1.66 .95

Palm frond 2,000 4.59 7.65 .23

Grass 667 2.12 10.07 .15

Moss type—

Sphagnum 1,740 1.33 4.55 .40

Other moss 5,800 8.88 8.03 .18

Ground lichen 5,800 6.66 6.35 .27
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Fuel Characteristic Classification System Version 3.0: Technical Documentation

Inferred Variables by Species

Bulk density (ρb; lb/ft3)— 

Tree and shrub canopy bulk density data are taken from published sources such as 

the Natural Fuels Photo Series (Ottmar et al. 2004), Anderson (1969), and expert 

opinion of the authors.

Crown packing ratio (β; dimensionless)—

Packing ratio (β) of tree crowns by species; defined as the fraction of crown

volume occupied by fuel including needles and fine branches. 

Crown shape factor (CSF; dimensionless)—

A geometrical adjustment factor used to differentiate crown shapes of coniferous 

and broadleaf trees in volume and loading calculations. 

Flammability index (ηF; dimensionless)—

The flammability index allows for the designation of species with special properties with respect to fire. The flammability index is assigned by species based on 

information from the Fire Effects Information System (USDA FS 2012c), available 

literature, and expert opinion. Flammable species are those that have extractives 

(e.g., terpenes, fats, waxes, and oils, particularly terpenoid hydrocarbons and lipids, 

which provide a ready source of combustible volatiles. High heat of combustion, 

volatility, and lower limits of flammability are associated with flammable species 

(Pyne et al. 1996). Neutral species do not contribute to reaction intensity. 

Foliage particle density (ρf

; lb/ft3)—

Taken from standard sources (e.g., Hoadley 1991, Wenger 1984). A default 

value of 25.0 lb/ft3 is used when values are not available. 

Low fuel heat content (h; Btu/lb)—

Heat of a material produced by combustion (Byram 1959).

Rotten wood particle density (ρr; lb/ft3

)—

Taken from standard sources (e.g., Hoadley 1991, Wenger 1984). A default 

value of 18.7 lb/ft3 is used when values are not available. 

Sound wood particle density (ρb; lb/ft3

)—

Taken from standard sources (e.g., Hoadley 1991, Wenger 1984). A default 

value of 25.0 lb/ft3 is used when values are not available. 

Surface-area-to-volume ratio (σ; ft2

/ft3)—

The amount of surface area per unit volume of fuels (Fons 1946). 
8

GENERAL TECHNICAL REPORT PNW-gtr-887

Inferred Variables by Type (i.e., Litter Type)

Bulk density (ρb; lb/ft3)—

Based on expert opinion (Ottmar et al. 2007). 

Fuel area index (FAI; ft2

/ft2)—

A dimensionless measure of cover and the total fuel surface area per unit 

ground area. It is analogous to a two-sided leaf area index, which is often 

used in agricultural and ecological research. FAI is calculated for each species 

in FCCS but is inferred for type designations.

Low fuel heat content (h; Btu/lb)—

Heat of a material produced by combustion.

MaxFAI (dimensionless)—

Maximum fuel area index by LLM type (Ottmar 2005). 

MaxFlameWL (lb/ft3

)—

Maximum flame-available loading by LLM type.

Packing ratio (β; dimensionless)—

A measure of fuelbed compactness; the fraction of the fuel array volume occupied 

by fuel. Packing ratio is calculated for each species in FCCS but is inferred by type.

Particle density (ρ; lb/ft3

)—

Taken from standard sources (Hoadley 1991, Wenger 1984). 

Particle volume (PV; ft3

/ac)—

The volume of space filled by fuel particles. Whereas bulk volume is the volume 

of fuels and the interstitial airspace between fuels (e.g., volume of a tree crown), 

particle volume only considers the space containing fuel particles. It is calculated 

as dry weight biomass (lb/ac) divided by particle density (lb/ft3

).

Relative packing ratio (β; dimensionless)—

The ratio of actual fuel packing ratio to optimum packing ratio. This can also be 

calculated as the ratio of optimum depth to actual depth.

Surface-area-to-volume ratio (σ; ft2

/ft3)—

The amount of surface area per unit volume of fuels (Fons 1946). 
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Fuel Characteristic Classification System Version 3.0: Technical Documentation

Calculation of Fuelbed Characteristics

FCCS organizes fuels into six horizontal fuelbed strata including canopy, shrubs, 

herb, woody fuels, LLM, and ground fuels (fig. 1). Strata are further divided into 

categories and subcategories (table 3). The fuelbed categories and subcategories 

have common combustion characteristics. For example, the canopy stratum includes 

three categories: trees, snags, and ladder fuels. The tree category is divided into 

three subcategories: overstory, midstory, and understory trees. 

• Canopy stratum: includes trees, snags, and ladder fuels.

• Shrub stratum: primary and secondary shrub layers.

• Herb stratum: herb (i.e., grasses, sedges, rushes and forbs) in primary 

and secondary layers.

• Woody fuels stratum: includes all downed and dead wood, sound wood, 

rotten wood, and stumps.

Table 3—User-input FCCS fuelbed variables used in the calculation of physical 

characteristics and properties of wildland fuels 

Strata, category, and subcategory Variables

Canopy:

Total canopy— Percentage covera

Trees— Percentage covera,c

 Overstory, midstory, understory Height (ft)a,c

 Height to live crown (ft)a,c

 Density (number of stems/ac)a,c

 Diameter at breast height (in)c

 Species and relative cover (percentage)a,c

Snags—

 Class 1 snags with foliage Height (ft)a,c

 Height to snag crown base (ft)a

 Density (number of stems/ac)a,c

 Diameter at breast height (in)c

 Species and relative cover (percentage)a,c

 Subcategory: class 1 (without foliage), class 2, Height (ft)c

 class 3 Diameter (in)c

 Density (number of stems/ac)c

 Species and relative cover (percentage)c

Ladder fuels—

 Type: arboreal lichens and mosses, climbing Minimum height (ft)

 ferns and other epiphytes, dead branches, Maximum height (ft)

	 	 	 leaning snags, stringy or fuzzy bark, tree 	 Is there vertical continuity sufficient to carry

	 	 	 regeneration, and vines-liana	 	fire between the canopy and lower strata?a

Shrubs:

Primary layer, secondary layer Percentage covera,b,c

 Height (ft)a,b,c

 Percentage livea,b

 Loading (tons/ac)a,b,c— optional

 Species and relative cover (percentage)a,b,c

	 Needle drape	 Is needle drape on shrubs sufficient to affect

	 	 	 	 	fire behavior?a
10

GENERAL TECHNICAL REPORT PNW-gtr-887

Table 3—User-input FCCS fuelbed variables used in the calculation of physical 

characteristics and properties of wildland fuels (continued)

Strata, category, and subcategory Variables

Herbs:

Primary layer, secondary layer Percentage covera,b

 Height (ft)a,b

 Percentage live

a

 Loading (tons/ac)a,b,c

 Species and relative cover (percentage)a,b

Woody fuels:

All woody— Total cover (percentage)a,b

 Depth (ft)a,b

Sound wood— Species and relative cover (percentage)a,b

 0 to 0.25 in Loading (tons/ac)a,b,c

 >0.25 to 1 in Loading (tons/ac)a,b,c

 >1 to 3 in Loading (tons/ac)a,b,c

 >3 to 9 in Loading (tons/ac)a,b,c

 >9 to 10 in Loading (tons/ac)a,b,c

 >20 in Loading (tons/ac)a,b,c

Rotten wood— Species and relative cover (percentage)a,b

 >3 to 9 in Loading (tons/ac)a,b,c

 >9 to 10 in Loading (tons/ac)a,b,c

 >20 in Loading (tons/ac)a,b,c

Stumps— Species and relative cover (percentage)

c

 Sound, rotten, lightered/pitchy Height (ft)

c

 Diameter (in)

c

 Density (number per acre)

c

Piles— Loading (tons/acre)

c

Litter-lichen-moss:

	 Litter—	 Litter arrangement (fluffy, normal, perched)a,b,c

 Type: Short needle pine, long needle pine, Litter type and relative cover (percentage)a,b,c

 other conifer, broadleaf deciduous, Depth (in)a,b,c

 broadleaf evergreen, palm frond, grass Percentage of cover (%)a,b,c

 Loading (tons/ac)a,b,c —optional

Lichen— Depth (in)a,b,c

 Percentage of cover (%)a,b,c

 Loading (tons/ac)a,b,c —optional

Moss— Depth (in)a,b,c

 Type: spaghnum, other moss Percentage of cover (%)a,b,c

 Loading (tons/ac)a,b,c —optional

Ground fuels:

Upper duff— Depth (in)

c

	 	 Derivation: dead litter and moss; fibric peat 	 Percentage cover (%)

c

 Loading (tons/ac)

c

 —optional

Lower duff— Depth (in)

c

 Derivation: humus or muck; humic peat Percentage of cover (%)

c

 Loading (tons/ac)

c

 —optional

Basal accumulations— Depth (in)

c

 Derivation: bark slough, branches, broadleaf Radius (ft)

c

 deciduous, broadleaf evergreen, grass, Percentage of trees affected (%)

c

 needle litter, palm fronds Loading (tons/ac)

c

 —optional

Squirrel middens— Depth (in)

c

 Radius (ft)

c

 Density (number per acre)

c

 Loading (tons/ac)

c

 —optional

Superscripts a, b, and c indicate variables important to crown fire potentials, surface fire behavior algorithms, 

and carbon accounting, respectively.
11

Fuel Characteristic Classification System Version 3.0: Technical Documentation

• Litter-lichen-moss stratum: litter, lichen, and moss layers.

• Ground fuels stratum: includes duff, basal accumulations, and 

squirrel middens.

The FCCS fuel characteristics calculator provides summaries of fuelbed inputs, 

such as percentage cover and fuelbed depth, and calculates additional fuel characteristics such as live and dead fuel loading, FAI, and packing ratio by stratum, 

category, and subcategory. Inputs specific to individual categories or strata are also 

summarized (e.g., tree density, dbh, and height to live crown in the canopy stratum). 

Weighted Average of Inferred Variables

Inferred variables, including surface-area-to-volume ratio, flammability index, 

and particle density, are often summarized by category and stratum to be used in 

surface and crown fire behavior calculations. 

Weighted average by species loading (Yk; dimensionless)—

Calculated for each inferred variable:

(1)

where

y = inferred variable,

k = subscript reference to stratum,

j = subscript reference to category or subcategory,

i = subscript reference to species, 

wi

 = loading by species (tons/ac), and

wj = total loading by category or subcategory (tons/ac).

Generalized equations for FAI, packing ratio, depth, and fuel loading are 

detailed in the following sections.

Fuel area index (FAI; dimensionless)—

To assimilate the heterogeneous fuel structures represented by FCCS fuelbeds, 

FCCS calculates the total fuel surface area for all size classes of fuels including live 

foliar biomass. The FAI is the total fuel surface area per unit ground area (ft2

/ft2). 

Analogous to a two-sided leaf area index, FAI is used to calculate the FCCS fire 

potentials (Riccardi et al. 2007b). In each stratum, category, and subcategory, FCCS 

calculates live fuel FAI, dead fuel FAI, FAI of very fine (flash) fuels, and total FAI. 

yk= yj = yi

j=1 i=1

wi

wj
12

GENERAL TECHNICAL REPORT PNW-gtr-887

Fuel area index is calculated by species or type (i.e., litter type) and then summarized by category, subcategory, and stratum: 

(2) 

where

k = subscript reference to stratum,

j = subscript reference to subcategory or category,

i = subscript reference to species or type,

wi

 = loading by species or type (lb/ft2),

σi = surface-area-to-volume ratio inferred by species or type (ft2/ft3) 

(tables 1 and 2), and

ρi = particle density inferred by species or type (lb/ft3) (tables 1 and 2).

Packing ratio (β; dimensionless)—

The proportion of fuelbed stratum volume occupied by fuel particles (Rothermel 

1972) and is an important factor in predicting fire behavior. At low packing ratios 

(or bulk fuel densities), fire intensity is limited by excessive heat loss. At high packing ratios, lack of oxygen limits fuel combustion. Packing ratio (β; dimensionless) is

calculated by species or type and is summed by category and stratum. 

(3)

where 

k = subscript reference to stratum,

j = subscript reference to subcategory or category,

i = subscript reference to species or type,

wi

 = loading by species (lb/ft2),

ρi

 = particle density inferred by species or type (lb/ft3) (tables 1 and 2), and

δk = stratum depth (ft). 

Depth (δ; ft)—

A basic measure of the vertical structure of a fuelbed stratum, category, or subcategory. Based on common usage, the term height (ft) is used as an input in 

canopy, shrub, and herb strata, whereas depth (in) is used for woody fuels, LLM, 

and ground fuels strata. The woody fuels stratum contains downed and dead woody 

fuels, stumps, and tree cones. For consistency, FCCS considers only the depth of 

fine woody fuels (<3 in in diameter) in the woody fuels stratum. Depth of each 

stratum or category is calculated as:

δk = HeightTop – HeightBottom (4)

βk = βj = wi

ρiδk j=1 i=1

FAI k = FAIj = wiσi

ρi

j=1 i=1
13

Fuel Characteristic Classification System Version 3.0: Technical Documentation

where

k = subscript reference to stratum,

HeightTop = top height of a fuel stratum or category (ft), and

HeightBottom = bottom height of fuel stratum or category (ft). 

For canopy tree subcategory layers, depth is calculated as the distance between 

input values of height to live crown and tree height. For surface fuels (shrubs, 

herb, woody fuels, andLLM depth is equal to the input height or depth of each fuel 

category or subcategory. For ground fuels, depth is equal to the input depth of each 

ground fuel category.

Fuel loading (w; tons/ac)—

The dry weight biomass of fuels, which is important in many FCCS calculations 

including FAI, packing ratio, total carbon, surface fire behavior, crown fire potentials, and available fuel potentials. Loading calculations are handled differently in 

each stratum and category as detailed below:

Canopy stratum—Canopy stratum loading is the sum of all trees, snags, and ladder 

fuels and only includes the foliage and fine branch (<1.5 in) loading of tree crowns.

Tree crown loading (wTreeCrown j; tons/ac)—Calculated by species and summed by 

tree subcategory (i.e., overstory, midstory, and understory): 

(5)

where

j = subscript reference to tree subcategory (overstory, midstory, understory),

0.0005 = conversion factor (tons/lb),

i = subscript reference by tree species,

ACVi

 = bulk crown volume (ft3/ac) adjusted for species relative cover and crown 

shape factor (eq. 5a), and

CrownBDi = crown bulk density inferred by species (lb/ft3).

Adjusted crown volume (ACVTree; ft 3

/ac)—The calculated volume of tree crowns 

by species:

(5a) 

where

i = subscript reference to tree species,

j = subscript reference to tree subcategory (overstory, midstory, understory),

ΥTree j = bulk crown volume (ft3

/ac) of each tree subcategory, calculated in 

equation (5b),

wTreeCrown. j = 0.0005 ACV CrownBD i

i=1

ACVTree i = YTree j

RelCovi

100 CSFi
14

GENERAL TECHNICAL REPORT PNW-gtr-887

RelCovi = percentage of total cover by tree species, and

CSFi = crown shape factor (conifer or broadleaf) inferred by tree species.

Bulk crown volume (YTree j; ft3

/ac)—The total volume of each tree subcategory, 

including interstitial spaces between tree crowns. It is calculated as:

(5b)

where 

j = subscript reference to tree subcategory (overstory, midstory, understory),

θj

 = percentage cover of each tree subcategory (%),

Heightj = height of each tree subcategory (ft),

HLCj = height to live crown of each tree subcategory (ft), and

43560 = square feet per acre.

Total aboveground tree loading (wTree; tons/ac)—Total aboveground tree loading 

is calculated for each tree subcategory by using the equations in table 4. Equations 

numbers are assigned to each tree species in the FCCS inferred variables database. 

The equations yield aboveground biomass in kilograms per tree and are converted 

to tons per acre with a conversion factor to tons and input tree density.

wj

 = 0.0011 ABj Densityj (6) 

where

j = subscript reference to tree subcategory (overstory, midstory, understory),

0.0011 = conversion of kilograms to tons,

ABj

 = calculated aboveground biomass (kilograms) (table 4), and

Densityj = input tree density by tree subcategory (number/ac).

Class 1 snags with foliage loading (canopy stratum) (wSnagC1F; tons/ac)—Crown 

loading for class 1 snags with foliage is calculated similarly to that of tree crown 

loading:

(7)

where

i = subscript reference to species,

0.0005 = conversion factor to tons (tons/lb),

ACVi

 = bulk crown volume (ft3/ac) adjusted species relative cover and crown shape 

factor (eq. 7a), and

CrownBDi = crown bulk density inferred by species (lb/ft3).

YTree j = 43560

θj

100 Height – HLC j

wSnagC1F = 0.0005 ACV CrownBD i

i=1
15

Fuel Characteristic Classification System Version 3.0: Technical Documentation

Table 4—Total aboveground biomass equations by major tree species groupsa

 Range of

Equation diameter at

number Species group A B C Equation form breast height Reference

Centimeters

1 Aspen/alder/cottonwood/willow -2.2094 2.3867 kg = e[A+B ln(dbh)] 1–70 Jenkins et al. (2003)

2 Soft maple/birch -1.9123 2.3651 kg = e[A+B ln(dbh)] 1–66 Jenkins et al. (2003)

3 Mixed hardwood -2.48 2.4835 kg = e[A+B ln(dbh)] 1–56 Jenkins et al. (2003)

4 Hard maple/oak/hickory/beech -2.0127 2.4342 kg = e[A+B ln(dbh)] 1–73 Jenkins et al. (2003)

5 Cedar/larch -2.0336 2.2592 kg = e[A+B ln(dbh)] 1–250 Jenkins et al. (2003)

6	 Douglas-fir	 -2.2304	 2.4435	 	 kg = e

[A+B ln(dbh)] 1–210 Jenkins et al. (2003)

7	 True fir/hemlock	 -2.5384	 2.4814	 	 kg = e

[A+B ln(dbh)] 1–230 Jenkins et al. (2003)

8 Pine -2.5356 2.4349 kg = e[A+B ln(dbh)] 1–180 Jenkins et al. (2003)

9 Spruce -2.0773 2.3323 kg = e[A+B ln(dbh)] 1–250 Jenkins et al. (2003)

10 Juniper/oak/mesquite -0.7152 1.7029 kg = e[A+B ln(dbh)] 1–78 Jenkins et al. (2003)

11 Cactus .001 3.2327 kg = A dbhB n/a Sampaio and Silva 

 (2005)

12 Palm 24.559 4.921 1.017 kg = A + (B Height) 0.2–14.5 Penman et al. (2003)

 + (C Height2)

a

 Equations are in metric units, and input tree diameter must be converted from inches to centimeters. An equation number is assigned to each tree 

species in the Fuel Characteristic Classification System inferred variables database.

Adjusted crown volume (ACVSnagC1F; ft3/ac)—Calculated as:

(7a)

where

i = subscript reference to class 1 snag species,

ΥSnagC1F = bulk crown volume (ft3

/ac) of class 1 snags with foliage, as calculated in 

equation 7b, 

RelCovi

 = percentage of total class 1 foliage snag cover by tree species, and

CSFi = crown shape factor (conifer or broadleaf) by tree species.

Bulk crown volume (YSnagC1F; ft3

/ac)—Calculated as:

(7b)

where

43560 = conversion factor (ft2

/ac),

θSnagC1F = input percentage cover of class 1 snags with foliage (percentage),

HeightSnagC1F = input height of class 1 snag category (ft), and

HCBSnagC1F = input height to crown base of class 1 snags with foliage (ft).

YSnagC1F = 43560

θSnagC1F

100 Height – HCB SnagC1F

ACVSnagC1F = YSnagC1F

RelCovi

100 CSFi
16

GENERAL TECHNICAL REPORT PNW-gtr-887

Snag wood loading (canopy stratum) (wSnagWood; tons/ac)—Snag wood loading 

is calculated by species and then summed by snag subcategory (i.e., class 1, class 2, 

and class 3 snags). 

(8)

where

j = subscript reference to snag subcategory,

i = subscript reference to species,

0.0005 = conversion factor to tons (tons/lb),

0.25 = constant in volume equation for a cylinder,

Diameterj

 = diameter at breast height of snag subcategory (ft), 

π = pi,

Heightj

 = height of snag subcategory (ft), 

Densityj

 = stems per acre of snag subcategory,

ρi

 (wood particle density) = particle density inferred by species and snag category 

(i.e., a sound wood density is applied to class 1 snags and rotten wood density is 

applied to class 2 and 3 snags) (lb/ft3

) (table 1), and 

RelCovi

 = percentage of total snag subcategory cover by species. 

Ladder fuels loading (canopy stratum) (wLadder; tons/ac)—Ladder fuels loading 

is inferred by ladder fuel type (i.e., arboreal lichens and moss, climbing ferns and 

other epiphytes, dead branches, leaning snags, profuse epicormic sprouting, stringy 

or fuzzy bark, tree regeneration, and vines-liana) (table 1).

Shrub loading (wShrub; tons/ac)—Total aboveground shrub loading can be either 

input or calculated in FCCS. The FCCS calculates primary and secondary layer 

shrub loading based on input percentage cover and height by species (table 5). 

Shrub stratum loading is the sum of the primary and secondary layer loadings (wj

).

(9)

where

j = subscript reference to primary and secondary shrub categories,

i = subscript reference to shrub species,

wj

 = primary and secondary shrub loading,

A = equation coefficient by species (table 5),

B = equation coefficient by species (table 5),

θj

 = input percentage cover by primary or secondary category, 

Heightj

 = input height of primary or secondary category (ft) (used only in 

eqs. 26 through 29), and 

N = conversion factor to tons/ac.

WSnagWood j = 0.0005 0.25 Diameterj

2

π Heightj Densityj ρi

RelCovi

100

i=1

wShrub = wj= (A+B)iθj

i=1

2

j=1

HeightjNi
17

Fuel Characteristic Classification System Version 3.0: Technical Documentation

Shrub crown loading (wShrubCrown; tons/ac)—FCCS estimates the proportion of 

crown loading by species (table 5) and uses a calculated shrub crown loading in 

surface fire behavior and crown fire potential calculations.

(9a)

where

wShrub = total aboveground shrub stratum loading, and

PercentCrown = conversion from total aboveground biomass to crown loading of 

foliage and small branches. 

Table 5—Biomass equations used in Fuel Characteristic Classification System to calculate shrub loading 

(w; tons/ac)

BioEqIDa Source species A B X1b X2 Nc Percentaged Reference

1 Diervilla lonicera 0.8601 0.3238 θj 0.0248 35 Ohmann et al. (1981)

2 Ledum groenlandicum 3.341 .0001 θj 0.0248 50 Ohmann et al. (1981)

3 Lonicera canadensis 0.9266 .2727 θj 0.0248 35 Ohmann et al. (1981)

4 Rosa spp. (Minnesota) 0.7867 .3964 θj 0.0248 35 Ohmann et al. (1981)

5 Rubus parviflorus 0.7885 .2508 θj 0.0248 60 Ohmann et al. (1981)

6 Rubus pubescens 1.6346 .0214 θj 0.0248 35 Ohmann et al. (1981)

7 Vaccinium angustifolium 1.2458 .1496 θj 0.0248 60 Ohmann et al. (1981)

8 Vaccinium myrtilloides 0.9340 .3747 θj 0.0248 35 Ohmann et al. (1981)

9 Rubus spectabilis 14.4183 12.561 θj 0.0045 45 Alaback (1986)

10 Vaccinium alaskense -7.2200 17.307 θj 0.0045 35 Alaback (1986)

11 Gaultheria shallon 11.6990 3.5180 θj 0.0045 60 Alaback (1986)

12 Mahonia nervosa 14.2180 1.9840 θj 0.0045 50 Gholz et al. (1979)

13 Ephedra torreyana 0 627.00 θj/100 0.0045 50 Gholz et al. (1979)

14 Flourensia cernua 0 1425.0 θj/100 0.0045 50 Smith and Ludwig (1976)

15 Larrea tridentata 0 1504.0 (θj/100)2 0.0045 35 Smith and Ludwig (1976)

16 Prosopis glandulosa 0 287.00 (θj/100)2 0.0045 50 Smith and Ludwig (1976)

17 Yucca elata 0 1435.0 θj/100 0.0135 100 Smith and Ludwig (1976)

18 Arctostaphylos patula 0 .1410 θj 1 50 Martin et al. (1981)

19 Ceanothus velutinus 0 .1080 θj 1 50 Martin et al. (1981)

20 Purshia tridentata 0 .0500 θj 1 35 Martin et al. (1981)

21 Artemisia tridentata 0 230.586 θj 0.0005 35 Ottmar et al. (2000)

22 Ceanothus chaparral 0 .3720 θj 1 50 Ottmar et al. (1998)

23 Adenostoma fasciculatum 0 .1700 θj 1 40 Ottmar et al. (1998)

24 Arctostaphylos uva-ursi 0 .0022 θj 1 50 Alexander (1978)

25 Yucca elata (brevifolia) 0 1435.0 (θj/100)2 0.0135 100 Smith and Ludwig (1976)

26 Juniper shrubs 0 .1000 (θj/100)2 Heightj

/2 1 65 Ottmar et al. (2000)

27 Broadleaf deciduous 0 2000.0 θj/100 Heightj 0.0005 40 Ottmar et al. (2002)

shrubs

28 Broadleaf evergreen 0 3000.0 θj/100 Heightj 0.0005 50 Ottmar et al. (1998)

shrubs

29 Pocosin shrubs 0 4000.0 θj/100 Heightj 0.0005 65 Ottmar and Vihnanek 

 (2000)

a

 BioEqID is an inferred variable assigned by species. 

b θ = input percent cover by primary and secondary shrub layer.

c

 N is a conversion factor .

d

 Percentage of aboveground biomass that is from the crown. This includes shrub foliage and small branches. 

wShrubCrown = wShrubPercentCrown
18

GENERAL TECHNICAL REPORT PNW-gtr-887

Herb loading (wHerb; tons/ac)—Required input variable by primary and 

secondary layer. 

Sound and rotten woody fuel loading (woody fuels stratum) (wj

; tons/ac)—

Required input variables by diameter size class. 

Stump loading (woody fuels stratum) (wStumpj; tons/ac)—Calculated for each

subcategory (i.e., sound, rotten or lightered/pitchy).

(10)

where

j = subscript reference to stump subcategory,

i = subscript reference to species,

0.0005 = conversion factor to tons (ton/lb),

0.25 = constant in calculation of the volume of a cylinder,

Diameterj

 = input diameter of each stump subcategory (in), 

12 = conversion factor from inches to feet, 

Heightj

 = input height of each stump subcategory (ft), 

Densityj

 = stems per acre by stump subcategory, and

ρi

 = wood particle density based on stump species (lb/ft3). 

Sound wood particle densities (ρs

) by species are used for sound stumps, and 

rotten wood particle densities (ρr) are used for rotten and lightered/pitchy stumps. 

Tree cone loading (woody fuels stratum) (wj

; tons/ac)—Required input variable. 

Pile loading (wPile; tons/ac)—Total pile loading is calculated in the Pile Calculator 

(USDA FS 2012e) and can be entered in FCCS fuelbeds and used in available fuel 

potential and total carbon calculations.

Litter-lichen-moss stratum loading (wLLM; tons/ac)—Stratum loading is the sum 

of LLM loading, which are calculated separately, based upon input depth, percentage cover, and inferred bulk density (table 1):

(11)

where

j = subscript reference to LLM;

i = subscript reference to type (i.e., litter type or moss type);

43560 = conversion factor (ft2

/ac);

2000 = conversion factor (lb/ton);

wLLM = wj

3

j=1

= 43560

2000

i=1

ρbiδj θj

wStump j = 0.0005 0.25

Diameterj

12

2

π HeightjDensityjρi

i=1
19

Fuel Characteristic Classification System Version 3.0: Technical Documentation

ρbi = bulk density by lichen, moss, or litter type (lb/ft3) (table 1);

δj

 = depth of LLM (ft). Litter depth is weighted by the relative cover of each litter 

type (i.e., longleaf pine, shortleaf pine); and

θj

 = percentage cover of LLM.

Ground fuels loading (wGroundFuels; tons/ac)—The sum of all ground fuel types, 

including duff (upper and lower duff), squirrel middens, and basal accumulations. 

Duff loading (ground fuels stratum) (wDuff; tons/ac)—The sum of upper and lower duff loading, which are calculated based on inferred bulk density by duff derivation (table 1) as well as input depth and percentage cover of each duff layer.

(12)

where

j = subscript reference to duff subcategory (i.e., upper or lower duff layer),

43560 = conversion factor (ft2

/ac),

2000 = conversion factor (lb/ton),

ρbj = bulk density by duff derivation of upper (lb/ft3) (table 1), 

δj = input depth of the duff subcategory weighted by its relative cover (ft), and

θj = percentage cover of the duff layer.

Basal accumulation loading (ground fuels stratum) (wBA; tons/ac)—Estimated 

based on input dimensions and inferred bulk density by basal accumulation derivation (e.g., bark slough or needle litter). The input radius of basal accumulations is 

measured from the edge of the tree to the outside edge of the basal accumulation. 

Bulk volume is estimated as a paraboloid shape minus a cylindrical volume of the 

tree bole. The FCCS assumes that only overstory trees have basal accumulations.

(13)

where

YBA j (ft3

/ac) = estimated basal accumulation volume, calculated as the difference 

between the gross volume of a parabaloid minus the cylindrical volume of the tree 

bole (eq. 13a);

DensityBA (number/ac) = calculated as the product of overstory tree density and 

input percentage affected of basal accumulations divided by 100; 

ρb BA = inferred bulk density of basal accumulations (lb/ft3

) (table 1); and

2000 = conversion factor (lb/ton).

wj =

43560

2000 ρbj

δj θj

wBA = YBA DensityBA

ρb BA

2000
20

GENERAL TECHNICAL REPORT PNW-gtr-887

Basal accumulation volume (YBA.j; ft3/ac)—Calculated as:

where

DepthBA (ft) = input basal accumulation depth converted to feet,

WidthBA (ft) = calculated as the sum of the basal accumulation and tree diameters, 

DiameterOverstory (ft) = input overstory tree diameter,

DepthBA (ft) = input basal accumulation depth converted to feet, and

DensityOverstory (number/ac) = input overstory density.

Squirrel midden loading (ground fuels stratum) (wSM; tons/ac)—Estimated 

based on input dimensions and an inferred bulk density (table 1).

(14)

where

YSM (ft3

/ac) = estimated squirrel midden volume, calculated as the volume 

of a parabaloid (eq. 14a);

DensitySM (number/ac) = input density of squirrel middens; 

ρb SM = inferred bulk density of squirrel middens (lb/ft3) (table 1); and

2000 = conversion factor (lb/ton).

Squirrel midden volume (YSM; ft3

/ac)—Calculated as:

(14a)

where

Depth (ft) = input squirrel midden depth converted to ft, 

Width (ft) = calculated as 2RadiusSM, and

Density (middens/acre) = input number of squirrel middens per acre.

Live versus dead loading (w; tons/ac)—After total fuel loading is calculated, live 

and dead loadings are based on designation of live or dead by an inferred internal 

variable (table 6) or input variable (percentage live) in the shrub and herb strata. 

YBA (13a) Wj = 

π DepthBA WidthBA

2

8 – π

DiameterOverstory

2

2

DepthBA DensityOverstory

wSM = YSM DensitySM

ρb SM

2000

YSM =

π DepthSM WidthSM

2

Density

8
21

Fuel Characteristic Classification System Version 3.0: Technical Documentation

Table 6—Inferred live and dead designations

Fuel category Live Dead

Trees X

Snags X

Ladder fuels—

Arboreal lichen X

Dead branches X

Climbing ferns/other epiphytes X

Stringy or fuzzy bark X

Vines-liana X

Leaning snags X

Tree regeneration X

Shrubs X

Herb X

Woody fuels X

Litter-lichen-moss—

Litter X

Lichen X

Moss X

Ground fuels X

Total carbon (C; tons/ac)—Estimated to be between 48 and 52 percent of plant 

biomass (Eggleston et al. 2006, McGroddy et al. 2004). The FCCS reports total 

carbon by stratum, category, and subcategory by using conversion factors listed in 

table 7.

(15)

where

PropC = proportion of dry weight biomass that is carbon, and

wj

 = loading by stratum, category, and subcategory (ton/acre).

Table 7—Proportion of carbon by fuelbed category and subcategory

 Proportion of dry weight

Stratum Category/subcategory biomass that is carbon (PropC)

Canopy Trees 

Overstory (total) 0.5

Midstory (total) .5

Understory (total) .5

Snags

Class 1 foliage (crown only) .5

Class 1 wood .5

Class 2 wood .4

Class 3 wood .4

Ladder fuels .5

Shrubs Primary and secondary layers .5

Needle drape .5

Cj PropC wj
22

GENERAL TECHNICAL REPORT PNW-gtr-887

Surface Fire Behavior 

The FCCS surface fire behavior calculator predicts surface fire reaction intensity 

(Btu/ft2/min), rate of spread (ft/min), and flame length (ft) based on fuelbed characteristics and a user-specified moisture scenario, midflame windspeed, and slope 

gradient. Sandberg et al. (2007b) present a spread model formulation based on 

Rothermel (1972) with logical improvements and corrected analytical interpretations of data reported by Frandsen (1973) and Wilson (1990). In this section, we 

have expanded the documentation to better inform users and developers. We also 

present changes made to the model since 2007 as a result of field testing and expertuser feedback on model performance.

The reformulated Rothermel model in the FCCS inherits many of the same 

assumptions as the original Rothermel (1972) fire spread model (table 8). Predictions are made based on the assumption of a steady-state head fire and model 

spread in the flaming front, which is dominated by fine dead and live fuels. The 

main difference in the FCCS model is its modification to allow for heterogeneous 

surface fuel inputs (Sandberg 2007a). Heat source and heat sink terms of the FCCS 

rate of spread equation are calculated by surface fuel stratum (shrubs, herbs, woody 

fuels, and the LLM stratum) and summed. Because FCCS allows for heterogeneous 

surface fuels, it does not rely on stylized fire behavior fuel models and can more 

realistically represent actual surface fuel characteristics. This change shifts the 

potential applications from one of strictly modeling surface fire behavior to allowing predictions of both fire behavior and effects (e.g., fuel consumption and emissions) using the same inputs. It also allows for objective comparison of predicted 

fire behavior in realistic fuelbeds, such as comparison in treated versus untreated 

Table 7—Proportion of carbon by fuelbed category and subcategory (continued)

Proportion of dry weight

Stratum Category/subcategory biomass that is carbon (PropC)

Herb Primary and secondary layers .5 

Woody fuels Sound wood (all size classes) .5

Rotten wood (all size classes) .4

Stumps

Sound .5

Rotten .4

Lightered .5

Woody fuel accumulations .5

Litter, lichen, and moss Litter, lichen, and moss .5

Ground fuels Duff (upper and lower layers) .4

Basal accumulations .5

Squirrel middens .5
23

Fuel Characteristic Classification System Version 3.0: Technical Documentation

fuels. Fuelbeds in FCCS can be quite complex and reflect changes in fuel characteristics from vegetation succession over time, natural disturbances, and active 

management such as forest harvesting and prescribed burning (Ottmar et al. 2007). 

Several weaknesses remain in the physical theory underlying the modified 

spread model in FCCS. For example, we are limited by the reliance on empirical 

data to estimate the propagating flux ratio as a single term, rather than the more 

appropriate treatment of flux as a set of dependent variables that consider convective, radiative, and brand transfer modes individually. Although the FCCS model 

now resolves the fire behavior contribution of heterogeneous mixtures of fuelbed 

categories within strata and mixtures of strata within surface layers, it remains 

weak in the ability to predict the interaction of fires in multiple layers such as 

dependent crown fires or surface fires driven by the contribution of litter. Finally, 

we are confident that FCCS manages the influence of live fuel moisture on surface 

more logically than earlier models, but we still base predictions on our hypotheses 

of live moisture dynamics rather than solid scientific evidence.

Table 9 presents an overview of the surface fire behavior equations in FCCS 

and how they differ from the Rothermel (1972) spread model. With few exceptions, 

the main difference between the FCCS (2007) modeling approach and the Rothermel (1972) model is the treatment of more complex fuelbeds with four surface fuel 

strata. The LLM stratum is generally much more densely packed and constitutes a 

different combustion environment than other surface fuels (shrubs, herb, and woody 

fuels). For this reason, the LLM stratum is often treated separately from the other 

surface fuels.

Table 8—Comparison of the original Rothermel (1972) and Fuel Characteristic Classification System 

(FCCS) (2007) fire spread models

Basis of comparison Rothermel (1972) FCCS (2007)

Assumptions Steady state head fire	 Same

Flaming front (fine fuels)	 Same

Uniform fuel moisture, wind, slope Same

Homogenous surface fuels (fuel model) Heterogeneous surface fuels (four strata) 

Objective	 Support fire management operations in standard	 Compare fire behavior between varying

fuels under changing environmental conditions fuelbed characteristics and changing

 (i.e., weather and topography) environmental conditions

Applications	 Surface fire behavior modeling Surface fire behavior modeling

 Fire effects modeling

 Fuel treatment assessment

 Habitat assessments 

Fuel characteristics crosswalk Stylized fuel models Measured fuel characteristics 

Fuel models cannot be crosswalked to realistic fuels Crosswalk from fuelbeds to fuel models 

 (original 13 and standard 40) 
24

GENERAL TECHNICAL REPORT PNW-gtr-887

Type of equation Rothermel equation

Rate of spread (R) 

(equation 25)

 where

 IR = surface reaction intensity 

 (British thermal unit [BTU]/ft2

/min),

	 	 ξ = propagating flux ratio,

	 	 φw = wind coefficient,

	 	 φs = slope coefficient,

	 	 ρb = oven dry particle density (lb/ft3

),

	 	 ε = effective heating number, and

 Qig = heat of preignition (BTU/lb).

R = IR 1 + φw φk

ρb

εQig

ξ +

IR = Г'wnh ηΜηK

Fuel Characteristic Classification System

where

m = subscript reference to rate of spread calculated for all

 surface fuels (RSurf) and the litter, lichen, and moss

 (LLM) stratum (RLLM);

IR = surface reaction intensity (BTU/ft2

/min), calculated

 individually for the LLM stratum and summed for the

 other strata (shrub, herb, woody fuels);

ξ = propagating flux ratio calculated by stratum;

φw = wind coefficient;

φs = slope coefficient; and

q = surface heat sink, individually calculated for each

 stratum and then summed.

R = max(RSurf , RLLM ) Where R = IR 1 + φw φ ξ + s

q

Table 9—Comparison of equation forms in Rothermel (1972) and Fuel Characteristic Classification System 

(FCCS) (2007)

ηβ' η

A

wo h M K F

k

4

k=1

I η η R = Г'

ηβ'

A

where

k = subscript reference to surface fuel stratum;

= reaction efficiency, used to modify reaction velocity

 as in Rothermel but expressed explicitly here;

Г’ = maximum reaction velocity;

wo = oven dry fuel loading. The FCCS does not adjust for 

 mineral content, but the difference is slight (5 percent).

h = fuel particle low heat content (BTU/lb),

ηM = moisture damping coefficient of live and dead fuels,

ηK = mineral damping coefficient (constant = 0.42), and

ηF = flammability index assigned by species and LLM type

(2 = flammable, 1 = neutral).

Surface fire reaction

intensity (IR) 

(equation 16) where

	 	 Г’ = optimum reaction velocity, calculated

 as the product of maximum reaction 

	 	 velocity and reaction efficiency;

 wn = net fuel loading (ovendry fuel loading 

 wo divided by 1 + ST (total mineral 

 content);

 h = fuel particle low heat content (BTU/lb);

	 	 ηM = moisture damping coefficient, and

	 	 ηK = mineral damping coefficient (calculated

 from fuel particle mineral content).

Reaction efficiency	 Embedded within the optimum reaction

 velocity term and expressed as:

(equation 17)

 where

	 	 A = Rothermel’s A is an exponential term

	 	 that lessens the influence of relative

	 	 packing ratio on reaction efficiency in

	 	 very fine fuels and is calculated as

 = relative packing ratio, calculated as the 

 ratio of packing ratio to optimum 

 packing ratio.

ηβ'

A

ηβ'

A

= max

β

βop

A

e

A

1–β

βop

β

βop

A =

 1

(4.774σ0.1 – 7.27)

Reaction efficiency is calculated in the FCCS as:

where

A = Rothermel’s A, Ai = 133σk

-0.7913 and

β' = relative packing ratio, calculated as the ratio of optimum 

 depth and average depth of surface fuels (effective depth).

 This is a comparable calculation of relative packing ratio

 to Rothermel but uses depth instead of packing ratio.

ηβ'

A

= β'

e1–β'

 

A
25

Fuel Characteristic Classification System Version 3.0: Technical Documentation

Table 9—Comparison of equation forms in Rothermel (1972) and Fuel Characteristic Classification System 

(2007) (continued)

Propagating 

flux ratio (ξ)

(equation 28) where

	 	 β = packing ratio.

Type of equation Rothermel equation

ξ = (192 + 0.2595σ)-1 e[(0.792 + 0.681σ0.5)(β + 0.1)

Fuel Characteristic Classification System

ξ = (0.03 + 2.5βε)

This new equation form represents a recalculation of 

Rothermel’s (1972) propagating flux ratio and regresses

empirical data on effective packing ratio (βε) calculated as 

the ratio of the reactive fuel volume to fuel depth (Sandberg 

et al. 2007b).

Heat sink (q) Heat sink denominator term, 

(equation 32) 	 expressed as ρbεQig

 where

	 	 ρb = ovendry particle density (lb/ft3

),

	 	 ε = effective heating number, and

 Qig = heat of preignition (Btu/lb).

φwm = 8.8 

B

1

β'

E

m

U

BMU

where

8.8 = windspeed multiplication constant (see Sandberg 

 et al. 2007 for details),

 = ratio of input windspeed to benchmark 4 mph 

 windspeed,

B = 1.2 (intermediate of published values (Sandberg 

 et al. 2007),

β' = relative packing ratio,

E = 0.55 – 0.2 (FAISurf – FAIWoody )

, and

m = subscript reference to all surface fuels and the 

 LLM stratum.

U

UBMU

FAISurf

Slope coefficient	 φS = 5.275β-0.3tan(Slope)2

(φS) (equation 31)

 where

	 	 β = fuelbed packing ratio, and

 Slope = input slope (°)

φw = CUB -E β

βop

Wind coefficient

(φw) (equation 30)

 where

 C = 747e-0.133σ0.55

 

B = 0.02526σ0.54

 E = 0.715e-3.59σ 104

where

β = packing ratio,

m = subscript reference to all surface fuels and the 

 LLM stratum, and

Slope = input slope (percent).

φSi = 5.275

2 Slope

100 βm

-0.3

where

m = subscript reference to shrub, herb, woody fuels, and 

LLM strata;

ηβ' = reaction efficiency (dimensionless) (equation 18);

RVi

 = the reactive volume of each fuel stratum (ft3/ft2) 

(equation 21a);

PDi

 = particle density summarized by stratum (lb/ft3);

Qigi

 = heat of ignition by stratum (Btu/lb) (equation 34a, 

34b); and

MinEffδi

 = area-weighted average depth of each surface 

fuel stratum (equation 31), capped at a 1 ft maximum in 

order not to reduce the calculated heat sink in deep (i.e., 

greater than 1 ft) stratum.

RV PD Qig

2

m=1

βm m

η ' MinEffδ qSurf =
26

GENERAL TECHNICAL REPORT PNW-gtr-887

Reaction Intensity

Reaction intensity (IR; Btu/ft2/min)—

The heat release rate per unit area of the surface fire flaming front (Rothermel 

1972). In FCCS, it is calculated as the sum of component reaction intensities for 

each stratum including shrubs, herbaceous vegetation, woody fuels, and LLM. 

The LLM stratum differs from other surface fuels in its fuel packing ratio and 

configuration, and its potential reaction intensity is calculated somewhat differently 

from that of other surface fuel strata. Reaction intensity is calculated as the sum of 

component reaction intensities (IR Comp) by surface fuel stratum:

(16)

where

k = subscript reference to a single fuelbed stratum (shrubs, herbs, and woody fuels);

IR Compk

 = component reaction intensity by stratum;

ηβ' = total reaction efficiency of other surface fuels (shrubs, herbs, and woody 

fuels) or LLM (dimensionless) (eq. 17);

A = Rothermel’s A; exponent that reduces the influence of relative packing ratio

on reaction efficiency in very fine fuels (eq. 21);

Г'

max = maximum reaction velocity that would exist at optimum fuelbed depth 

with no fuel moisture or mineral content (1/min) (eq. 22);

wo = flame-available loading of each surface fuel stratum (lb/ft2); includes shrub 

crowns, herbs, flame-available LLM, and a proportion of small woody fuels 

including all 1-hr fuels, 25 percent of 10-hr fuels, and 12.5 percent of 100-hr fuels 

and is equivalent to wn in Rothermel (1972) in that it is net of silica content and 

represents the material available for flaming combustion;

h = low fuel heat content of each surface fuel stratum (Btu/lb) (eq. 23);

ηM = moisture damping coefficient, which reduces reaction velocity 

(dimensionless) (eq. 24);

ηK = mineral damping coefficient, which reduces reaction velocity (0.42, 

dimensionless) and corresponds to a conventional value for silica-free ash 

content of 1 percent established by Rothermel (1972);

IR =

4

k=1

IR Compk

3

k=1

IR Compk = + ηβ' η

A

Г'

max wo h M ηKηF

k

ηβ' η

A

Г'

max wo h MηKηF

LLM
27

Fuel Characteristic Classification System Version 3.0: Technical Documentation

ηF= flammability index,1 weighted by species loading by stratum (eq. 1)

(dimensionless); and

LLM = subscript reference to the LLM stratum. 

Reaction efficiency (ηβ'; dimensionless)—

Reaction efficiency is calculated as a proportional value between 0 and 1 and 

represents the damping effect of inefficiently packed fuels in the reaction intensity 

equation (eq. 16). As fuels approach an optimum packing ratio, reaction efficiency 

approaches 1. All components within a stratum are assumed to burn within a 

single flame envelope and have one reaction efficiency. Strata within a fuel layer 

(i.e., all surface fuels and low surface fuels) are assumed to burn with one reaction 

efficiency. For example, because shrubs rarely burn without lower surface fuels, 

the reaction efficiency of the all surface fuels layer includes shrubs, herbs, and 

woody fuels. Low surface fuels may carry flames without involving shrubs, so 

are assumed to burn with a single reaction efficiency determined by the combined 

characteristics of only the herb and woody fuels strata. Reaction efficiency is 

calculated separately for the LLM stratum and is weighted by LLM type (table 2). 

It is calculated as follows for all surface and lower surface fuel layers:

(17)

where

m = subscript reference to all surface fuels (includes shrubs, herbs, and woody 

fuels), and lower surface fuels (herbs and woody fuels), and

β'm = relative packing ratio (dimensionless) (eq. 18).

Relative packing ratio (β'm; dimensionless)—The ratio of optimum depth to 

actual depth (β'm, dimensionless), used in calculating reaction efficiency (eq. 17):

(18)

where

m = subscript reference to upper surface fuels (includes shrubs, herbs, and woody 

fuels) and lower surface fuels (herbs and woody fuels only),

δopt m = optimum depth (ft) determines the depth that creates an optimum air to 

fuel ratio (eq. 19), and

Effδm = effective depth of surface fuel layer (ft) (eq. 20).

1

 Flammability index is a multiplier based on expert opinion applied to species that burn 

with more intensity than most, generally resulting from differences in fuel chemistry or 

live fuel dynamics. Flammable species = 2, neutral species = 1 (dimensionless).

βm

' = βm

' e1–βm

' η

βm

' = δopt

Effδ m
28

GENERAL TECHNICAL REPORT PNW-gtr-887

Optimum depth (δopt; ft)—The depth at which fuels are optimally packed for 

maximum reaction velocity:

(19)

where

m = subscript reference to upper surface fuels (shrubs, herbs, and woody fuels) and 

lower surface fuels (herbs and woody fuels only),

PVm = volume of particles in a square foot area (ft3/ft2 = ft) (eq. 19a), and

OptAirVolm = optimum air volume of surface fuels and low surface fuels 

(ft3/ft2 = ft) (eq. 19b).

Particle volume (PV; ft3

/ft2)—The volume of particles in a square foot area, 

calculated for upper surface fuels (including shrubs, herbs, and woody fuels), 

and lower surface fuels (herbs and woody fuels only). 

(19a)

where

m = subscript reference to upper surface fuels and lower surface fuels,

BVWoody = bulk volume of the woody fuel stratum (ft3

/ft2),

βWoody = packing ratio of the woody fuel stratum (dimensionless) (eq. 3),

k = subscript reference to shrub stratum and herb stratum, and

PVk = particle volume of the shrub stratum and herb, calculated as dry weight 

biomass (lb/ft2) divided by particle density for each stratum (lb/ft3).

Optimum air volume (OptAirVolm; ft3

/ft2)—The volume of air space between fuel 

particles that would result in maximum reaction intensity.

(19b)

where

m = subscript reference to upper surface fuels and lower surface fuels;

k = subscript reference to shrubs, herbs, and woody fuels for upper surface fuels 

and herbaceous and woody fuels only for lower surface fuels;

45 = is the optimum air to volume ratio of fuel particles (Sandberg et al. 2007b); 

and RVk = the reactive volume of each fuel stratum, calculated as flame-available 

loading divided by particle density (ft3

/ft2) for each stratum (eq. 19c). 

OptAirVolm = 45RVk

k=1

δoptm= PVm+ OptAirVolm

PVm = (BVWoody βWoody) PVk

2

k=1
29

Fuel Characteristic Classification System Version 3.0: Technical Documentation

Reactive volume (RVk; ft3/ft2)—The volume of fuel in each stratum that would 

be involved in flaming combustion and used in calculating optimum air volume

(eq. 19b). Reactive volume of LLM fuels is capped at maximum reactive volume, 

which is calculated as MaxFlameLoading (inferred) divided by LLM particle 

density (lb/ft2). 

(19c)

where

k = subscript reference to shrubs, herbs, woody fuels, and LLM strata;

wFlameAvailable = loading available for flaming combustion, defined as: total shrub 

loading, total herbaceous loading, a portion of woody fuel loading (100 percent 

1-hr, 25 percent 10-hr, and 12.5 percent 100-hr woody timelag classes), and total 

LLM loading (lb/ft2) ; and 

25 = default particle density (lb/ft3).

This definition of RV, based on wFlameAvailable, is different than the “shell 

volume” introduced by Sandberg et al. (2007a), which in turn differs from the 

“surface area” approach inherent in Rothermel (1972). The latest definition recognizes that energy from the combustion of a woody fuel shell thicker than the 

pyrolysis thickness at ignition is available to feed flame intensity and spread.

Effective depth of upper and lower surface fuels (Effδm; ft)—Used in calculating 

relative packing ratio (eq. 18) and calculated as the depth (ft) of upper and 

lower surface fuel layers, weighted by reactive volume and percentage cover. 

(20)

where

m = subscript reference to upper surface fuels (shrubs, herbs, and woody fuels) 

or low surface fuels (herbs and woody fuels only),

k = subscript reference to surface fuel stratum, 

RVk = the reactive volume of each fuel stratum, calculated as flame-available 

loading divided by particle density (ft3/ft2) (eq. 19c),

δk = input depth of each surface stratum (ft), and

CovRatiok = cover of each stratum divided by 100.

RVk = wFlameAvailable k

25

k=1

Effδm = (RV δ CovRatio)k

RVk k=1
30

GENERAL TECHNICAL REPORT PNW-gtr-887

Rothermel’s A (A; dimensionless)—Modifies reaction efficiency in equation (16)

to account for the lower sensitivity of reaction efficiency to relative packing ratio in

flash fuels (or thermally thin fuels). It is set to 1.0 for woody fuels and LLM strata.

For shrub crowns and herbaceous fuels, it is calculated as: 

(21)

where

k = subscript reference to surface fuel stratum (shrubs, herbs, woody fuels, 

and LLM), 

133 = empirical constant (Rothermel 1972), 

σk = surface-area-to-volume ratio by stratum (ft2

/ft3), and

-0.7913 = empirical constant (Rothermel 1972).

Maximum reaction velocity (maxΓ'; per minute)—The reaction velocity that 

would exist at optimum fuelbed depth with no fuel moisture or mineral content; 

used in calculating reaction intensity (eq. 16). Reaction velocity is the ratio of the 

reaction zone efficiency to unit time (Rothermel 1972). Maximum reaction velocity

of woody fuels and LLM strata are 9.495/min and 15/min, respectively. Maximum 

reaction velocity is calculated for shrubs and herbs as:

(22)

where

9.495 = maximum reaction velocity of typical woody fuels (Rothermel 1972), 

k = subscript reference to shrub and herb strata,

σk = surface-area-to-volume ratio of shrubs and herbs (ft2

/ft3). Shrub σ is

calculated as the average of the σFoliage and σWoody, and

σWoody = surface-area-to-volume ratio typical of small woody fuels (488 ft2/ft3).

Equation 22 is a significant departure from the calculation of the Rothermel 

(1972) maximum reaction velocity, which was a curvilinear fit of empirical data 

that in effect reached a maximum value near 16/min for extremely fine fuels 

(thermally thin or flash fuels). This revised equation is benchmarked to Rothermel’s

data when the surface-area-to-volume ratio is 488/ft representative of small woody 

fuels but extrapolated by using an inverse function that allows maximum reaction 

velocity to exceed 16/min for thermally thin fuels. This equation is also a departure 

from the value used by Sandberg et al. (2007b). 

Ak = 133σk

-0.7913

maxΓk = 9.495

σk

σWoody
31

Fuel Characteristic Classification System Version 3.0: Technical Documentation

Heat content (h; Btu/lb)—The low fuel heat content of each surface fuel stratum 

used in calculation of reaction intensity (eq. 16). Heat content is adjusted for live 

foliar moisture content in shrub and herb strata. Woody fuel heat content is assumed to be 8000 Btu/lb, and LLM heat content is inferred by type and weighted 

by loading of each LLM type (e.g., litter type, moss type or lichen) (table 2).

hk = h – FMkV (23)

where

k = subscript reference to shrub and herb strata,

h = default low fuel heat content (8000 Btu/lb),

FMk = input foliar moisture content (percentage) for shrub and herb strata. Fuel 

moisture is expressed as percentage multiplied by 100 in this equation (e.g., 90 

rather than 0.9), and

V = latent heat of vaporization (11.16 Btu/lb).

Moisture damping coefficient (ηM; dimensionless)—The moisture damping 

coefficient reduces reaction velocity in equation (16) and is calculated as the

weighted average of live and dead fuels for each stratum by the following regression equation: 

(24)

where

k = subscript reference to surface fuel stratum (i.e., shrub, herb, woody 

fuels, LLM),

j = subscript reference to live or dead fuels in each stratum,

ηLive = live moisture damping coefficient (dimensionless),

ηDead = dead moisture damping coefficient (dimensionless),

PropDead k = ratio of dead loading to total loading (dimensionless),

FMj = percentage fuel moisture by dry weight of live fuels (live shrubs, 

live herbs) and dead fuels (woody fuels, LLM). 

Dead shrubs and herbs are assumed to have the same fuel moistures as 

small (<3 in diameter) woody fuels, and

Mxj

 = moisture content of extinction for live shrubs, live herbs, dead 

woody fuels and LLM.

Mk

= Live 1–PropDead k + DeadPropDead k η η η

j = 1 – 2.59

FMj

Mxj

+ 5.11

FMj

Mxj

2

– 3.52

FMj

Mxj

3

η
32

GENERAL TECHNICAL REPORT PNW-gtr-887

Rate of Spread 

Surface fire rate of spread (R; ft/min)—

Calculated as the ratio of heat source (defined as the surface fire energy propagated 

to unburned fuels) to surface fuel heat sink (defined as the energy required to 

preheat surface fuels). Owing to the difference in packing ratio between the LLM 

stratum and other surface fuels, litter-dominated fuelbeds may have substantially 

different rates of spread than other fuelbeds. For this reason, R is calculated separately for the LLM stratum. Surface rate of spread is calculated as the maximum 

of the rate of spread of all surface fuels (including shrubs, herbs, woody fuels, and 

LLM) and that of RLLM. Rate of spread is also capped at a maximum based on 

input windspeed and slope.

R = min(WindSlopeCap, RMax) (25)

where

WindSlopeCap = maximum rate of spread based on input windspeed and 

slope (ft/min) (eq. 26) and

RMax = maximum rate of spread (ft/min) (eq. 27).

WindSlopeCap (ft/min)—

Calculated as the maximum possible rate of spread based on input windspeed and 

slope and limits the final rate of spread (eq. 25).

WindSlopeCap = 88 U(1 + φS) (26)

where 

88 = conversion factor from mph to ft/min,

U = input midflame windspeed (mph), and

φS = slope coefficient (eq. 31).

Maximum rate of spread (RMax; ft/min)—

Calculated as the maximum of surface fuel and LLM rates of spread and used in 

equation (25). 

(27)

where

m = subscript reference to surface fuels (RSurf) and the LLM stratum (RLLM),

IRm = reaction intensity of all surface fuels and component IR for the LLM stratum 

(Btu/ft2/min) (eq. 16),

φ φ

RMax = max(RSurf, RLLM) 

Rm = 

IRm m 1+ W+ S m

qSurf m

ξ
33

Fuel Characteristic Classification System Version 3.0: Technical Documentation

ξm = propagating flux ratio, the proportion of IR transferred to unburned fuels 

(dimensionless) (eq. 28),

(1+ φW + φS)m = acceleration of the rate of spread owing to windspeed (φW, eq. 30) 

and slope (φS, eq. 31) (dimensionless), and

qSurf m = heat sink term of all surface fuels and the LLM stratum (Btu/ft3) (eq. 32).

Propagating flux ratio (ξSurf; dimensionless)—A regression equation that estimates the proportion of the reaction intensity that contributes to the forward rate 

of spread (eq. 27). The FCCS equation (Sandberg et al. 2007b) is a slight departure 

from Rothermel (1972) because we assume that the efficiency of heat transfer to the

unburned reactive fuels is directly proportional to the bulk density of those fuels. 

The new equation was recalculated by regressing empirical data from Rothermel 

(1972) on effective packing ratio, defined here as the ratio of reactive volume to

depth. 

(28)

where

0.03 = y-intercept of the regression equation for the propagating flux ratio 

(dimensionless),

2.5 = slope of the regression equation for the propagating flux ratio (percentage),

min = minimum expression,

0.06 = maximum value allowed, which limits propagating flux ratio to ≤ 0.18

(dimensionless),

k = subscript reference to each surface fuel stratum,

RVk = reactive volume of each surface fuel stratum (ft3

/ft2) (eq. 19c), and

δSurfHeatSink = depth of surface fuel heat sink (ft) (eq. 29). 

Propagating flux ratio for the LLM stratum (ξLLM; dimensionless)—

Calculated as:

(28a)

where

RVLLM = reaction volume of the LLM stratum (eq. 19c), and

DepthLLM = total depth of the LLM stratum (ft).

Surf

 = 0.03 + 2.5 min 0.06,

RVk

4

k=1

δSurfHeatSink

ξ

LLM = 0.03 + 2.5min 0.06,

RVLLM

DepthLLM

ξ
34

GENERAL TECHNICAL REPORT PNW-gtr-887

Depth of surface heat sink (δSurfHeatSink; ft)—

Area-weighted average depth of each surface fuel stratum used in calculating 

propagating flux ratio (eq. 28). Calculated as the sum of each stratum depth (ft) 

multiplied by percentage cover, such that: 

(29)

where

k = subscript reference to shrub, herb, woody fuels, and LLM strata; 

δk = depth of each surface stratum (ft) (eq. 4); and

CovRatiok = input percentage cover (percentage) by stratum divided by 100 

(dimensionless). 

Wind coefficient (φW; dimensionless)—

Wind and slope coefficients (eq. 31) modify the heat source term of the rate of 

spread equation (eq. 27). Owing to differences in fuel characteristics and boundary 

conditions between the LLM stratum and other surface fuel strata, a wind coefficient is calculated separately for the LLM stratum. The wind coefficient term in 

the rate of spread equation (eq. 25) is a weighted average of the LLM and surface 

wind coefficients using the relative contribution of LLM and other strata to reaction 

intensity. 

(30)

where

IR.LLM = component reaction intensity of the LLM stratum (eq. 16),

IR = surface fire reaction intensity (eq. 16),

φwSurf) = surface fuel wind coefficient,

φwLLM = LLM wind coefficient,

m = subscript reference to surface fuels or LLM stratum,

8.8 = 

β'm = relative packing ratio of surface fuels or LLM (dimensionless) (eq. 18), 

E = exponential term representing a mild effect of large fuels in reducing the 

accelerating effect of wind on fire spread by attenuating the wind flow within the 

fuelbed (eq. 30a); this term is assumed to be the same for the LLM stratum 

as calculated for all surface fuels,

φw 1 –

IR.LLM

IR

φwSurf + 

IR.LLM

IR

φwLLM

φwm= 8.8

1

β'

m

E

U

BMU

B

1 when = = 1 φwU

BMU β'

E

δSurfHeatSink = δk

4

k=1

CovRatiok
35

Fuel Characteristic Classification System Version 3.0: Technical Documentation

U = input midflame windspeed (ft/min),

BMU = benchmark midflame windspeed constant = 352 ft/min, and

B = exponential response of wind coefficient to windspeed. A null value would 

indicate no response, and unity would indicate a linear response. Rothermel (1972) 

allows B to vary from 0.5 to 2.0. Beer (1993) recommends 1.0. The FCCS (Sandberg 

et al. 2007b) uses a constant value of 1.2 (dimensionless).

Exponential flash term (E; dimensionless)—

FAI-weighted value of surface fuels used in the wind coefficient calculation (eq. 

30). Assumed to be 0.35 for all flash fuels (i.e. σ >714/ft) and 0.55 for all thermally

thick woody fuels (i.e., σ <714/ft) (dimensionless),

(30a)

where

0.55 = y intercept of regression equation (dimensionless);

0.2 = slope term of regression equation (dimensionless);

FAISurf = sum of shrub, herb, and woody fuels FAI (dimensionless) (eq. 2), and

FAIWoody = FAI of woody fuels (dimensionless).

Slope coefficient (φS; dimensionless)—

Wind and slope coefficients modify the heat source term of the rate of spread 

equation (eq. 25). Owing to differences in fuel characteristics and boundary conditions between the LLM stratum and other surface fuel strata, a slope coefficient is 

calculated separately for the LLM stratum. The slope coefficient term in the rate 

of spread equation is a weighted average of the LLM and surface wind coefficients 

using the relative contribution of LLM and other strata to reaction intensity. 

(31)

where

IR.LLM = component reaction intensity of the LLM stratum (eq. 16),

IR = surface fire reaction intensity (eq. 16),

φSSurf = surface fuel slope coefficient,

φSLLM = LLM slope coefficient,

m = subscript reference to surface fuels or LLM stratum,

E = 0.55 – 0.2

FAISurf – FAIWoody

FAISurf

φS 1–

IR.LLM

IR

φSSurf +

IR.LLM

IR

φSLLM

φSm= 5.275 Slope

100

2

βm

-0.3
36

GENERAL TECHNICAL REPORT PNW-gtr-887

5.275 = Rothermel’s (1972) empirically derived minimum slope coefficient, i.e., fire

spread rate at 100 percent slope as a multiple of no-slope spread rate in extremely 

densely packed fuelbeds,

Slope/100 = input slope (percentage) divided by 100,

βm = packing ratio of LLM and all other surface fuels (calculated as the sum of 

shrub, herb, and woody packing ratios) (dimensionless), and

-0.3 = exponential constant derived by Rothermel (1972) to represent observations 

of the effect of fuelbed packing ratio on the influence of slope on fire spread. 

Heat sink (qSurf; Btu/ft3

)—

The heat sink term of the rate of spread equation (eq. 25) is calculated for each 

surface fuel stratum and then summed. This calculation is slightly different in 

its representation of the effect of fuelbed depth than the calculation described by 

Sandberg et al. (2007b). First, the calculated heat sink required for ignition is now 

reduced by the reaction-efficiency term. Second, the effective depth of each stratum 

included in the heat sink term is limited to 1 ft based on an assumption that it is 

seldom necessary to preheat more than 1 ft of depth within a stratum to achieve 

ignition of the stratum. Both corrections are applied to avoid unrealistic predictions 

of fire behavior in very dense or very tall fuelbeds. 

(32)

where

k = subscript reference to shrub, herbs, woody fuels, and LLM strata;

m = subscript reference to reaction efficiency of all surface fuels (applicable to the 

shrub stratum), low surface fuels (applicable to herbs and woody strata), and the 

LLM stratum (ηβ'm, eq. 17);

RVk = the reactive volume of each fuel stratum (ft3

/ft2) (eq. 19c);

PDk = particle density summarized by stratum (lb/ft3

);

Qigk = heat of ignition by stratum (Btu/lb) (eq. 32a,b); and

δSurfHeatSink = depth of surface fuel heat sink (ft) (eq. 29). 

Heat of ignition (Qigk; Btu/lb)—

Heat of pre-ignition, the amount of heat required to ignite 1 lb of fuel. Used in 

the heat sink calculation (eq. 32) and calculated by stratum, heat of ignition is 

calculated as a weighted average of live and dead fuels in shrubs and herb:

Qig (32a) k = QigLive 1 – PropDead k+ QigDeadPropDead k

qSurf = β'

m

RV PD Qig

δSurfHeatSink k

4

k=1

η
37

Fuel Characteristic Classification System Version 3.0: Technical Documentation

where

k = subscript reference to each surface fuel stratum,

QigLive = live fuel heat of ignition (Btu/lb), 

PropDead = proportion of dead fuel loading to total fuel loading, and

QigDead = dead fuel heat of ignition (Btu/lb).

Component heat of ignition (Qigj

; Btu/lb)—

Individually calculated for live shrubs, live herbs, and woody fuels. Dead shrubs, 

dead herbaceous vegetation, and the LLM stratum are assumed to have the same 

heat of ignition as woody fuels:

(32b)

where

j = subscript reference to live shrubs, live herb, and woody fuels,

250 = heat of preignition of dry cellulose (Btu/lb),

V = latent heat of vaporization (11.16 Btu/lb), and

FMj = fuel moisture content (percentage).

Flame Length

Flame length (FL; ft)—

Defined as the distance between the flame tip and the midpoint of the flame depth 

at the base of the flame (Albini 1976, Byram 1959). In the FCCS, flame length is 

calculated as the product of reaction intensity and rate of spread:

(33)

where

0.45 = constant of proportionality (Byram 1959),

IR = surface reaction intensity (Btu/ft2

/min) (eq. 16),

R = rate of spread (ft/min) (eq. 25),

tR = flame residence time (min) (eq. 34), and

0.46 = constant (Fons 1946).

Flame residence time (tR; min)—Defined as the time fuels contribute to

propagating flux and is estimated as:

(34)

where

192 = unit conversion based on the commonly used assumption that woody fuels 

consume at rate of 8 min/in, as limited by thermal diffusivity (Anderson 1969),

k = subscript reference to shrubs, herb, woody fuels, and LLM strata,

IRk

 = contribution to reaction intensity of each fuelbed stratum (Btu/ft2/min) 

(eq. 16), and

FL = 0.45 IR R tR

0.46

tR = 192

IR RT k

4

k=1

IR

Qig j = 250 + V FM j
38

GENERAL TECHNICAL REPORT PNW-gtr-887

RTk = reaction thickness of each stratum (ft) (eq. 35).

Reaction thickness (RT; ft)—

Reaction thickness, used to estimate flame residence time (eq. 34), is the approximate thickness of a fuel element shell that contributes to reaction intensity. Reaction 

thickness is estimated to be 0.0028 ft in thermally-thick fuel elements (Sandberg et. 

al. 2007b) based on data from Frandsen (1973). When the diameter of a fuel element 

is less than twice the reaction thickness, the entire fuel element contributes to reaction intensity, and reaction thickness is simply the radius (or one-half the thickness) 

of the fuel element.

(35)

where

k = subscript reference to shrubs, herbs, woody fuels, and LLM strata, 

0.0028 = assumed maximum reaction thickness (ft),

2/σ = radius (or one-half the thickness) of the fuel element, and

σk = surface-area-to-volume ratio (ft2

/ft3). Shrub surface-area-to-volume ratio 

is adjusted as the average between inferred shrub and woody surface-area-tovolume ratio to represent woody portions and foliar portions of shrub components.

Fuel Model Crosswalks 

As part of each fuelbed calculation, FCCS provides a static crosswalk to one of the 

13 original fire behavior prediction system (FBPS) fuel models (Anderson 1982) 

and one of the 40 standard fire behavior fuel models (Scott and Burgan 2005). The 

crosswalk is invalid under any other environmental scenario and is generally not 

appropriate for simulating fire spread across landscapes, as in FARSITE (Finney 

1998). Fuel model crosswalks may be used in FlamMap (Finney 2006) as long 

as input wind, slope, and fuel moistures remain the same as in the original FCCS 

prediction.

A crosswalk to one of the original 13 FBPS fuel models is first calculated based 

on vegetation type grouping and closest match to predicted rate of spread (ft/min) 

and flame length (ft). Predicted values of R and FL of the original fuel models were 

calculated in BEHAVE v3.0 using a D2L2 moisture scenario (Andrews et al. 2003) 

and 4 mph midflame windspeed and 0 percent slope inputs, and they were then 

compared to the same values calculated for fuelbeds in FCCS 2.2. The suggested 

crosswalk to the standard 40 fuel models is only a recommendation because there 

are multiple standard models that may have predicted fire behavior similar to any 

FCCS fuelbed. 

RTk = min 0.0028, 2

σk
39

Fuel Characteristic Classification System Version 3.0: Technical Documentation

FBPS Fuel Models 

Fire behavior predictions are first grouped into three main categories based on the 

ratio of rate of spread (R) and flame length (FL).

• Forest: ratio <3:1 (i.e., R = 3 FL)

•	 Brush and timber slash or litter: 3:1 ≤ ratio <6:1

•	 Grass: ratio ≥ 6:1

Fuelbeds are then assigned a fuel model based on their predicted FL or R, or 

both (table 10). Fuel model 7 is never selected because it is basically identical to 

fuel model 6 but with a different moisture content of extinction. 

Table 10—Fuel model crosswalk to the original 13 fire behavior fuel models

Fuelbed type (based on

ratio rate of spread/ Fuel

flame length [R:FL])	 model(s)	 Flame length grouping

Forest (ratio <3:1) 8 FL < 2 ft

	 	 9	 2 ft ≤ FL < 4 ft (2–4 ft)

	 	 10	 4 ft ≤ FL < 6 ft (4–6 ft)

	 	 12	 6 ft ≤ FL < 8 ft (6–8 ft)

	 	 13	 FL ≥ 8 ft (> 8 ft)

Brush and timber slash or 	 4	 FL ≥ 14 ft (> 14 ft)

	 litter (3:1 ≥ ratio <6:1)	 5	 6 ft ≤ FL < 14 ft (6-14 ft)

 6 (or 7)a	 2 ft ≤ FL < 6 ft and R ≥ 12 ft/min (2–6 ft)

	 	 11	 2 ft ≤ FL < 4 ft and R < 12 ft/min (2–4 ft)

 8 FL < 2 ft

Grass (ratio ≥ 6:1)	 1	 36 ft/min ≤ R < 84 ft/min (36–84 ft/min)

 2 R < 36 ft

	 	 3	 R ≥ 84 ft/min

a

 Suggested fuel model crosswalks do not include fuel model 7, but this fuel model could be selected in place of 

fuel model 6 in shrub fuels with particularly high moisture contents of extinction.

Standard Fuel Models

The FCCS crosswalk to standard fuel models follows the procedure outlined by 

Scott and Burgan (2005). The first step is to crosswalk to one of the 13 original 

models as outlined above (table 10). The second step is to calculate the departure 

of predicted R and FL between FCCS fuelbed calculations and fuel models used in 

BehavePlus (Andrews et al. 2003) estimates at D2L2 and 4 mph windspeed. The 

FCCS calculates a ratio value of each fuelbed's predicted FL and R to FBPS fuel 

model benchmarks (table 11). It then makes a crosswalk assignment to one of the 40 

standard fuel models (table 12). In many cases, FCCS selects one of several close 

matches. Users are encouraged to consider alternative matches by comparing FCCS 

and BehavePlus outputs that may provide better coincidence of fire behaviors over 

the range of expected fuel and weather conditions.
40

GENERAL TECHNICAL REPORT PNW-gtr-887

Table 11—Adjective assignment of the percentage 

difference between predicted rate of spread (R) and 

flame length (FL) values and benchmark values

Adjective class to be used in

the table 12 look up table Ratio rangea

Low <0.50

Slightly lower 0.50 to <0.85

Comparable 0.85 to <1.15

Slightly higher 1.15 to <1.50

Higher 1.50 to <2.00

Much higher 2.00 to <3.00

High > 3.00

a Ratio values are calculated as the ratio of predicted R or FL to 

benchmark values. A ratio value of 1 indicates an exact match.

(36)

(37)

where

FLFCCS = predicted FCCS flame length (ft),

FLFuelModel = benchmark FBPS flame length (ft),

RFCCS = predicted FCCS rate of spread (ft/min), and

RFuelModel = benchmark FBPS flame length (ft/min).

Table 12—Crosswalk matrix from original Fire Behavior Prediction System (FBPS) fuel model assignment to 

standard fuel models

Ratio of FCCS	 	 Ratio of FCCS flame length to benchmark flame length

rate of spread

to benchmark Slightly Slightly Much

Original FBPS fuel modela rate of spreadb Low lower Comparable higher Higher higher High

Standard fuel model codec

1—Short grass (1 ft) Low GR1 GR1 GR1 GR4 GR7 GR9 GR9

 Slightly lower GR1 GR1 GR2 GR4 GR7 GR9 GR9

 Comparable GR1 GR2 GR2 GR4 GR7 GR9 GR9

 Slightly higher GR1 GR2 GR2 GR5 GR8 GR9 GR9

 Higher GR1 GR2 GR3 GR5 GR8 GR9 GR9

 Much higher GR2 GR3 GR3 GR6 GR9 GR9 GR9

 High GR2 GR3 GR3 GR6 GR9 GR9 GR9

2—Timber (grass and understory) Low GS1 GR1 GR3 GR3 GR4 GR5 GR8

 Slightly lower GS1 GR1 GR3 GR4 GR4 GR5 GR8

 Comparable GS1 GR2 GR3 GR4 GR4 GR5 GR8

 Slightly higher GS2 GR2 GR3 GR4 GR4 GR5 GR8

 Higher GS2 GR3 GR3 GR4 GR5 GR6 GR9

 Much higher GR3 GR3 GR3 GR4 GR5 GR7 GR9

 High GR3 GR3 GR3 GR4 GR5 GR7 GR9

FuelModelFL = closest match = 1.0 ≈ FLFCCS

FLFuelModeli

FuelModelR = closest match = 1.0 ≈ RFCCS

RFuelModeli
41

Fuel Characteristic Classification System Version 3.0: Technical Documentation

Table 12—Crosswalk matrix from original fire behavior prediction system (FBPS) fuel model assignment to 

standard fuel models (continued)

Ratio of FCCS	 	 Ratio of FCCS flame length to benchmark flame length

rate of spread

to benchmark Slightly Slightly Much

Original FBPS fuel modela rate of spreadb Low lower Comparable higher Higher higher High

Standard fuel model codec

3—Tall grass (2.5 ft) Low GR3 GR4 GR5 GR7 GR8 GR9 GR9

 Slightly lower GR4 GR4 GR6 GR7 GR8 GR9 GR9

 Comparable GR4 GR4 GR6 GR7 GR8 GR9 GR9

 Slightly higher GR4 GR4 GR6 GR7 GR8 GR9 GR9

 Higher GR4 GR4 GR6 GR8 GR8 GR9 GR9

 Much higher GR4 GR4 GR6 GR8 GR9 GR9 GR9

 High GR4 GR4 GR6 GR8 GR9 GR9 GR9

4—Chaparral Low SH5 SH6 SH6 SH6 SH6 SH6 SH6

 Slightly lower SH5 GR8 SH6 SH6 SH6 SH6 SH6

 Comparable SH6 GR8 GR8 GR8 GR8 SH9 SH9

 Slightly higher SH7 SH7 GR8 GR8 GR8 GR8 GR8

 Higher SH8 SH8 GR8 GR8 GR8 GR8 GR9

 Much higher GR8 GR8 GR8 GR8 GR8 GR9 GR9

 High GR8 GR8 GR8 GR8 GR9 GR9 GR9

5—Brush Low SH1 GR1 TU5 TU5 SH7 SH7 SH7

 Slightly lower SH1 GR1 TU5 TU5 SH7 SH7 SH7

 Comparable SH1 GS2 TU5 TU5 SH7 SH7 SH7

 Slightly higher SH1 GS2 TU5 TU5 SH7 SH7 SH7

 Higher SH1 GS2 TU5 TU5 SH7 SH7 SH7

 Much higher SH2 GS2 TU5 SH5 SH5 SH5 SH5

 High SH2 GS2 TU5 GR8 GR8 GR8 SH5

6—Dormant brush, hardwood slash Low SH2 SH2 SH4 SH4 SH6 SH6 SH6

 Slightly lower SH2 SH2 SH4 SH4 SH6 SH6 SH6

 Comparable SH2 SH2 SH4 SH4 SH6 SH6 SH9

 Slightly higher SH2 SH2 SH4 SH4 SH6 SH9 SH9

 Higher SH2 SH4 SH4 SH6 SH6 SH9 SH5

 Much higher SH2 SH4 SH4 SH6 SH5 SH5 SH5

 High SH2 SH4 SH4 SH5 SH5 SH5 GR8

7—Southern rough Low SH3 SH3 SH4 SH4 SH7 SH9 SH9

 Slightly lower SH3 SH3 SH4 SH4 SH7 SH9 SH9

 Comparable SH3 SH3 SH4 SH4 SH7 SH9 SH9

 Slightly higher SH3 SH3 SH4 SH4 SH7 SH9 SH9

 Higher SH3 SH3 SH4 SH4 SH7 SH9 SH5

 Much higher SH3 SH4 SH4 SH7 SH7 SH5 SH5

 High SH3 SH4 SH4 SH7 SH9 SH5 GR8

8—Closed timber litter Low TL1 TL3 TL3 TL3 TL7 TL8 TL8

 Slightly lower TL1 TL3 TL3 TL4 TL7 TL8 TL8

 Comparable TL1 TL3 TL3 TL4 TL7 TL8 TL8

 Slightly higher TL1 TL3 TL3 TL4 TL7 TL8 TL8

 Higher TL1 TL3 TL3 TL4 TU1 TL6 TL8

 Much higher TL3 TL3 TL3 TL4 TL5 TL6 TL8

 High TL3 TL3 TL4 TL4 TL5 TL6 TL8
42

GENERAL TECHNICAL REPORT PNW-gtr-887

Table 12—Crosswalk matrix from original fire behavior prediction system (FBPS) fuel model assignment to 

standard fuel models (continued)

Ratio of FCCS	 	 Ratio of FCCS flame length to benchmark flame length

rate of spread

to benchmark Slightly Slightly Much

Original FBPS fuel modela rate of spreadb Low lower Comparable higher Higher higher High

Standard fuel model codec

9—Hardwood (long-needle pine) litter Low TL2 TL3 TL5 TL8 TL9 TL9 TL9

 Slightly lower TL3 TL3 TL6 TL8 TL9 TL9 TL9

 Comparable TL3 TL3 TL6 TL8 TL9 TL9 TL9

 Slightly higher TL3 TL3 TL6 TU2 TL9 TL9 TL9

 Higher GR1 GR1 TL6 TU2 TU4 SH6 SH8

 Much higher GR1 GR1 GR1 TU2 SH6 SH8 SH8

 High GR1 GR1 GR1 GR2 SH6 SH8 SH8

10—Timber (litter and understory) Low TU1 TU1 TU2 TU5 TU5 TU5 TU5

 Slightly lower TU1 TU1 TU2 TU5 TU5 TU5 TU5

 Comparable TU1 TU2 TU2 TU5 TU5 TU5 TU5

 Slightly higher TU1 TU2 TU4 TU5 TU5 TU5 TU5

 Higher TU1 TU2 TU4 TU3 TU3 TU3 TU3

 Much higher TU1 TU2 TU4 TU3 TU3 TU3 TU3

 High TU2 TU2 TU4 TU3 TU3 TU3 TU3

11—Light slash Low TL5 TL5 SB1 SB1 SB2 SB2 SB2

 Slightly lower TL5 TL5 SB1 SB1 SB2 SB2 SB2

 Comparable TL5 TL5 SB1 SB1 SB2 SB2 SB2

 Slightly higher TL5 TL5 SB1 SB1 SB2 SB2 SB2

 Higher TL5 TL5 SB1 SB1 SB2 SB2 SB2

 Much higher TL5 SB1 SB1 SB2 SB2 SB2 SB2

 High TL5 SB1 SB2 SB2 SB2 SB2 SB2

12—Medium slash Low SB1 SB2 SB3 SB3 SB3 SB3 SB3

 Slightly lower SB1 SB2 SB3 SB3 SB3 SB3 SB3

 Comparable SB1 SB2 SB3 SB3 SB3 SB3 SB3

 Slightly higher SB1 SB2 SB3 SB3 SB3 SB3 SB3

 Higher SB2 SB2 SB3 SB3 SB3 SB3 SB3

 Much higher SB2 SB2 SB3 SB3 SB3 SB3 SB3

 High SB2 SB3 SB3 SB3 SB3 SB3 SB3

13—Heavy slash Low SB2 SB2 SB3 SB3 SB3 SB4 SB4

 Slightly lower SB2 SB2 SB3 SB3 SB3 SB4 SB4

 Comparable SB2 SB2 SB3 SB3 SB3 SB4 SB4

 Slightly higher SB2 SB2 SB3 SB3 SB3 SB4 SB4

 Higher SB2 SB2 SB3 SB3 SB3 SB4 SB4

 Much higher SB2 SB2 SB3 SB3 SB3 SB4 SB4

 High SB2 SB3 SB3 SB3 SB4 SB4 SB4

Note: FCCS = Fuel Characteristic Classification System. a Anderson (1982).

b

 Scott and Burgan (2005).

c Adjective classes are defined in table 11.
43

Fuel Characteristic Classification System Version 3.0: Technical Documentation

FCCS Fire Potentials

The FCCS fire potentials are a set of relative values that rate the intrinsic properties 

of a wildland fuelbed, including its potential for surface fire behavior, crown fire 

behavior, and available fuel for consumption under a benchmark set of windspeed 

and fuel moisture conditions (90 percent foliar moisture of shrubs; 60 percent 

foliar moisture of herbaceous fuels; 6 percent fuel moisture of 1-hr, 7 percent fuel 

moisture of 10-hr, and 8 percent fuel moisture of 100-hr woody fuels; no slope, 4 

mph windspeed) (Sandberg et al. 2007a). They are intended for use in mapping fire 

hazard, categorizing fuelbeds on the basis of predicted fire behavior, and communicating degree of fire hazard. They can also be used in conjunction with known 

environmental conditions to predict fire behavior and effects (i.e., scaled downward 

to reflect fuel moisture conditions, or scaled upward to represent windspeed or 

slope). The FCCS potentials calculator reports nine FCCS fire potentials for each 

fuelbed arranged into three summary FCCS fire potentials (fig. 2). 

Reaction

potential

Spread

potential

Flame

length

potential

Crown fire

initiation

potential

Crown-to-crown

transmissivity

potential

Crown fire

rate of spread

potential

Flame

available

fuel

Smoldering

available

fuel

Residual

available

fuel

Fire behavior

potential

Crown fire

potential

Available fuel

potential

416

FCCS Potential

4 = Modest surface fire behavior (9.8-ft flames, 13.3 ft min-1 rate of spread) 

1 = Low crowning potential (1 on scale of 9) 

6 = Above-average fuel loading (60 tons/acre)

Figure 2—Diagram of Fuel Characteristic Classification System (FCCS) potential.
44

GENERAL TECHNICAL REPORT PNW-gtr-887

The FCCS fire potentials are organized in three categories:

Surface fire behavior summary potential (SFP; dimensionless)—

Surface fire behavior potential is calculated as the maximum of the FL and R 

potentials, scaled to values of 0 to 9 (dimensionless). The three components of 

FBP are:

• Reaction potential (RP) (eq. 39)—Represents the reaction intensity 

(energy release per unit area per unit time) under dry fuel conditions, scaled 

to an index value. 

• Spread potential (SP) (eq. 40)—Is proportional to the R (distance per unit 

time at benchmark environmental conditions) in surface fuels.

• Flame length potential (FP) (eq. 41)—Provides a measure of predicted FL 

(in feet, at benchmark environmental conditions) proportional to fireline 

intensity or FL.

Crown fire summary potential (CFP; dimensionless)—

Crown fire summary potential derives from application of the FCCS crown fire 

model described by Schaaf et al. (2007), which uses concepts and algorithms from 

Van Wagner (1977), Alexander (1988), Scott and Reinhardt (2001), and Sandberg 

et al. (2007b). Crown fire potential is scaled from 0 to 9 (dimensionless) and is 

calculated as three subpotentials:

• Crown fire initiation (IC; dimensionless)—Is the ratio of surface fireline 

intensity to critical fireline intensity (eq. 43). Represents the likelihood that 

a surface fire will propagate into a crown fire.

• Crown-to-crown transmissivity (TC; dimensionless)—Represents the 

potential for fire to carry through a forest canopy (eq. 49).

• Crown fire rate of spread (RC; dimensionless)—Is a relative index of 

crown fire spreading potential (eq. 54).

Available fuel summary potential (AFP; dimensionless)—

Available fuel potential estimates the mass of fuel present within the outside shell of 

layers of canopy, surface, and ground fuel elements that are potentially combustible 

under dry conditions in the flaming, smoldering, and residual smoldering phases 

of combustion. The summary index and subpotentials are expressed in units of 10 

tons/ac. The summary index is confined to 0 to 9, but subpotentials can exceed 9 

to represent actual available loading values. Depths of woody fuel shell (1⁄2 in, 2 in, 

and 4 in) involved in each combustion phase were estimated from fuel consumption 

models within Consume (Prichard et al. 2007).

• Flame available fuel (FA; 10 tons/ac)—Is the sum of mass within 1⁄2 in of 

the surface of the fuel element available for the flaming stage of combustion. This variable is the sum of three subcomponents:
45

Fuel Characteristic Classification System Version 3.0: Technical Documentation

▪ Flame-reactive surface available fuel (FAR)—Is the mass of fuel 

consumed in the flaming front of a spreading surface fire. FAR contributes to forward energy transfer and is the mass of thermally thin 

fuel elements plus a thin skin of larger fuel elements with a thickness that represents the depth of the pyrolysis zone. The surface fuel 

includes shrub crowns, herb, woody fuels, and LLM strata.

▪ Flame-available postreactive surface fuels (FAP)—Is the remainder 

of flame-available surface fuel after the passage of the flaming front. 

This variable represents the flaming consumption that occurs behind 

the flaming front and does not contribute to FL or propagating flux.

▪ Flame-available canopy fuel (FAC)—I s the mass of tree canopy and 

fine twigs.

• Smoldering available fuel (SA; 10 tons/ac)—Is the sum of mass between 

1⁄2 and 2 in of a fuel surface.

• Residual available fuel (RA; 10 tons/ac)—Is the sum of mass between 

2 and 4 in of a fuel surface.

Surface Fire Potentials

FCCS calculates the summary surface fire behavior summary potential (SFP) as 

the maximum of the spread potential and FL potential, scaled to values of 0 to 9. 

This summary potential value does not include reaction potential. The surface fire 

behavior potential is calculated as:

(38)

where

SP = spread potential (eq. 39) and

FP = flame length potential (eq. 40).

Spread potential (SP; dimensionless)—

Spread potential is proportional to the R (ft/min) in surface fuels at benchmark 

environmental conditions. Rate of spread is a function of reaction intensity, propagating energy flux, and heat sink provided by unburned fuels in advance of the 

spreading flame. 

(39)

where

NSP = scaling factor = 2√R) (dimensionless) and

R = surface rate of spread (ft/min) (eq. 25). 

SFP = min[9, max (SP, FP)]

SP = min 9, NSPR
46

GENERAL TECHNICAL REPORT PNW-gtr-887

FP = NFP FL

Flame length potential (FP; dimensionless)—

Flame length potential is a scaled measure of predicted FL (ft) at benchmark 

environmental conditions. 

(40)

where

NFP = scaling factor = 2√FL), and

FL = flame length (ft) (eq. 33).

Reaction potential (RP; dimensionless)—

Reaction potential represents reaction intensity, expressed as energy release per unit 

area and time at benchmark environmental conditions, and is a function of volume 

of fuels per unit of ground surface, depth of the surface fuelbed strata, heat of combustion, damping coefficients due to moisture and mineral content, and a scaling 

factor. Although RP is included as a subpotential, it is not used in the calculation 

of the surface fire behavior summary potential. Reaction potential is the reaction 

intensity (Btu/ft2

/min) times a scaling factor.

(41)

where

NRP = reaction potential scaling factor = 0.08√IR) (dimensionless) and

IR = reaction intensity (Btu/ft2/min) (eq. 16).

Crown Fire Potentials

The FCCS crown fire potentials (CFP) include crown fire initiation, crown-tocrown transmissivity, and crown fire rate of spread. They are calculated within 

the FCCS CFP calculator. The FCCS CFP build on predictions in FCCS 2.2 that 

incorporate equations published by Schaaf et al. (2007). The FCCS crown fire 

modeling framework is theoretical and would require further scientific development and verification to provide actual predictions of crown fire intensity and RP. 

The FCCS currently ranks the relative potential for crown fire initiation and spread 

on as indexed values between 0 and 9 under default windspeed and fuel moisture 

conditions (Sandberg et al. 2007a). Crown fire initiation and spread depends on a 

widely varying combination of fuels, weather, and topographic conditions (Cruz et 

al. 2006, Cruz and Alexander 2010). Important predictors of crown fire initiation 

and spread include surface fire intensity, canopy closure, crown density, presence of 

ladder fuels, and crown base height (Schaaf et al. 2007). 

RP = NRP(IR) 
47

Fuel Characteristic Classification System Version 3.0: Technical Documentation

Crown fire summary potential (CFP; dimensionless)—

The crown fire summary potential scales the three subpotentials to an index value 

between 0 and 9. It places more emphasis on crown fire initiation and rate of spread 

than crown-to-crown transmissivity (Schaaf et al. 2007). 

(42)

where

0.4286 = scaling factor to limit CFP between 0 and 9 (dimensionless),

IC = crown fire initiation potential (dimensionless) (eq. 43), 

TC = crown-to-crown transmissivity potential (dimensionless) (eq. 49), and

RC = crown fire rate of spread potential (dimensionless) (eq. 53).

Crown fire initiation potential (IC; dimensionless)—

The FCCS crown fire initiation potential represents the likelihood of a surface 

fire torching into single or multiple trees. If there are no flammable species in the 

overstory or understory, crown fire initiation is assumed to be zero.

(43)

where

FAIFlammUC = FAI (eq. 2) of flammable upper canopy subcategories, including 

overstory and midstory trees and class 1 snags with-foliage. Flammable species are 

defined as those that contribute energy to the fire. Neutral species, including many 

broadleaf deciduous trees, are assumed to not contribute to the flammable canopy,

9 = maximum allowable value for IC index,

4 = scaling factor (dimensionless),

ICq = crown fire initiation quotient (dimensionless), and

0.2 = scaling factor (dimensionless).

Crown fire initiation quotient (ICq; dimensionless)—

Ratio of surface fireline intensity to critical fireline intensity. 

(44)

where

IS = surface fireline intensity (Btu/ft2

/min) (eq. 45) and

I' = critical fireline intensity (Btu/ft2/min) (eq. 46).

ICq = IS

I'

If FAI FlammUC = 0, then IC = 0 

Else, IC = min 9, 4 ICq

0.2 

CFP = 0.4286 IC + TC

3 + RC
48

GENERAL TECHNICAL REPORT PNW-gtr-887

Surface fireline intensity (IS; Btu/ft/sec)—Numerator term in the crown fire initiation quotient (eq. 44). Surface fireline intensity is the product of surface reaction

intensity, flame residence time, and R:

(45)

where

IR = surface reaction intensity (Btu/ft2

/min) (eq. 16),

tR = flame residence time (min) (eq. 34),

R = surface fire R (ft/min) (eq. 25), and

60 = unit conversion (s/min).

Critical fireline intensity (I'; Btu/ft/sec)—Denominator term in the crown fire

initiation quotient (eq. 44). Critical fireline intensity is the intensity required for a

surface fire to transition into a crown fire (Scott and Reinhardt 2001).

(46)

where

FMCAdj = adjusted canopy fuel moisture (percentage) (eq. 47),

GapCanopy = canopy gap (ft) (eq. 48),

3/2 = assumed power-law vertical rate of temperature decrease in convective plume 

(Schaaf et al. 2007) (dimensionless), and

1718.2 = unit conversion (kJ/m/sec to BTU/ft/sec).

Adjusted canopy fuel moisture (FMCAdj; percentage)—Used in the critical 

fireline intensity equation (eq. 46) and calculated as:

(47)

where

460 = empirical constant in Van Wagner’s (1977) adjustment of energy required for

canopy ignition (dimensionless),

25.9 = empirical slope of Van Wagner’s (1977) adjustment of energy required for

canopy ignition (dimensionless), and

FMC = input canopy fuel moisture (default = 90 percent).

FMCAdj = 460 + 25.9

FMC

100

IS = IR tR

R

60

I

'

 =

FMCAdj GapCanopy

3

2

1718.2
49

Fuel Characteristic Classification System Version 3.0: Technical Documentation

Canopy gap (GapCanopy; ft)—Used in the critical fireline intensity equation (eq.

46), canopy gap is a function of the gap between surface and canopy fuels and an 

assigned ladder fuel rating (table 13). Individual gap terms are calculated for the 

upper canopy (overstory and midstory trees) and understory trees. A decision rule is 

employed to determine whether canopy gap equals the overstory gap or understory 

gap term, such that:

where

GapIntermediate = intermediate gap term (ft) (eq. 48a),

2.5 = hypothetical threshold of the vertical gap to FAI ratio that determines whether 

the understory acts as an effective ladder fuel to the overstory,

FAIUnderstory = Fuel area index of understory trees (dimensionless) (eq. 2),

GapUC = upper canopy gap term (ft) (eq. 48), and

GapUnderstory = understory tree gap term (ft) (eq. 48b).

Intermediate gap term (GapIntermediate; ft)—Estimated as the difference between 

the height to live crown of overstory and midstory trees and the maximum height of 

understory trees, shrubs or the effective surface depth.

where

HLCUC = minimum height to live crown of overstory and midstory trees and 

height to crown base of class 1 snags with foliage (ft),

HeightUnderstory = height of understory trees (ft),

δShrub = depth of shrub stratum (ft),

EffδSurf = effective depth of surface fuels (ft) (eq. 20), and

LadderFuelRating = inferred by ladder fuel type (dimensionless) (table 13).

Individual canopy gap terms (Gapm; ft)—Upper canopy and understory gap 

terms are calculated by tree subcategory: 

(48b)

If Gap Intermediate < 2.5

FAI Understory

3π

, then GapCanopy = max 1, min Gap UC, Gap Understory

Else GapCanopy = GapUC

(48)

GapIntermediate = min 0,

HLC UC – max HeightUnderstory , δShrub ,Eff δsurf

LadderFuelRating + 1

(48a)

Gapm = min 0,

HLCmin – max (δShrub ,Eff δsurf

LadderFuelRating + 1

)
50

GENERAL TECHNICAL REPORT PNW-gtr-887

Crown-to-crown transmissivity potential (TC; dimensionless)—

Crown-to-crown transmissivity is an index of the relative likelihood of a crown fire 

to spread horizontally through the canopy. If the fuel area index of the flammable 

canopy is less than a threshold FAI, transmissivity is assumed to be zero. Otherwise, it is a function of canopy cover and windspeed. Windspeed is included here as 

a placeholder for future versions where input windspeed may modify CFPs.

(49)

where

FAIUC = fuel area index (eq. 2) of the flammable upper canopy subcategories, 

including overstory and midstory trees and class 1 snags with foliage 

(dimensionless),

TFAI = threshold FAI value (eq. 50),

3π = hypothesized lowest threshold TFAI that will support crown-to-crown

fire spread (dimensionless),

10 = scaling factor (dimensionless), and

TCq = efficiency of crown-to-crown heat transfer as a proportion of 

efficiency when canopy cover is 100 percent (eq. 51).

where

m = subscript reference to upper canopy and understory trees,

HLCmin = minimum height to live crown of overstory, midstory trees, and height to 

crown base of Class 1 snags for the upper canopy; input height to live crown for the 

understory (ft),

δShrub = depth of shrub stratum (ft),

EffδSurf = effective depth of surface fuels (ft) (eq. 20), and

LadderFuelRating = inferred by ladder fuel type (dimensionless) (table 13).

TC = 

If FAIUC < 

TFAI

3π , then TC = 0

Else TC = min 9, 10TCq

Table 13—Ladder fuel rating by type

Rating

Type Box uncheckeda Box checked

Arboreal lichen 1 3

Dead branches 0 2

Climbing ferns/other epiphytes 0 2

Stringy or fuzzy bark 1 3

Vines-liana 0 2

Leaning snags 0 2

Tree regeneration 1 3

a If the “Is there vertical continuity between the canopy and lower strata?” box is

checked, ladder fuel rating are rated higher than if there is no vertical continuity 

between surface and crown fuels.
51

Fuel Characteristic Classification System Version 3.0: Technical Documentation

Threshold FAI (TFAI; dimensionless)—Used to determine whether crown-tocrown transmissivity (eq. 49) is zero or is equal to the TC quotient (eq. 51).

(50)

where

A = 2.6296 if σUC ≤ 2,000 (ft2

/ft3) and 3.2868 if σUC >2,000 (ft2/ft3),

σUC = surface-area-to-volume ratio of upper canopy (overstory trees, midstory 

trees, and class 1 snags with foliage),

-0.0019 = coefficient describing the reduction in threshold FAI at higher 

windspeeds after Schaaf et al. (2007), and

U = input midflame windspeed converted to ft/min.

TCq (dimensionless)—Used in calculating transmissivity potential (eq. 48). 

TCq estimates the efficiency of crown-to-crown heat transfer as a proportion

of maximum efficiency at 100 percent canopy cover.

(51)

where

CovC = input total canopy cover (percentage),

WAF = canopy windspeed adjustment factor, set to 1 for the benchmark windspeed 

of 4 mph (ft/min) (eq. 52),

40 = threshold canopy cover to initiate dependent crown spread (percentage), and

0.3 = coefficient describing the assumed effect of crown cover on transmissivity at 

benchmark windspeed. 

Canopy windspeed adjustment factor (WAF; dimensionless)—The canopy windspeed adjustment factor increases or decreases effective canopy cover depending 

on whether input midflame windspeeds are higher or lower than the benchmark

windspeed. Currently, this value is set to 1 because a default windspeed of 4 mph is 

used. It is calculated as a function of input windspeed, vertical stack velocity, and 

benchmark windspeed, such that:

(52)

where

U = input windspeed (352 ft/min),

VS = vertical stack velocity (900 ft/min), and

BMU = benchmark midflame windspeed (352 ft/min).

TCq = 

max 0,(CovCWAF – 40)0.3

(100WAF – 40)0.3

WAF =

U U2

+VS2 ⁄

BMU BMU2 +VS2 ⁄

TFAI = Ae-0.0019U
52

GENERAL TECHNICAL REPORT PNW-gtr-887

Crown fire rate of spread potential (RC; dimensionless)—

Index of the crown fire R. With little available empirical data, crown fire behavior 

is difficult to model, and actual Rs currently are not possible to predict. The potential index is calculated as:

(53)

where

2.5 = scaling factor and

RC = crown fire R ratio (eq. 54).

Crown fire rate of spread (RC; dimensionless)—The ratio of canopy heat source to 

canopy heat sink: 

(54)

where

IR = surface fire reaction intensity (Btu/ft2

/min) (eq. 16),

IR C = crown fire reaction intensity (Btu/ft2/min) (eq. 55),

ξC = canopy propagating flux ratio (dimensionless) (eq. 58),

WAF = canopy windspeed adjustment factor, set to 1 for the 4 mph benchmark 

windspeed (dimensionless) (eq. 52), and

qC = canopy heat sink (Btu/ft3) (eq. 59).

Crown fire reaction intensity (IR.C; Btu/ft2

/min)—Analogous to surface fire

reaction intensity (eq. 16) and is calculated as:

(55)

where

j = subscript reference to overstory trees, midstory trees, understory trees, and class 

1 snags with foliage,

ηβ' UC = upper canopy reaction efficiency (dimensionless) (eq. 56),

maxΓ' = maximum reaction velocity of canopy fuels (15/min),

ηK = mineral damping coefficient, which reduces reaction velocity 

(0.42; dimensionless),

Wo j

 = Fuel loading of flammable trees (overstory, midstory, and understory) 

and class 1 snags with foliage (lb/ft2), and

hAdj j = Low fuel canopy heat content adjusted for fuel moisture content 

(Btu/lb) (eq. 57). Because only one crown fuel moisture is input, this value 

will always be the same for all tree layers.

RC = 

HeatSourceC

HeatSinkC

= IR+ IR C CWAF

qC

ξ

IRC = β'

UC

max ' K Woj hAdj

4

j=4

η Г η

RC = min 9, 2.5RC

1

e
53

Fuel Characteristic Classification System Version 3.0: Technical Documentation

Reaction efficiency, upper canopy (ηβ'UC; dimensionless)—Modifies crown

fire reaction intensity (eq. 55). Reaction efficiency represents the damping effect

of inefficiently packed fuels in the upper canopy.

(56)

where

β'UC = relative packing ratio (dimensionless) (eq. 56a).

Relative packing ratio, upper canopy (β'UC; dimensionless)—Calculated as the 

ratio of optimum depth to mean depth of the upper canopy. This is equivalent to the 

proportion of canopy space that is occupied relative to the optimum packing ratio.

(56a)

where

δopt UC = optimum depth of the upper canopy (ft) (eq. 56b) and

δUC = depth of the upper canopy, calculated as the maximum height minus the 

minimum height to live crown of the overstory, midstory tree layers, and class 1 

snags with foliage (ft).

Optimum depth, upper canopy (δopt.UC; ft)—The canopy depth that would result 

in the maximum reaction velocity, assuming that canopy fuels behave similarly to 

surface fuels and have the same optimum air volume (eq. 19b). 

(56b)

where

0.4 = optimum spacing between canopy fuels (ft),

FAIUC = fuel area index of the flammable upper canopy layers (defined eq. 43), 

βC = packing ratio of the upper canopy stratum, including overstory and midstory 

trees and class 1 snags with foliage (eq. 3),

ACVUC = bulk crown volume of overstory and midstory tree layers and class 1 

snags with foliage, adjusted for species relative cover and crown shape factor 

(ft3/ac) (eq. 5a), and

43,560 = conversion factor to ft (ft2/ac).

β'

UC

= βUC

' e1– βUC

'

η

βUC

' = 

δopt UC

δUC

δopt UC = 0.4FAIUC + βC

ACVUC

43,560
54

GENERAL TECHNICAL REPORT PNW-gtr-887

Low fuel heat content, upper canopy (hAdj; Btu/lb)—Contributes to crown fire reaction intensity (eq. 55). Upper canopy heat content is adjusted for each tree subcategory based on input canopy fuel moisture and a constant latent heat of evaporation:

(57)

where

h = inferred low fuel heat content of each canopy layer (Btu/lb),

j = subscript reference to overstory, midstory, and understory tree layer 

subcategory,

FMC = input tree crown fuel moisture; one input applies to all canopy tree 

layers (percentage), and

V = latent heat of vaporization (11.16) (Btu/lb).

Canopy propagating flux ratio (ξC; dimensionless)—Represents the proportion 

of the canopy reaction intensity that contributes to a crown fire's forward rate of

spread (eq. 54).

(58)

where

FAIUC = fuel area index of flammable upper canopy trees (eq. 43) (dimensionless),

4 = coefficient defining the rate of attenuation of radiant energy from fires in 

vegetative canopies (Butler et al. 2004), and

δUC = depth of the upper canopy, calculated as the maximum height minus the 

minimum height to live crown of the overstory, midstory tree layers, and class 1 

snags with foliage (ft).

Canopy heat sink (qC; Btu/ft3

)—Incorporates the component heat sinks of canopy 

branch wood, canopy foliage, and surface fuels. 

(59)

where

qCanopyWood = heat sink of canopy branches (eq. 59a; Btu/ft3

),

qCanopyFoliage = heat sink of canopy foliage (eq. 59a; Btu/ft3), and

qSurf = heat sink of surface fuels (eq. 32; Btu/ft3).

hAdj

j

= hj – FMCV

C= 1 – e- FAIUC

ξ 4δUC

qC = qCanopyWood + qCanopyFoliage+ qSurf
55

Fuel Characteristic Classification System Version 3.0: Technical Documentation

Individual heat sink terms (qj; Btu/ft3)—Canopy heat sink terms that comprise 

the total canopy heat sink (eq. 59). 

(59a)

where

j = subscript reference to canopy branch wood or foliage,

0.5 = division term to divide the upper canopy FAI into two equal components: 

canopy branch wood and foliage (dimensionless),

FAIUC = fuel area index of the flammable upper canopy layers (eq. 43), 

ςj = ignition thickness of canopy branch wood or foliage, assumed to be 

equivalent to reaction thickness of herbaceous and woody surface fuels, 

respectively (eq. 35; ft).

ρp = default particle density (25 lb/ft3), 

QigC= canopy heat of ignition equaation 959b); Btu/lb),

θC = input canopy total percentage cover divided by 100, and

δUC = depth of the upper canopy, calculated as the maximum height minus 

the minimum height to live crown of the overstory, midstory tree layers, and 

class 1 snags with foliage (ft).

Canopy heat of ignition (QigC; Btu/lb)—The amount of heat required to 

ignite 1 lb of canopy fuel.

(59b)

where

250 = heat of ignition of dry cellulose (Btu/lb),

V = latent heat of vaporization (11.16) (Btu/lb), and

FMC = input tree crown fuel moisture; one input applies to all canopy tree 

layers (percentage).

Canopy foliage ignition thickness (ςCFoliage; ft)—Ignition thickness of the canopy 

foliage is the approximate thickness of a fuel element shell that contributes to the 

canopy heat sink (eq. 59a). 

(59c)

where

0.0028 = assumed maximum ignition thickness (ft),

2/σ = radius (or one-half the thickness) of the fuel element, and

σCFoliage = average surface-area-to-volume ratio (ft2

/ft3) of upper canopy foliage 

(overstory and midstory trees and class 1 snags with foliage) weighted by loading.

 qj =

0.5 FAIUC ςj ρp QigC

θC δUC

QigC = 250 + (V FMC)

ςCFoliage = min 0.0028, 2

σCFoliage
56

GENERAL TECHNICAL REPORT PNW-gtr-887

Available Fuel Potentials

Available fuel potentials are calculated in the FCCS AFP Calculator and approximate the combustible biomass under extremely dry moisture conditions in each of 

three phases of combustion (flaming, smoldering, and residual smoldering). They 

represent the upper limit of biomass consumption at extreme conditions, so act 

as an initial step for predicting biomass consumption, emissions, and related fire 

effects. 

Available fuel potential (AFP; 10 tons/ac)—

By default, FCCS calculates AFP as the sum of the three component potentials. 

Available fuel potentials are scaled by 10 tons/ ac for convenient conversion to 

actual tons/ac. The AFP summary potential is capped at 9 (90 tons/ac), but no 

limits are placed on subpotentials so that actual fuel loads are represented. For 

example, a RA of 13 represents 130 tons/ac of fuel available for residual smoldering 

combustion.

(60)

where

FA = flame available fuel potential (10 tons/ac) (eq. 61),

SA = smolder available fuel potential (10 tons/ac) (eq. 62), and

RA = residual smolder available fuel potential (10 tons/ac) (eq. 63).

Flame available fuel potential (FA; 10 tons/ac)—

The sum of fuel loadings expected to be consumed in all flaming phases of fire 

(mass of fuels within 1⁄2 in of the surface of the fuel element). Class loadings 

(class w; tons/ac) are listed in table 14.

(61)

where

NFA = 0.1; scaling factor to confine range of available fuel potential to 0 to 10,

Class w1 = sum of canopy flame available fuel (class C) and reactive surface 

available fuel (Class R) (tons/ac),

Class w2 = components that may consume a portion in flames (tons/ac), and

Class w3 = components that may consume a shell to a constant depth in flames 

(tons/ac).

AFP = min 9, FA + SA + RA

FA = NFA Class w 1, 2, 3

3

i=1
57

Fuel Characteristic Classification System Version 3.0: Technical Documentation

Table 14—Available fuel potential (AFP) calculations by fuel loading classa

Fuel

loading Fuel consumed Consumable carbon

class	 Definition	 Equation/list	 (yi

)

b equation (C)

Percentage Tons/ac

Class 1 Components that may WFlammable overstory crown y1Canopy C = 0.5W (y/100)

 consume entirely

	 	 in flames (lb/ft2

) WLadder fuels y1Canopy C = 0.5W (y/100)

 WFlammable understory crown y1Canopy C = 0.5W (y/100)

 WNeedle drape y1Canopy C = 0.5W (y/100)

 WFlammable shrub crown y2Shrub C = 0.5W (y/100)

 WNeutral shrub crown y2Shrub C = 0.5W (y/100)

 WLive herb y3Herb C = 0.5W (y/100)

 WDead herb y41-hr C = 0.5W (y/100)

 W0–1/4 in sound wood y41-hr C = 0.5W (y/100)

 W1/4–1 in sound wood y510-hr C = 0.5W (y/100)

 WFlame LLM = min [WLLM, max flame loading]	 y12Litter C = 0.5W (y/100)

Class 2 Components that may W100-hr < 1/2 in = 0.8 W1–3 in sound wood y7100 hr flame

 consume a proportion

	 	 in flames (lb/ft2

) WLive shrub stems < 3in = 0 y2Shrubs C = 0.5W (y ⁄100)

 W15% rotten stump y91000-hr C = 0.5W (y/100)

 W5% class 2 and 3 snags y91000-hr C = 0.5W (y/100)

 W35% lightered stump y12Litter C = 0.5W (y/100)

C = 0.5W (y ⁄100)

0.8

Smolder available fuel potential (SA; 10 tons/ac)—

The sum of fuel loadings expected to be consumed in the smoldering phase of a 

fire (mass of fuels between 1⁄2 and 2 in of a surface). Class loadings (class w; 

tons/ac) are listed in table 14.

(62)

where

NSA = 0.1; scaling factor to confine range of available fuel potential to 0-10,

Class w4 = components that may consume a shell to a constant depth in smoldering 

(tons/ac), and Class w5 = components that may consume a proportion in smoldering 

(tons/ac).

SA = NSA Class w 4, 5

2

i=1
58

GENERAL TECHNICAL REPORT PNW-gtr-887

Table 14—Available fuel potential (AFP) calculations by fuel loading classa (continued)

Fuel

loading Fuel consumed Consumable carbon

class	 Definition	 Equation/list	 (yi

)

b equation (C)

Percentage Tons/ac

Class 3 Components that may W1000-hr < 1⁄2in = 25(FAISound wood >3 inc/24) y91000-hr flame C = 0.5(W1000-hr < 1/2 in

 consume a shell to a + W1/2 in < 1000 < 2 in )

 constant depth in

	 	 flames (lb/ft2

) WRotten < 1 in = 18.72(FAIRotten wood /12) y510 hr C = 0.4W (y ⁄100)

Class 4 Components that may W1 in < Rotten < 2 in = 18.72 lb/ft3

 (FAIRotten wood /12) y13Duff C = 0.4W (y ⁄100)

 consume a shell to a 

 constant depth in WDuff < 4 in depth y13Duff C = 0.4W (y ⁄100)

 smoldering (lb/ft2

)

 W1/2in< 1000 < 2in = 24.96 lb/ft3

(FAISound wood >3in /24) y111000-hr smolder C = 0.5 (W1000-hr < 1/2 in

 + W1⁄2 in < 1000 < 2 in

 (y/100)

Class 5 Components that may WSmolder LLM = min (0, WLLM – WFlame LLM) y12Litter C = 0.5W (y ⁄100)

 consume a proportion

 in smoldering (lb/ft2

) W100-hr > 1/2 in = 0.2W1–3 in Sound wood y8100 hr smolder

 W30% Jackpot y16Piles C = 0.5W (y ⁄100)

 W30% Piles y16Piles C = 0.5W (y ⁄100)

Class 6 Components that may WSquirrel middens y14residual duff	 C = 0.5W (y ⁄100)

 consume totally in

 residual smoldering WBasal accumulation y15residual rotten C = 0.5W (y ⁄100)

 (lb/ft2

)

Class 7 Components that may W20% Jackpot y15residual rotten C = 0.4W (y ⁄100)

 consume a proportion

 in residual smoldering W85% Rotten stump y15residual rotten C = 0.4W (y ⁄100)

 (lb/ft2

)

 W90% class 2 and 3 snag y15residual rotten	 C = 0.4W (y ⁄100)

 W65% lightered stump y15residual rotten	 C = 0.4W (y ⁄100)

Class 8 Components that may W1000 > 2 in = max [0, (WSound wood > 3 in y14residual duff C = 0.4W (y ⁄100)

 consume a shell to a – W1000-hr < 1⁄2 in – W1/2 in < 1000 < 2 in)]

 constant depth in

 residual smoldering W4 in < Duff < 12 in = max [0, (WDuff – WDuff < 4 in) y14residual duff C = 0.4W (y ⁄100)

	 "C = 0.4W" ("y" ⁄"100" )

 (lb/ft2

) Maximum allowable value is 128,000 lb/ac

 WRotten > 2 in= max [0, (WRotten wood y14residual duff C = 0.4W (y ⁄100)

 – W1–2 in Rotten – WRotten < 1 in)]

a

 W = fuel loading (tons/ac), y = predicted fuel consumption (table 15).

b

 See table 15 for percentage of fuel consumed (y) equations.

c

 If depth > 4 in, then . WDuff >4 = 4

DepthDuff

WDuff

C = 0.5W (y ⁄100)

0.2
59

Fuel Characteristic Classification System Version 3.0: Technical Documentation

Residual smolder available fuel potential (RA; 10 tons/ac)—

The sum of fuel loadings expected to be consumed in the residual smoldering or 

“glowing” phase of a fire (mass of fuels between 2 and 4 in of a surface). Class 

loadings (class w, tons/ac) are listed in table 14.

(63)

where

NRA = 0.1; scaling factor to confine range of available fuel potential to 0 to 10,

Class w6 = components that may consume totally in residual smoldering (tons/ac),

Class w7 = components that may consume a proportion in residual smoldering 

(tons/ac), and

Class w8 = components that may consume a shell to a constant depth in residual 

smoldering (tons/ac).

RA = NRA Class w 6, 7, 8

2

i=1

Table 15—Combustible carbon equations

 Benchmark Percentage

 fuel consumed at

 moisture benchmark

EquationID Component A B Ind Var (percent) Equation form moisture 

y1 Canopy 260 -0.0185 CanopyFM 90 y = A[exp(B*IndVar)] 49

y2 Shrub -0.731 107 ShrubFM 90 y = A(IndVar + B) 41

y3 Herbaceous -.891 107 HerbFM 60 y = A(IndVar + B) 54

y4 1-hr .00025 4 1hrFM 6 y = 100 – [A(IndVarB] 100

y5 10-hr .160 2 10hrFM 7 y = 100 – [A(IndVarB)] 92

y6 100-hr (total) -3.330 100 100hrFM 8 y = A(IndVar + B) 73

y7	 100-hr (flame)	 	 	 100hrFM	 8	 y = total - smolder	 59

y8 100-hr (smolder) .027 30 100hrFM 8 y = A[(IndVar-B)2

] 14

y9 1000-hr (total) -1.800 74.4 1000hrFM 12 y = A(IndVar + B) 53

y10	 1000-hr (flame)	 	 	 1000hrFM	 12	 y = total - smolder	 27

y11 1000-hr (smolder) .022 41 1000hrFM 12 y = A[(IndVar-B)2

] 25

y12 Litter .356 1.75 10hrFM 7 y = 100 – [A(IndVarB)] 89

y13 Duff 99.2 -.00572 DuffFM 50 y = A[exp(B*IndVar)] 75

y14 Residual duff -.006 94 DuffFM 50 y = A[(IndVar2

) + B] 79

y15 residual rotten -.000536 98 1000hrFM 150 y = A[(IndVar2

) + B] 86

y16 Piles -.99 94 1000hrFM 12 y = A(IndVar + B) 82

y17 Slash (total) -3 124 1000hrFM 12 y = A(IndVar + B) 88

y18	 Slash (flame)	 .036	 41	 1000hrFM	 12	 y = total – smolder	 58

y19 Slash (smolder) .036 41 1000hrFM 12 y = A(IndVar – B)2 30
60

GENERAL TECHNICAL REPORT PNW-gtr-887

Fuel Consumption and Combustible Carbon

FCCS estimates biomass consumption for each available fuel category, expressed as 

a percentage of available fuel. The equations are approximate in that they each only 

include a single fuel moisture as an independent variable, neglecting the influence 

of fuel loading, fire behavior, wind, and the condition of other categories of fuel 

(table 15). They are only intended to provide a reasonable midrange estimate of 

consumption, emissions, carbon yield, and fire effects. For fuelbed-specific and 

project-specific estimates of fuel consumption, we recommend the use of CONSUME (Prichard et al. 2007) or other more complex algorithms. 

Available fuel consumption, wConsumed, (tons/ac) is estimated as:

(64)

where

j = subscript reference to fuelbed category,

yj

 = fuel consumed (percentage) by fuelbed category, from table 15, and

AFPj = available fuel per fuelbed category (10 tons/ac).

Combustible carbon (carbon) (tons/ac) is estimated as:

(65)

where

j = subscript reference to fuelbed category,

cj

 = carbon mass fraction of biomass (dimensionless), normally assumed to equal 

0.5 for all sound or living biomass and equal to 0.4 for all rotten and duff categories 

(Penman et al. 2003), and

wConsumed j = estimated fuel consumption by fuelbed category (tons/ac).

Future Model Development and Validation

To date, the FCCS surface fire behavior predictions have been compared to values 

obtained in BehavePlus for the 13 and 40 fire behavior fuel models. The FCCS 

predictions are within the range of fuel model predictions and reflect greater heterogeneity in fuelbed loadings and structure represented in fuelbeds as compared 

to fuel models (Sandberg et al. 2007a). The FCCS fire behavior predictions have 

not been validated against published fire behavior or in field-based trials. This is 

an important next step in model validation. Future development of the FCCS will 

include validating calculated canopy loading and surface fire behavior predictions 

with measured fire behavior in wildland fires.

wConsumed j = 10 y AFP

19

j=1

Ccarbon = cjwConsumed
61

Fuel Characteristic Classification System Version 3.0: Technical Documentation

Metric Equivalents

When you know: Multiply by: To find:

Inches (in) 2.54 Centimeters (cm)

Feet (ft) .305 Meters (m)

Acres (ac) .405 Hectares (ha)

Square feet (ft2

) .0929 Square meters (m2)

Cubic feet (ft3

) .0283 Cubic meters (m3)

Pounds (lb) .454 Kilograms (kg)

Pounds per square feet (kg/m2

) 4.88 Kilograms per square meter (lb/ft2)

Pounds per cubic feet (lb/ft3

) 16.0184 Kilograms per cubic meter (kg/ha)

Pounds per acre (lb/ac) 1.12 Kilograms per hectare (kg/ha)

Number per acre (number/ac) 2.471 Number per hectare (number/ha)

Feet per minute (ft/min) .305 Meters per minute (m/min)

Miles per hour (mph) 1.609 Kilometers per hour (kph)

British thermal units (Btu) 1,050 Joules (J)

British thermal units per 2326 Joules per kilogram (J/kg)

pound (Btu/lb)

British thermal units per 189.14 Joules per square meter per

square feet per minute second (J/m2

/s)

(Btu/ft2

/min)

Acknowledgments

We thank Geoff Cushon, Jennifer Long, Cynthia Riccardi and Mark Schaaf for 

scientific development; Ella Elman, Preetma Kooner, and Karen Kopper for fuelbed 

development; Paul Campbell, Rabi Lahiri, Tao Long, and Jeff Silverman for 

software engineering, and team members of the Fire and Environmental Research 

Applications for insightful reviews and software testing. Research and development was sponsored by the U.S. Department of Agriculture, Forest Service, Pacific 

Northwest Research Station and National Fire Plan; U.S. Department of the Interior, 

Bureau of Land Management, National Park Service, and Fish and Wildlife Service; 

and the Joint Fire Science Program. 
62

GENERAL TECHNICAL REPORT PNW-gtr-887

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creating fuelbeds for resource planning. Canadian Journal of Forest Research. 

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66

GENERAL TECHNICAL REPORT PNW-gtr-887

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conceptual framework for ranking crown fire potential in wildland fuelbeds.

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Fuel Characteristic Classification System Version 3.0: Technical Documentation

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of Agriculture, Forest Service, Intermountain Forest and Range Experiment 

Station. 13 p.
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GENERAL TECHNICAL REPORT PNW-gtr-887

Appendix—Summary of Equations

Equation Abbreviation/ Source

number	 Variable	 symbol	 equation	 Unit

1 Weighted average of inferred variables yk

2 Fuel area index FAI Dimensionless

3	 Packing ratio	 β	 	 Dimensionless

4	 Depth	 δ	 	 ft or inch

5 Loading (tree crowns) wTreeCrown tons/ac

5a Adjusted crown volume (trees) ACVTree 5 ft3/ac

5b Bulk crown volume YTree 5 ft3/ac

6 Loading (aboveground trees) wTree tons/ac

7 Loading (class 1 snags with foliage crowns) wSnagC1 tons/ac

7a Adjusted crown volume (class 1 snags with foliage) ACVSnagC1 7 ft3

/ac

7b Bulk crown volume (class 1 snags with foliage) YSnagC1 7 ft3/ac

8 Loading (snag wood) wSnagWood tons/ac

9 Loading (shrubs) wShrub tons/ac

9a Loading (shrub crowns) wShrubCrowns 9 tons/ac

10 Loading (stumps) wStump tons/ac

11 Loading (LLM) wLLM tons/ac

12 Loading (duff) wDuff tons/ac

13 Loading (basal accumulation) wBA tons/ac

13a Bulk volume (basal accumulation) YBA 13 ft3/ac

14 Loading (squirrel middens) wSM tons/ac

14a Bulk volume (squirrel middens) YSM 14 ft3/ac

15 Total carbon C tons/ac

16 Reaction intensity IR Btu/ft2/min

17	 Reaction efficiency	 ηβ' 16 Dimensionless

18	 Relative packing ratio	 β'	 17	 Dimensionless

19	 Optimum depth	 δopt 18 ft

19a Particle volume PV 19 ft3/ft2

19b Optimum air volume OptAirVol 19 ft3/ft2

19c Reactive volume RV 19b ft3/ft2

20	 Effective depth	 Effδ	 18	 ft

21 Rothermel’s A A 16 Dimensionless

22	 Maximum reaction velocity	 max Γ'	 16	 1/min

23 Heat content h 16 Btu/lb

24	 Moisture damping coefficient	 ηM 16 Btu/lb

25 Rate of spread R ft/min

26 Wind/slope cap WindSlopeCap 25 ft/min

27 Maximum rate of spread RMax 25 ft/min

28	 Propagating flux ratio (surface)	 ξSurf 27 Dimensionless

28a	 Propagating flux ratio (LLM)	 ξLLM 27 Dimensionless

29	 Depth of surface heat sink	 δSurfHeatSink 28 ft

30	 Wind coefficient	 φW 27 Dimensionless

30a	 Exponential flash term	 E	 30	 Dimensionless
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Fuel Characteristic Classification System Version 3.0: Technical Documentation

Equation Abbreviation/ Source

number	 Variable	 symbol	 equation	 Unit

31	 Slope coefficient	 φS 27 Dimensionless

32 Heat sink qSurf 27 Btu/ft3

32a Heat of ignition Qig 32 Btu/lb

32b Heat of ignition by component Qigj 32 Btu/lb

33 Flame length FL ft

34 Flame residence time tR 33 min

35 Reaction thickness RT 34 ft

36 Flame length fuel model crosswalk FuelModelFL Dimensionless

37 Rate of spread fuel model crosswalk FuelModelR Dimensionless

38	 Surface fire behavior potential	 SFP	 	 Index (0–9)

39 Reaction potential RP Index (0–9)

40 Spread potential SP Index (0–9)

41 Flame length potential FP Index (0–9)

42	 Crown fire summary potential	 CFP	 	 Index (0–9)

43	 Crown fire initiation potential	 ICP	 	 Index (0–9)

44	 Crown fire initiation quotient	 Ic	 43	 Dimensionless

45	 Surface fireline intensity	 Is	 44	 Btu/ft2/min

46	 Critical fireline intensity	 I'	 44	 Btu/ft2/min

47 Adjusted canopy fuel moisture FMCAdj 46 Percentage

48 Canopy gap GapCanopy 46 ft

48a Intermediate gap term GapIntermediate 48 ft

48b Individual canopy gap terms Gap 48 ft

49 Crown-to-crown transmissivity potential TCP Index (0–9)

50 Threshold FAI TFAI 49 Dimensionless

51 Tc quotient TcQuotient 49 Dimensionless

52 Canopy wind speed adjustment factor WAF 51 Dimensionless

53	 Crown fire rate of spread potential	 RCP	 	 Dimensionless

54	 Crown fire rate of spread	 RC 53 ft/min

55	 Crown fire reaction intensity	 IR.C 54 Btu/ft2/min

56	 Reaction efficiency, upper canopy	 ηβ'UC 55 Dimensionless

56a	 Relative packing ratio, upper canopy	 β'UC 56 Dimensionless

56b	 Optimum depth, upper canopy	 δopt.UC 56a ft

57 Low fuel heat content, upper canopy hAdj 55 Btu/lb

58	 Canopy propagating flux ratio	 ξC 54 Dimensionless

59 Canopy heat sink qC 54 Btu/ft3

59a Individual canopy heat sink terms q 59 Btu/ft3

59b Canopy heat of ignition QigC 59 Btu/lb

59c	 Canopy foliage ignition thickness	 ςCFoliage 59a ft

60 Available fuel summary potential AFP 10 tons/ac

61 Flame available fuel potential FA 60 10 tons/ac

62 Smolder available fuel potential SA 60 10 tons/ac

63 Residual smolder available fuel potential RA 60 10 tons/ac

64 Available fuel consumption wConsumed tons/ac

65 Combustible carbon CCarbon tons/ac
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GENERAL TECHNICAL REPORT PNW-gtr-887

Glossary

Definitions taken directly from the National Wildfire Coordinating Group (2012) 

glossary of wildland fire terms are noted with an asterisk (*).

Adjusted crown volume—The volume of the overstory and midstory tree crowns 

and class 1 snag crown foliage that is flammable. 

Available fuel potential—The relative value of combustible biomass under ovendry moisture conditions during the flaming, smoldering, and residual phases of 

combustion, calculated as a multiple of the total fuel loading of all fuel elements 

within a set depth from the surface of the fuel component.

Basal accumulation—Needles, twigs, bark pieces, litter, and duff that accumulate 

and form over time at the base of trees, creating a deeper organic layer than exists 

in the surrounding area.

Broadleaf—A conventional term applied to trees and shrubs of the flowering 

plant class Angiospermae, in contrast to the class Gymnospermae, which generally 

includes needle-leaved and cone-bearing plants.

Broadleaf forest—A forest dominated by deciduous trees with a percentage 

canopy cover of greater than 60 percent and height exceeding 6 ft.

*Btu (British thermal unit)—Amount of heat required to raise 1 lb of water 1 °F 

(from 59.5 to 60.5 °F), measured at standard atmospheric pressure.

Bulk density—The weight per unit volume of wood or foliage that includes the 

bulk air space between individual pieces of wood or foliage (lb/ft3

).

Canopy cover—The proportion of area covered by a vertical projection of the 

outermost perimeter of the natural spread of trees, including small openings within 

the canopy.

Canopy reaction efficiency—The reaction intensity (energy release per unit area 

per unit time) from the canopy fuels expressed as the ratio to the optimum reaction 

intensity (<1).

Canopy stratum—The fuel layer in the FCCS that characterizes fuels in the tree, 

snag, and ladder fuel categories.

Category—Each stratum is divided into at least two categories. For example, the 

canopy stratum has tree, snag, and ladder fuel categories, and the shrub stratum has 

a primary and secondary layer. Many categories in FCCS are further divided into 

subcategories.
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Fuel Characteristic Classification System Version 3.0: Technical Documentation

Combustion—The process of burning. Fuel combustion consists of three phases in 

the FCCS: flaming, smoldering and residual smoldering.

Conifer forest—A forest dominated by needle-leaf trees with a percentage canopy 

cover of greater than 60 percent and height exceeding 6 ft.

Critical fireline intensity—The intensity required for a surface fire to transition 

into a crown fire (Scott and Reinhardt 2001).

Crown—The part of a tree or woody plant bearing live branches and foliage.

Crown bulk density—The density of the tree crowns including the bulk air space 

between branches and foliage (lb/ft3

).

*Crown fire—A fire that advances from top to top of trees or shrubs more or less 

independent of a surface fire. Crown fires are sometimes classed as running or 

dependent to distinguish the degree of independence from the surface fire.

Crown fire potential—The three crown fire potentials include crown fire initiation, crown-to-crown transmissivity, and crown fire rate of spread. The FCCS 

crown fire summary potential scales the three subpotentials to an index value 

between 0 and 9. It places more emphasis on crown fire initiation and rate of spread 

than crown-to-crown transmissivity. 

Crown packing ratio—Packing ratio (β) of tree crowns by species; defined as the

fraction of crown volume occupied by fuel, including needles and fine branches. 

Crown shape factor—A geometrical adjustment factor used to differentiate crown 

shapes of coniferous and broadleaf trees in volume and loading calculations. 

Decay class—A class of wood decay from recently killed trees (decay class 1) to 

highly decomposed wood (decay class 3). 

Density—Amount per unit area. For example, the number of trees per acre.

Depth—The vertical extent of a fuel layer. For example, the depth of duff or 

downed woody fuels.

Diameter at breast height (d.b.h.)—A standard measurement of tree diameter 

4.5 feet above the average ground level.

Downed woody fuel—Dead, woody fuel that has fallen on the ground.

Duff—The partially decomposed organic material above mineral soil that lies 

beneath freshly fallen twigs, needles, and leaves. Sometimes referred to as F 

(fermentation) and H (humus) layers.
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GENERAL TECHNICAL REPORT PNW-gtr-887

Duff derivation—The parent material from which duff was derived. Examples of 

the parent material include needles, leaves, moss, bark flakes, or woody debris.

Evergreen—Plants that remain green all year by retaining at least some of their 

leaves at all times.

FCCS—Fuel characteristic classification system. A catalog of inherent physical 

properties of wildland fuelbeds and associated calculations of fuel characteristics 

and fire potentials.

FCCS fire potential—A set of relative values that rate the intrinsic physical capacity of any wildland fuelbed to release energy, spread, crown, consume, and smolder 

under benchmark dry fuel conditions, 4 mph midflame windspeeds and flat 

ground. These potentials are calculated from the loading, heat content, bulk density, 

and characteristic thickness of fuel elements without varying moisture content or 

environmental conditions. 

*Fire behavior—The manner in which a fire reacts to the influences of fuel, 

weather, and topography.

Fire behavior fuel model—The formulation of fuelbed inputs into fuel models that 

allow realistic estimation of fire behavior. 

Flame available fuel—Fuel available for consumption during the flaming combustion phase.

*Flame length—The distance between the flame tip and the midpoint of the flame 

depth at the base of the flame. An indicator of fire intensity.

Flame length potential—The potential flame length of a fuelbed. Proportional to 

fireline intensity or flame length.

*Flaming combustion—Phase of a fire in which the fuel is ignited and consumed 

by flaming combustion.

*Flash fuels—Highly combustible fine fuels such as grass, leaves, draped pine 

needles, fern, tree moss and some kinds of slash, which ignite readily and are 

consumed rapidly when dry. 

Flammability index—An assigned flammability index of 2 for accelerant species 

that contain volatile compounds that contribute to fire behavior (Pyne et al. 1996) 

and 1 for neutral species that do not contribute to fire behavior.

Foliar moisture content—Quantity of moisture in live leaves and needles 

expressed as a percentage of dry weight biomass.
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Fuel Characteristic Classification System Version 3.0: Technical Documentation

Fuel—combustible material. In wildland fuels, fuel can include any organic material, including live and dead vegetation, litter, and duff.

Fuel area index (FAI)—Total fuel surface area per unit ground area (ft2

/ft2)

Fuelbed—The inherent physical characteristics of fuels that contribute to fire 

behavior and effects. The FCCS describes fuelbeds in 6 horizontal layers including 

canopy, shrubs, herb, woody fuels, litter-lichen-moss, and ground fuels. Each layer, 

or stratum, is further divided into one or more categories to represent the complexity of wildland and managed fuels. A fuelbed can represent any scale that the user 

considers to be mostly uniform.

Fuelbed categories—Groupings of fuels within a fuel stratum that share common 

combustion characteristics. For example, the canopy stratum contains three categories—trees, snags, and ladder fuels. There are 16 fuelbed categories in total.

Fuelbed depth—Average height of surface or canopy fuels contained in the 

combustion zone of a spreading fire front.

Fuel loading—The dry weight biomass per unit area of combustible material 

(tons/ac). 

Fuel model—A mathematical description of fuels required for the solution of the 

Rothermel (1972) fire spread model.

Fuel model crosswalk—A static crosswalk to one of the 13 original fire behavior 

prediction system (FBPS) fuel models (Anderson 1982) and one of the 40 standard 

fire behavior fuel models (Scott and Burgan 2005) based on the closest match in 

surface fire rate of spread and flame length. 

Fuelbed subcategories—Further grouping of fuels within a fuelbed category with 

common combustion characteristics. For example, snags are further divided into the 

following subcategories—class 1 snags with foliage, class 1 snags without foliage, 

class 2 snags, and class 3 snags.

Gap—A separation in fuel continuity that affects the progression of the flames 

between fuel elements.

Grass—Member of the family Gramineae and characterized by hollow stems and 

circular cross sections and bladelike leaves arranged on the culm or stem in two 

ranks.

Ground fire—A fire that burns low-growing vegetation, litter, and organic material in the soil layer.
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GENERAL TECHNICAL REPORT PNW-gtr-887

Ground fuels stratum—Fuel layer within FCCS that characterizes the duff, 

squirrel midden, and basal accumulation fuel categories.

Heat content—Heat of a material produced by combustion expressed as the 

quantity of heat per unit weight of fuel (Btu/lb) (Byram 1959). Also referred to 

as low-fuel heat content and heat of combustion.

Heat of ignition—Heat of preignition, the amount of heat required to ignite one 

pound of fuel.

Heat sink—The energy required to preheat surface fuels, influenced by fuel 

loading and fuel moisture.

Height—Distance from the base to the top of a fuel type. For example, the height 

of a tree would be the distance from the base of the tree to the top of the tree.

Height to live crown—Distance from the ground surface to the continuous portion 

of tree crowns, including the branches and foliage.

Herb—A plant that lacks a permanent woody stem and dies back to the ground 

each year.

Herb stratum—The fuel layer within FCCS that characterizes grasses, sedges, 

and other herbaceous vegetation.

Inferred variables—Fuelbed characteristics (e.g. surface-area-to-volume ratio, 

heat of combustion, flammability rating, and particle density) inferred from species 

or other fuel attributes. 

*Ladder fuels—Fuels that provide vertical continuity between strata, thereby 

allowing fire to carry from surface fuels into the crowns of trees or shrubs. There 

are eight ladder fuel types in the FCCS: arboreal lichens and moss, climbing ferns, 

and other epiphytes, dead branches, profuse epicormic sprouting, stringy or fuzzy 

bark (e.g., eucalyptus bark), tree regeneration, and vines-lianas. 

Layer—Some strata and/or categories and subcategories are divided into layers. 

The duff category, for example, is divided into upper duff and lower duff layers.

Lichen—A composite plant of algae and fungi. The litter-lichen-moss stratum in 

FCCS includes ground lichen only.

Lightered stumps—Stumps that contain concentrations of pitch and other resinous 

volatiles and burn vigorously.
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Fuel Characteristic Classification System Version 3.0: Technical Documentation

*Litter—The top layer of forest floor, composed of loose debris of dead sticks, 

branches, twigs, and recently fallen leaves or needles; little altered in structure by 

decomposition.

Litter arrangement—The distribution of the litter layer. The FCCS includes three 

litter arrangement categories including fluffy (recently fallen), normal (litter that 

has not recently fallen or is not perched on grasses and other herbs), and perched 

(litter that has fallen mostly on an existing woody fuels or herb layer).

Litter-lichen-moss stratum—The fuel layer within FCCS that characterizes the 

litter, ground lichen and moss categories.

Live foliar moisture content—Water content of a live fuel (e.g., stems and leaves) 

expressed as a percentage oven-dry weight of the fuel.

Lower duff layer—The lower half of the duff layer, generally containing most of 

the humic or highly decayed material. The lower duff layer is denser than the upper 

duff layer.

*Midflame windspeed—the average wind from the top of the fuel bed to twice the 

height of the fuel bed (Andrews 2012).

Midstory—Intermediate and co-dominant trees in the canopy. The FCCS calculator uses the midstory and overstory data in calculating the torching potential.

Mixed forest—A mixed forest is one in which evergreen and deciduous species 

each generally contribute 25 to 75 percent of total tree cover in both the overstory 

and midstory tree layers.

Moisture content of extinction—the fuel moisture content at which predicted rate 

of spread approaches zero. 

Moss—A plant in the phylum Bryophyta; usually occurring in a moist habitat.

Moss type—The type of moss (e.g. sphagnum or other moss) that comprises the 

majority of mosses for a particular fuelbed.

Needle drape—Needles that fall and hang in shrubs or tree branches and provide 

vertical continuity between the surface fuels and crown fuels in a forested stand, 

thus contributing to the ease of torching and crowning.

Overstory—Dominant and emergent trees in the canopy. The FCCS uses the 

midstory and overstory data in calculating the torching potential.

Optimum depth—Used in calculating relative and optimum packing ratios, the 

canopy depth that would result in the maximum reaction velocity. 
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GENERAL TECHNICAL REPORT PNW-gtr-887

Optimum packing ratio—The packing ratio that would occur if fuel particles were 

optimally spaced (i.e., when the fuel particles each had 0.08 to 0.11 ft of air space 

from which to draw oxygen). Optimal fuel particle spacing for fine fuels is about 

0.08 ft and 0.11 ft for coarse fuels such as wood.

*Packing ratio—The fraction of a fuel bed occupied by fuels, or the fuel volume 

divided by bed volume.

Particle density—The density of wood or foliage that does not include the bulk air 

space between individual pieces of wood or foliage (lb/ft3

).

Particle volume—The volume of space filled by fuel particles. Whereas bulk volume is the volume of fuels and the interstitial airspace between fuels (e.g., volume 

of a tree crown), particle volume only considers the space containing fuel particles. 

It is calculated as dry weight biomass (lb/ac) divided by particle density (lb/ft3

).

Percentage cover—The percentage of a surface area covered by a fuelbed category. 

In FCCS, percentage cover is used to express the relative importance of a fuelbed 

category within a fuelbed type.

Percentage live—The percentage of a plant or fuel layer (e.g., primary shrubs) 

that is comprised of live biomass.

Percentage rotten—The percentage of downed, dead woody debris, snags, or 

duff that is composed of rotten, or partially decayed, material.

Piles—The accumulation of woody debris because of land use activities (e.g., 

timber harvest, thinning) and natural events (e.g., wind, avalanche, wildfire). The 

FCCS no longer supports woody fuel accumulations. Pile calculations are handled 

by the FERA pile calculator.

Primary herb layer—The main layer of vegetation within the herb stratum. Users 

can determine the primary layer based on a several selection criteria including 

species, height, mass, etc.

Primary shrub layer—The main layer of vegetation within the shrub fuelbed 

stratum. Users can determine the primary layer based on several selection criteria 

including species, height, mass, etc.

Propagating flux ratio—The proportion of the reaction intensity that contributes 

to the forward rate of spread.

Radius—The length of a line segment between the center and circumference of a 

log or tree bole.
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Fuel Characteristic Classification System Version 3.0: Technical Documentation

Rate of spread—The forward rate of spread (distance per unit time) of a surface or 

crown fire (ft/min).

*Reaction intensity—Rate of heat release per area of the flaming fire front, 

expressed as heat energy/area/time (Btu/ft2

/min).

Reaction potential—The reaction intensity of a fuelbed (energy release per unit 

area per unit time). It is a function of fuel surface area per unit ground surface, 

depth of the surface fuelbed stratum, heat of combustion, and a scaling factor based 

on flammability.

Reaction thickness—The approximate thickness of a fuel element shell that 

contributes to reaction intensity

Reactive volume—The flame-available volume of fuel (ft3

/ft2) in each stratum 

that would be involved in flaming combustion and used in calculating optimum air 

volume.

Relative cover—The relative percentage (0 to 100 percent) of a fixed area covered 

by a species or fuelbed category.

Relative packing ratio—Proportion of fuelbed space that is occupied relative to 

the optimum packing ratio (packing ratio/optimum packing ratio).

Residual available fuel—The fuel available for consumption during the residual 

phase of combustion.

Residual phase—The phase of combustion after the smoldering stage. Sometimes 

termed the “glowing phase.” Residual phase consumption may last for an extended 

period of time and generally involves woody fuels larger than 2 inches in diameter.

Rotten wood—Partially decomposed wood debris with obvious signs of decay such 

that the material falls apart when kicked. The three rotten wood diameter classes 

are 3 to 9 inches, 9 to 20 inches, and >20 inches.

Secondary herb layer—Optional herb layer with a different species composition 

and/or height than the primary herbaceous layer.

Secondary shrub layer—Optional shrub layer with a different species composition 

or height than the primary shrub layer.

Shrub—A woody perennial plant differing from a tree by its low stature and by 

generally producing several basal stems instead of a single bole.
78

GENERAL TECHNICAL REPORT PNW-gtr-887

*Slash—Debris resulting from such natural events as wind, fire, or snow breakage; 

or such human activities as road construction, logging, pruning, thinning, or brush 

cutting. It includes logs, chunks, bark, branches, stumps, and broken understory 

trees or brush.

Smoldering phase—The phase of combustion in which the overall reaction rate of 

combustion has diminished to a point at which concentrations of combustible gases 

above the fuel are too low to support a persistent flame envelope. The smoldering 

phase follows the flaming phase of combustion and is often characterized by large 

amounts of smoke. The smoldering phase is divided into two phases in FCCS: a 

smoldering phase representing short-term smoldering and a residual smoldering or 

“glowing” phase representing long-term smoldering of large wood and duff layers.

Snag—A standing dead tree or part of a dead tree >4.5 feet tall. Snags are classified into four decay class categories—snag class 1 with foliage, snag class 1 without 

foliage, snag class 2, and snag class 3.

Snag decay class 1 with foliage present—Standing, recently dead trees greater 

than 4.5 feet tall, predominantly sound with fine and coarse branches, top and 

foliage intact.

Snag decay class 1 with foliage absent—Standing, recently dead trees greater 

than 4.5 feet tall, predominantly sound with fine and coarse branches, but with no 

foliage present.

Snag decay class 2 with branches and bark present—Standing dead trees greater 

than 4.5 feet tall with coarse branches and bark intact.

Snag decay class 3, rotten and with no branches or bark—Standing dead trees 

greater than 4.5 feet tall that are predominantly rotten with no bark intact.

Sound wood—Woody debris that has minimal decay. The six sound wood diameter 

classes in FCCS include 0 to 1⁄4 inch, 1⁄4 to 1 inch, 1 to 3 inches, 3 to 9 inches, 9 to 

20 inches, and >20 inches.

Spread potential—The potential for fire to spread in a fuelbed, calculated as a 

function of reaction potential and the abundance of very fine fuels less than 1 inch 

in diameter.

Squirrel middens—A mound of cone scales and other cone debris accumulated 

over time from squirrels extracting seeds. The mounds are composed exclusively of 

organic matter that can burn for extended periods of time. Squirrel middens are a 

ground fuel category in FCCS.
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Fuel Characteristic Classification System Version 3.0: Technical Documentation

Strata/stratum—Vertical layers within the fuelbed that represent unique combustion characteristics. Six strata are recognized—tree, shrubs, herb, woody fuels, 

litter-lichen-moss, and ground fuels. 

Stringy or fuzzy bark—Ladder fuel type and qualitative term as used in the 

Australian Bushfire Prediction System to account for the role of such bark in many 

tree species (although few such species are found in the United States and Canada) 

to act as a ladder fuel or source of firebrand material.

Stump—The remaining part of a tree stem after the trunk is severed.

Subcategory—Categories in the FCCS are often further divided into subcategories. 

For example, the tree category is divided into overstory, midstory, and understory 

subcategories.

Surface fire behavior potential—A combination of reaction potential, spread 

potential, and flame length potential scaled to between 0 and 10.

*Surface-area-to-volume ratio—the amount of surface area per unit volume of 

fuels (Fons 1946). The smaller the particle, the more quickly it can become wet, dry 

out, or become heated to combustion temperature during a fire.

Tree—A tall, perennial woody plant greater than 4.5 feet having a main trunk 

and branches forming a distinct elevated crown; includes both gymnosperms and 

angiosperms.

Understory—Tree seedlings and saplings in the canopy layer separated from the 

overstory and midstory by a gap.

Upper duff layer—The upper half of the duff layer that generally contains a 

fermentation (F) layer and may contain a portion of the humic (H) layer. The upper 

duff layer is less dense than the lower duff layer.

Woody fuels stratum—A fuel layer within FCCS that characterizes sound and 

rotten woody fuels, stumps, and piles.

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