BehavePlus 

fire modeling system, 

version 4.0: 

Variables

Patricia L. Andrews

United States

Department

of Agriculture

Forest Service

Rocky Mountain

Research Station

General Technical Report 

RMRS-GTR-213WWW

July 2008
Rocky Mountain Research Station

Publishing Services

Telephone (970) 498-1392

FAX (970) 498-1122

E-mail rschneider@fs.fed.us

Web site http://www.fs.fed.us/rm/publications

Mailing Address Publications Distribution

Rocky Mountain Research Station

240 West Prospect Road

Fort Collins, CO 80526

Andrews, Patricia L. 2008. BehavePlus fire modeling system, version 4.0: 

Variables. Gen. Tech. Rep. RMRS-GTR-213WWW. Fort Collins, CO: Department of Agriculture, Forest Service, Rocky Mountain Research Station. 107 p. 

http://www.fs.fed.us/rm/pubs/rmrs_gtr213.pdf

Abstract

The BehavePlus fire modeling system is a computer program based on mathematical models that describe wildland fire behavior and effects and the fire environment. 

It is a flexible system that produces tables, graphs, and simple diagrams. It can be 

used for a host of fire management applications including projecting the behavior of 

an ongoing fire, planning prescribed fire, fuel hazard assessment, and training. The 

BehavePlus program automatically creates a worksheet that requests the required 

input variables based on the modules, output variables, and options selected by the 

user. This is a reference paper that describes the 181 variables in BehavePlus, with 

information on input and output relationships. A User’s Guide describes operation of 

the program and can be accessed at http://www.fs.fed.us/rm/pubs/rmrs_gtr106.html.

Keywords: Fire behavior, fire modeling, fire spread, fire intensity, computer program

Cover Art: “Wildfire” © 1992, an original acrylic painting by Monty Dolack. 

Trumpeter swans take refuge in the air as a threatening wildfire 

rages through the forest habitat. This image is from a 29” x 24” 

poster commissioned by the National Wildfire Foundation and used 

here by permission of the artist. 

All images used in BehavePlus and its associated manuals and 

training materials are from original work by Monte Dolack and 

appear by permission of the artist.

Author Profile

Patricia L. Andrews is a Research Physical Scientist at the Fire Sciences Laboratory 

in Missoula, MT. She received a B.A. in mathematics and chemistry from Montana 

State University, Billings in 1970, an M.A. in mathematics and computer science in 

1973, and a Ph.D. in Forestry in 2005 from the University of Montana, Missoula. She 

has been at the Fire Sciences Laboratory since 1973, serving as Project Leader of the 

Fire Behavior Research Work Unit from 1991 to 1996.
Acknowledgments

We acknowledge funding from the Joint Fire Science Program under Project JFSP 

05-4-1-23 for development of BehavePlus training material, including this document.

The Help system was developed through contracts with Systems for Environmental 

Management. Version 4.0 Help system was updated based on material in this document

by Faith Ann Heinsch. Earlier Help system material was developed by Don Carlton, 

Joe Scott, and Collin Bevins.

Preface

This Variables paper describes the many input and output variables used in the 

BehavePlus fire modeling system. A User’s Guide covers operation of the BehavePlus 

software.

These are RMRS online publications. They can be downloaded from the Rocky Mountain Research 

Station publications web page, http://www.fs.fed.us/rm/publications/titles/rmrs_gtr.html, or 

from the BehavePlus web page, http://www.firemodels.org/. They are updated as 

features and modeling capabilities are added to the program.

The Variables paper and the User’s Guide are also an integral part of the BehavePlus 

system, serving as online help.

The BehavePlus system is supported by 

US Forest Service

Fire and Aviation Management

800-253-5559 or 208-387-5290

E-mail at fire_help@fs.fed.us

BehavePlus 4.0 Variables 

Contents 

Introduction .....................................................................................................1

Alphabetical List of Variables ........................................................................4

Input Variable Table ......................................................................................10

Output Variable Table ...................................................................................16

Variable Descriptions....................................................................................23

Surface Fire Spread and Intensity............................................................25

Surface Rate of Spread.......................................................................................25

Flame Length ......................................................................................................26

Fireline Intensity ..................................................................................................28

Heat per Unit Area...............................................................................................29

Reaction Intensity................................................................................................29

Dead Fuel Reaction Intensity ..............................................................................29

Live Fuel Reaction Intensity ................................................................................30

Direction of Maximum Spread .............................................................................30

Surface Spread Distance ....................................................................................31

Spread Direction..................................................................................................31

Wind/Slope/Spread Direction Diagrams..............................................................31

Fire Characteristics Chart....................................................................................32

Wind Factor.........................................................................................................33

Slope Factor........................................................................................................33

Heat Source ........................................................................................................34

Heat Sink.............................................................................................................34

Flame Residence Time .......................................................................................34

Surface Fuel ...............................................................................................35

Fuel Model...........................................................................................................35

First Fuel Model...................................................................................................35

First Fuel Model Coverage..................................................................................35

Second Fuel Model .............................................................................................36

Fuel Model Type..................................................................................................36

Fuel Bed Depth ...................................................................................................37

Dead Fuel Moisture of Extinction ........................................................................38

US Forest Service RMRS-GTR-213WWW. 2008. i 
BehavePlus 4.0 Variables

Live Fuel Moisture of Extinction ..........................................................................38

Dead Fuel Heat Content......................................................................................38

Live Fuel Heat Content........................................................................................39

1-h Fuel Load ......................................................................................................39

10-h Fuel Load ....................................................................................................40

100-h Fuel Load ..................................................................................................40

Fuel Load Transfer Portion..................................................................................41

Dead Herbaceous Fuel Load ..............................................................................41

Live Herbaceous Fuel Load ................................................................................42

Live Herbaceous Fuel Load Remainder..............................................................42

Live Woody Fuel Load.........................................................................................43

Total Dead Fuel Load..........................................................................................43

Total Live Fuel Load............................................................................................43

Dead Fuel Load Portion ......................................................................................44

1-h Surface-Area-to-Volume Ratio......................................................................44

Live Herbaceous Surface-Area-to-Volume Ratio ................................................44

Live Woody Surface-Area-to-Volume Ratio ........................................................45

Characteristic Surface-Area-to-Volume Ratio .....................................................45

Relative Packing Ratio ........................................................................................45

Bulk Density ........................................................................................................46

Packing Ratio ......................................................................................................46

Aspen Fuel Model ...............................................................................................47

Aspen Curing Level.............................................................................................47

Aspen Fire Severity .............................................................................................48

P-G Age of Rough...............................................................................................48

P-G Height of Understory ....................................................................................48

P-G Palmetto Coverage ......................................................................................49

P-G Overstory Basal Area...................................................................................49

P-G Dead Fine Fuel Load ...................................................................................49

P-G Dead Medium Fuel Load..............................................................................50

P-G Dead Foliage Fuel Load...............................................................................50

P-G Live Fine Fuel Load .....................................................................................50

P-G Live Medium Fuel Load................................................................................51

P-G Live Foliage Fuel Load.................................................................................51

P-G Litter Fuel Load............................................................................................51

ii US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Surface Fuel Moisture ...............................................................................52

1-h Moisture ........................................................................................................52

10-h Moisture ......................................................................................................52

100-h Moisture ....................................................................................................52

Dead Fuel Moisture.............................................................................................53

Live Herbaceous Moisture...................................................................................53

Live Woody Moisture...........................................................................................54

Live Fuel Moisture...............................................................................................54

Moisture Scenario ...............................................................................................54

Characteristic Dead Fuel Moisture......................................................................55

Characteristic Live Fuel Moisture ........................................................................55

Wind............................................................................................................56

20-Ft Wind Speed ...............................................................................................56

10-Meter Wind Speed .........................................................................................57

Midflame Wind Speed .........................................................................................58

Effective Wind Speed..........................................................................................58

Maximum Effective Wind Exceeded?..................................................................59

Effective Wind Speed Limit .................................................................................59

Canopy Cover .....................................................................................................59

Wind Adjustment Factor......................................................................................60

Crown Fill Portion................................................................................................60

WAF Calculation..................................................................................................61

Direction of Wind Vector (from Upslope).............................................................62

Wind Direction (from North).................................................................................63

Slope...........................................................................................................64

Aspect .................................................................................................................64

Number of Contour Intervals ...............................................................................65

Contour Interval...................................................................................................65

Map Distance ......................................................................................................65

Map Representative Fraction ..............................................................................66

Slope Horizontal Distance...................................................................................66

Slope Elevation Change......................................................................................67

Slope Steepness .................................................................................................67

US Forest Service RMRS-GTR-213WWW. 2008. iii 
BehavePlus 4.0 Variables

Crown Fire..................................................................................................67

Foliar Moisture.....................................................................................................67

Canopy Base Height ...........................................................................................68

Canopy Bulk Density...........................................................................................68

Critical Surface Intensity......................................................................................68

Critical Surface Flame Length.............................................................................68

Transition Ratio ...................................................................................................69

Transition to Crown Fire?....................................................................................69

Crown ROS .........................................................................................................69

Critical Crown ROS .............................................................................................70

Active Ratio .........................................................................................................70

Active Crown Fire?..............................................................................................70

Fire Type .............................................................................................................71

Crown Spread Distance ......................................................................................71

Crown Fire Area ..................................................................................................71

Crown Fire Perimeter ..........................................................................................72

Crown Fire Length-to-Width Ratio.......................................................................72

Safety Zone ................................................................................................72

Number of Personnel ..........................................................................................72

Area per Person ..................................................................................................72

Number of Heavy Equipment ..............................................................................73

Area per Heavy Equipment .................................................................................73

Safety Zone Separation Distance........................................................................73

Safety Zone Radius.............................................................................................74

Safety Zone Size.................................................................................................74

Fire Size and Shape...................................................................................75

Elapsed Time ......................................................................................................75

Area or Fire Size at Report..................................................................................76

Perimeter.............................................................................................................76

Length-to-Width Ratio .........................................................................................77

Forward Spread Distance....................................................................................78

Backing Spread Distance....................................................................................78

Fire Length ..........................................................................................................78

Maximum Fire Width ...........................................................................................79

iv US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Fire Shape Diagrams ..........................................................................................79

Containment...............................................................................................79

Suppression Tactic..............................................................................................79

Line Construction Offset......................................................................................80

Resource Name ..................................................................................................80

Resource Line Production Rate ..........................................................................80

Resource Arrival Time.........................................................................................81

Resource Base Cost ...........................................................................................81

Resource Hourly Cost .........................................................................................82

Resource Duration ..............................................................................................82

Fire Area at Initial Attack .....................................................................................82

Perimeter at Initial Attack ....................................................................................83

Contain Status.....................................................................................................83

Time from Report ................................................................................................83

Contained Area ...................................................................................................84

Fireline Constructed ............................................................................................84

Number of Resources Used................................................................................84

Cost of Resources Used .....................................................................................85

Containment Diagram .........................................................................................85

Spotting Distance ......................................................................................86

Ridge-to-Valley Elevation Difference...................................................................86

Ridge-to-Valley Horizontal Distance....................................................................86

Spotting Source Location ....................................................................................87

Canopy Height.....................................................................................................87

Flame Height from a Burning Pile .......................................................................88

Number of Torching Trees ..................................................................................88

Spot Tree Species...............................................................................................89

Cover Height Applied for Spotting Distance Torching Trees...............................89

Steady State Flame Height from Torching Trees ................................................89

Tree Height/Flame Height Ratio for Torching Trees ...........................................90

Steady State Flame Duration from Torching Trees.............................................90

Firebrand Height from Torching Trees ................................................................90

Flat Terrain Spotting Distance from Torching Trees ...........................................90

Cover Height Applied for Spotting Distance from a Burning Pile ........................91

US Forest Service RMRS-GTR-213WWW. 2008. v 
BehavePlus 4.0 Variables

Firebrand Height from a Burning Pile..................................................................91

Flat Terrain Spotting Distance from a Burning Pile .............................................91

Cover Height Applied for Spotting Distance from a Wind-Driven Surface Fire ...92

Firebrand Height from a Wind-Driven Surface Fire .............................................92

Firebrand Drift Distance from a Wind-Driven Surface Fire..................................92

Flat Terrain Spotting Distance from a Surface Fire .............................................92

Spotting Distance from Torching Trees...............................................................93

Spotting Distance from a Burning Pile.................................................................94

Spotting Distance from a Wind-Driven Surface Fire............................................94

Scorch Height ............................................................................................95

Air Temperature ..................................................................................................95

Scorch Height......................................................................................................96

Tree Mortality .............................................................................................97

Tree Height..........................................................................................................97

Crown Ratio.........................................................................................................97

D.B.H...................................................................................................................98

Bark Thickness....................................................................................................98

Mortality Equation................................................................................................98

Mortality Tree Species.........................................................................................99

Tree Crown Length Scorched .............................................................................99

Tree Crown Volume Scorched ............................................................................99

Probability of Mortality .......................................................................................100

Probability of Aspen Mortality............................................................................100

Probability of Ignition..............................................................................101

Fuel Shading from the Sun................................................................................101

Probability of Ignition from a Firebrand .............................................................101

Lightning Ignition Fuel Type ..............................................................................101

Lightning Duff and Litter Depth..........................................................................102

Lightning Strike Type.........................................................................................102

Probability of Ignition from Lightning .................................................................102

References...................................................................................................103

Appendix A—Mathematical models in BehavePlus .................................106

vi US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Introduction 

The BehavePlus fire modeling system is a computer program based on mathematical 

models that describe wildland fire and the fire environment. It is a flexible system that 

produces tables, graphs, and simple diagrams. It can be used for a host of fire 

management applications including projecting the behavior of an ongoing fire, planning 

prescribed fire, fuel hazard assessment, and training (Andrews 2007, Andrews and others 

2008). Successful application of the system is dependent upon decisions made by the 

user. An educated fire behavior person understands the fire modeling foundation with the 

associated assumptions and limitations as well as the influence of various input variables 

on calculated values. 

This document is a reference guide that describes the many input and output variables in 

BehavePlus. It is based on information that was previously available only in the help 

windows of the program. This document can be printed and kept in a notebook as a 

reference. It is, however, most useful when viewed on a computer utilizing the many 

links and bookmarks. This paper is also packaged with the BehavePlus program and can 

be opened through the help system. It can also be accessed at http://www.firemodels.org/

or http://www.fs.fed.us/rm/publications/titles/rmrs_gtr.html. 

The many fire behavior, fire effects, and fire environment models in BehavePlus are 

grouped according to modules (Table 1). The CROWN module, for example, is based on 

models for crown fire spread rate (Rothermel 1991), transition from surface to crown fire 

(Van Wagner 1977), and conditions for an active crown fire (Van Wagner 1993). A table 

of the mathematical models (complete with references) that form the basis for 

BehavePlus is found in Appendix A. 

The BehavePlus program automatically creates a worksheet that requests the required 

input variables based on the modules, output variables, and options selected by the user. 

Using indentation, the module selection screen (Figure 1) indicates that some modules 

can be linked, with output from one being used as input to another. For example, Flame 

Length can be calculated in SURFACE and used in SCORCH. Or the user can enter the 

Flame Length directly into SCORCH. Notes on linkages are included in Table 1. 

Information on the 181 variables in BehavePlus, version 4.0 is organized in several ways 

in this document to facilitate finding the needed information. The Contents section lists 

variables according to logical groupings used to order the description of each variable. 

Table 2 is an alphabetical list of the variables, with links to the variable description, input 

table, and/or output table. Table 3 lists the input variables sorted according to the 

categories used on the worksheet (Fuel/Vegetation, Fuel Moisture, Weather, and so 

forth.) with indication of the modules in which the variable is used and valid input 

ranges. Table 4 provides the output variables according to module. Some variables, such 

as Flame Length as discussed above, are in both the input and the output table. 

US Forest Service RMRS-GTR-213WWW. 2008. 1 
BehavePlus 4.0 Variables

The bulk of this document is a description of each variable. These are brief definitions 

and are not meant to be comprehensive. This is just one document that supports effective 

use of fire modeling for fire management applications. 

Table 1—Modules in BehavePlus are groupings of related mathematical fire models. 

Module Calculations Linkages 

SURFACE • Surface fire rate of spread 

• Fireline intensity and flame length 

• Reaction intensity and heat per unit area 

• Intermediate values: heat source, heat 

sink, characteristic dead fuel moisture, 

relative packing ratio, and so forth 

• Standard, custom, and special case fuel 

models 

• Wind adjustment factor 

CROWN • Transition from surface to crown fire 

• Crown fire rate of spread 

• Crown fire area and perimeter 

• Fire type: surface, torching, conditional 

crown, crowning 

• Surface fireline intensity or flame 

length can come from SURFACE 

SAFETY • Safety zone size based on flame length 

• Area, perimeter, separation distance 

• Head fire flame length can come 

from SURFACE 

SIZE • Elliptically shaped point source fire 

• Area, perimeter, shape 

• Head fire rate of spread and 

effective wind speed can come 

from SURFACE 

CONTAIN • Fire containment success for single or 

multiple resources given line construction 

rate, arrival time, resource duration, head 

or rear attack, direct or parallel attack 

• Final area and perimeter, fire size at initial 

attack, fireline constructed 

• Head fire rate of spread can come 

from SURFACE 

• Length-to-width ratio and fire size 

at report can come from SIZE 

SPOT • Maximum spotting distance from torching 

trees, burning piles, or wind-driven 

surface fire 

• Head fire flame length can come 

from SURFACE for wind-driven 

surface fire 

SCORCH • Crown scorch height from surface fire 

flame length 

• Surface fireline intensity or flame 

length can come from SURFACE 

MORTALITY • Probability of mortality from bark thickness 

and crown scorch 

• Scorch height can come from 

SCORCH 

IGNITE • Probability of ignition by firebrands 

• Probability of ignition from lightning strikes 

2 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Figure 1—Module selection for BehavePlus. Indentation indicates that modules can be 

linked, with output from one being used as input to the other. For example, flame length 

calculated in SURFACE can be used to calculate scorch height in the SCORCH module. 

If SURFACE is not selected, the user enters values for flame length. 

US Forest Service RMRS-GTR-213WWW. 2008. 3 
BehavePlus 4.0 Variables

Alphabetical List of Variables 

An alphabetical list of the 181 variables in BehavePlus version 4.0 is given in Table 2. 

Each variable name is linked to its description in the body of the document. The --I--

takes you to the input variable table, and the --O-- to the output variable table. Note that 

some variables can play the role of either input or output depending on the modules and 

run options selected. 

Table 2—Alphabetical list of all input and output variables and an indication of whether 

each serves as input and/or output. 

Variable name Input and/or 

output 

1-h Fuel Load --I-- --O--

1-h Moisture --I--

1-h Surface-Area-to-Volume Ratio --I-- --O--

10-Meter Wind Speed --I--

10-h Fuel Load --I--

10-h Moisture --I--

100-h Fuel Load --I--

100-h Moisture --I--

20-Ft Wind Speed --I--

Active Crown Fire? --O--

Active Ratio --O--

Air Temperature --I--

Area Per Person --I--

Area or Fire Size at Report --I-- --O--

Area per Heavy Equipment --I--

Aspect --I--

Aspen Curing Level --I--

Aspen Fire Severity --I--

Aspen Fuel Model --I--

Backing Spread Distance --O--

Bark Thickness --I-- --O--

Bulk Density --O--

Canopy Base Height --I--

Canopy Bulk Density --I--

Canopy Cover --I--

Canopy Height --I--

4 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Variable name Input and/or 

output 

Characteristic Dead Fuel Moisture --O--

Characteristic Live Fuel Moisture --O--

Characteristic Surface-Area-to-Volume 

Ratio

--O--

Contain Status --O--

Contained Area --O--

Containment Diagram --O--

Contour Interval --I--

Cost of Resources Used --O--

Cover Height Applied for Spotting 

Distance from a Burning Pile

--O--

Cover Height Applied for Spotting 

Distance from a Wind-Driven Surface 

Fire

--O--

Cover Height Applied for Spotting 

Distance Torching Trees

--O--

Critical Crown ROS --O--

Critical Surface Flame Length --O--

Critical Surface Intensity --O--

Crown Fill Portion --O--

Crown Fire Area --O--

Crown Fire Length-to-Width Ratio --O--

Crown Fire Perimeter --O--

Crown ROS --O--

Crown Ratio --I-- --O--

Crown Spread Distance --O--

D.B.H. --I--

Dead Fuel Heat Content --I--

Dead Fuel Load Portion --O--

Dead Fuel Moisture --I--

Dead Fuel Moisture of Extinction --I--

Dead Fuel Reaction Intensity --O--

Dead Herbaceous Fuel Load --O--

Direction of Maximum Spread --O--

Direction of Wind Vector (from Upslope) --I--

Effective Wind Speed --I-- --O--

Effective Wind Speed Limit --O--

Elapsed Time --I--

US Forest Service RMRS-GTR-213WWW. 2008. 5 
BehavePlus 4.0 Variables

Variable name Input and/or 

output 

Fire Area at Initial Attack --O--

Fire Characteristics Chart --O--

Fire Length --O--

Fire Shape Diagrams --O--

Fire Size at Report (or Area) --I-- --O--

Fire Type --O--

Firebrand Drift Distance from a WindDriven Surface Fire--O--

Firebrand Height from a Burning Pile --O--

Firebrand Height from a Wind-Driven

Surface Fire

--O--

Firebrand Height from Torching Trees --O--

Fireline Constructed --O--

Fireline Intensity --I-- --O--

First Fuel Model --I--

First Fuel Model Coverage --I--

Flame Height from a Burning Pile --I--

Flame Length --I-- --O--

Flame Residence Time --O--

Flat Terrain Spotting Distance from a 

Burning Pile

--O--

Flat Terrain Spotting Distance from a 

Surface Fire

--O--

Flat Terrain Spotting Distance from 

Torching Trees

--O--

Foliar Moisture --I--

Forward Spread Distance --O--

Fuel Bed Depth --I-- --O--

Fuel Load Transfer Portion --I-- --O--

Fuel Model --I--

Fuel Model Type --I--

Fuel Shading from the Sun --I--

Heat per Unit Area --O--

Heat Sink --O--

Heat Source --O--

Length-to-Width Ratio --I-- --O--

Lightning Duff and Litter Depth --I--

Lightning Ignition Fuel Type --I--

6 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Variable name Input and/or 

output 

Lightning Strike Type --I--

Line Construction Offset --I--

Live Fuel Heat Content --I--

Live Fuel Moisture --I--

Live Fuel Moisture of Extinction --O--

Live Fuel Reaction Intensity --O--

Live Herbaceous Surface-Area-to-Volume 

Ratio 

--I--

Live Herbaceous Fuel Load --I-- --O--

Live Herbaceous Fuel Load Remainder --O--

Live Herbaceous Moisture --I--

Live Woody Fuel Load --I-- --O--

Live Woody Moisture --I--

Live Woody Surface-Area-to-Volume 

Ratio 

--I-- --O--

Map Distance --I--

Map Representative Fraction --I--

Maximum Effective Wind Exceeded? --O--

Maximum Fire Width --O--

Midflame Wind Speed --I-- --O--

Moisture Scenario --I--

Mortality Equation --I--

Mortality Tree Species --I--

Number of Contour Intervals --I--

Number of Heavy Equipment --I--

Number of Personnel --I--

Number of Resources Used --O--

Number of Torching Trees --I--

P-G Age of Rough --I--

P-G Dead Fine Fuel Load --O--

P-G Dead Foliage Fuel Load --O--

P-G Dead Medium Fuel Load --O--

P-G Height of Understory --I--

P-G Litter Fuel Load --O--

P-G Live Fine Fuel Load --O--

P-G Live Foliage Fuel Load --O--

P-G Live Medium Fuel Load --O--

P-G Overstory Basal Area --I--

US Forest Service RMRS-GTR-213WWW. 2008. 7 
BehavePlus 4.0 Variables

Variable name Input and/or 

output 

P-G Palmetto Coverage --I--

Packing Ratio --O--

Perimeter --O--

Perimeter at Initial Attack --O--

Probability of Ignition from Lightning --O--

Probability of Ignition from a Firebrand --O--

Probability of Aspen Mortality --O--

Probability of Mortality --O--

Reaction Intensity --O--

Relative Packing Ratio --O--

Resource Arrival Time --I--

Resource Base Cost --I--

Resource Duration --I--

Resource Hourly Cost --I--

Resource Line Production Rate --I--

Resource Name --I--

Ridge-to-Valley Elevation Difference --I--

Ridge-to-Valley Horizontal Distance --I--

Safety Zone Radius --O--

Safety Zone Separation Distance --O--

Safety Zone Size --O--

Scorch Height --I-- --O--

Second Fuel Model --I--

Slope Elevation Change --O--

Slope Factor --O--

Slope Horizontal Distance --O--

Slope Steepness --I-- --O--

Spot Tree Species --I--

Spotting Distance from a Burning Pile --O--

Spotting Distance from a Wind-Driven

Surface Fire 

--O--

Spotting Distance from Torching Trees --O--

Spotting Source Location --I--

Spread Direction --I--

Steady State Flame Height from Torching 

Trees

--O--

Suppression Tactic --I--

Surface Rate of Spread --I-- --O--

8 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Variable name Input and/or 

output 

Surface Spread Distance --O--

Time from Report --O--

Total Dead Fuel Load --O--

Total Live Fuel Load --O--

Transition Ratio --O--

Transition to Crown Fire? --O--

Tree Crown Length Scorched --O--

Tree Crown Volume Scorched --O--

Tree Height --I--

Tree Height / Flame Height Ratio for 

Torching Trees

--O--

WAF Calculation --O--

Wind/Slope/Spread Direction Diagrams --O--

Wind Adjustment Factor --I-- --O--

Wind Direction (from North) --I--

Wind Factor --O--

US Forest Service RMRS-GTR-213WWW. 2008. 9 
BehavePlus 4.0 Variables

Input Variable Table 

Input variables sorted according to the categories used on the worksheet are given in 

Table 3. Some serve as input for more than one module. I/O indicates input variables that 

can be output variables when modules are linked, with output from one module serving 

as input to another (for example, flame length). In addition, some run options allow 

variables to be either input by the user or calculated by BehavePlus (for example, bark 

thickness). The valid range for input values is given for English and metric units. 

Variable type of Continuous (C) indicates that a range of numerical values is meaningful 

(for example, 1-h fuel load). Discrete variables (D) have specific valid inputs (for 

example, fuel model). The variable name is linked to a description of the variable. 

Table 3—Input variables for BehavePlus version 4.0. (I=Input; O=Output; I/O=Input or 

Output depending on the selected modules and options; C=Continuous; D= Discrete) 

 

Input variable

SURFACE SAFETY SIZE CONTAIN SPOT SCORCH MORTALITY IGNITE CROWN Variable type 

English 

range 

and 

units 

Metric 

range 

and 

units 

Fuel/Vegetation, Surface/Understory 

Fuel Model I D N/A N/A 

First Fuel Model I D N/A N/A 

Second Fuel Model I D N/A N/A 

First Fuel Model 

Coverage

I C 0 to 100 

percent 

0 to 100 

percent 

Fuel Load Transfer 

Portion

I/

O 

C 0 to 100 

percent 

0 to 100 

percent 

Fuel Model Type I D Static, 

Dynamic 

Static, 

Dynamic 

1-h Fuel Load I C 0.0 to 31 

ton/ac 

0.0 to 69.5 

tonne/ha 

10-h Fuel Load I C 0.0 to 31 

ton/ac 

0.0 to 69.5 

tonne/ha 

100-h Fuel Load I C 0.0 to 31 

ton/ac 

0.0 to 69.5 

tonne/ha 

Live Herbaceous 

Fuel Load

I C 0.0 to 31 

ton/ac 

0.0 to 69.5 

tonne/ha 

Live Woody Fuel 

Load

I C 0.0 to 31 

ton/ac 

0.0 to 69.5 

tonne/ha 

10 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

 

Input variable

SURFACE SAFETY SIZE CONTAIN SPOT SCORCH MORTALITY IGNITE CROWN Variable type 

English 

range 

and 

units 

Metric 

range 

and 

units 

1-h Surface-Areato-Volume RatioI C 109 to 

4,000 

ft2

/ft3

358 to 

13,123 

m2

/m3

Live Herb SurfaceArea-to-Volume

Ratio

I C 109 to 

4,000 

ft2

/ft3

358 to 

13,123 

m2

/m3

Live Woody 

Surface-Area-toVolume Ratio

I C 109 to 

4,000 

ft2

/ft3

358 to 

13,123 

m

2

/m3

Fuel Bed Depth I C 0.05 to 

10.0 ft 

0.02 to 

3.05 m 

Dead Fuel Moisture 

of Extinction

I C 5 to 100 

percent 

5 to 100 

percent 

Dead Fuel Heat 

Content

I C 6,000 to 

12,000 

Btu/lb 

13,967 to 

27,934 

kJ/kg 

Live Fuel Heat 

Content

I C 6,000 to 

12,000 

Btu/lb 

13,967 to 

27,934 

kJ/kg 

P-G Age of Rough I C 1 to 25 

years 

1 to 25 

years 

P-G Height of 

Understory

I C 1 to 6 ft 0 to 2 m 

P-G Palmetto 

Coverage

I C 15 to 85 

percent 

15 to 85 

percent 

P-G Overstory 

Basal Area

I C 30 to 110 

ft2

/ac 

7 to 25 

m

2

/ha 

Aspen Curing Level I C 0 to 90 

percent 

0 to 90 

percent 

Aspen Fuel Model I D [5 types] [5 types] 

Lightning Ignition 

Fuel Type

I D [8 types] [8 types] 

Lightning Duff and 

Litter Depth

I C 0 to 12 in 0 to 30 cm 

US Forest Service RMRS-GTR-213WWW. 2008. 11 
BehavePlus 4.0 Variables

 

Input variable

SURFACE SAFETY SIZE CONTAIN SPOT SCORCH MORTALITY IGNITE CROWN Variable type 

English 

range 

and 

units 

Metric 

range 

and 

units 

Fuel / Vegetation, Overstory 

Canopy Cover I C 0 to 100 

percent 

0 to 100 

percent 

Canopy Height I I C 0 to 300 ft 0 to 91 m 

Tree Height I I C 10 to 300 

ft 

3 to 91 m 

Crown Ratio I/

O 

I C 0.1 to 1.0 0.1 to 1.0 

Canopy Base

Height

I C 0.1 to 100 

ft 

0 to 30.5 

m 

Canopy Bulk 

Density

I C 0.001 to 

0.062 

lb/ft3

0.016 to 

1.001 

kg/m3

Mortality Equation I D Nonspruce, 

Spruce 

Nonspruce, 

Spruce 

Mortality Tree 

Species

I D [206 

species] 

[206 

species] 

Spot Tree Species I D [14 

species] 

[14 

species] 

D.B.H. I I C 2 to 40 in 5 to 102 

cm 

Bark Thickness I/

O

C 0.1 to 3.3 

in 

0.3 to 8.4 

cm 

Fuel Moisture 

Moisture Scenario I D N/A N/A 

1-h Moisture I I I C 1 to 60 

percent 

1 to 60 

percent 

10-h Moisture I I C 1 to 60 

percent 

1 to 60 

percent 

100-h Moisture I I I C 1 to 60 

percent 

1 to 60 

percent 

Live Herbaceous 

Moisture

I C 30 to 300 

percent 

30 to 300 

percent 

12 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

 

Input variable

SURFACE SAFETY SIZE CONTAIN SPOT SCORCH MORTALITY IGNITE CROWN Variable type 

English 

range 

and 

units 

Metric 

range 

and 

units 

Live Woody 

Moisture

I I C 30 to 300 

percent 

30 to 300 

percent 

Dead Fuel Moisture I C 1 to 60 

percent 

1 to 60 

percent 

Live Fuel Moisture I C 30 to 300 

percent 

30 to 300 

percent 

Foliar Moisture I C 30 to 300 

percent 

30 to 300 

percent 

Weather 

Midflame Wind 

Speed

I/

O 

I C 0 to 60 

mi/h 

0.0 to 96.6 

km/h 

20-ft Wind Speed I I I C 0 to 99 

mi/h 

0 to 159 

km/h 

10-m Wind Speed I I I C 0 to 99 

mi/h 

0 to 159 

km/h 

Wind Adjustment 

Factor

I/

O 

C 0.1 to 1.0 0.1 to 1.0 

Effective Wind 

Speed

O I C 0 to 60 

mi/h 

0.0 to 96.6 

km/h 

Direction of Wind 

Vector (from 

Upslope)

I C 0 to 360 

deg 

0 to 360 

deg 

Wind Direction 

(from North)

I C 0 to 360 

deg 

0 to 360 

deg 

Air Temperature I I C -40 to 120 

deg F 

-40 to 49 

deg C 

Fuel Shading from 

the Sun

I C 0 to 100 

percent 

0 to 100 

percent 

Lightning Strike 

Type

I 

D

-, +, ? -, +, ? 

Terrain 

Slope Steepness I/

O 

C 0 to 604 

percent 

0 to 81 

deg 

0 to 604 

percent 

0 to 81 

deg 

US Forest Service RMRS-GTR-213WWW. 2008. 13 
BehavePlus 4.0 Variables

 

Input variable

SURFACE SAFETY SIZE CONTAIN SPOT SCORCH MORTALITY IGNITE CROWN Variable type 

English 

range 

and 

units 

Metric 

range 

and 

units 

Aspect I C 0 to 360 

deg 

0 to 360 

deg 

Ridge-to-Valley 

Elevation 

Difference

I C 0 to 4,000 

ft 

0 to 1,219 

m 

Ridge-to-Valley 

Horizontal 

Distance

I C 0 to 4.0 mi 0 to 6.4 

km 

Spotting Source 

Location

I D RT, MW, 

VB, ML 

RT, MW, 

VB, ML 

Fire 

Surface Rate of 

Spread 

(Maximum)

O I I C 0 to 500 

ch/h 

0.0 to 

167.6 

m/min 

Flame Length O I I I I C 0 to 200 ft 0 to 61 m 

Fireline Intensity O I I C 0 to 

10,000 

Btu/ft/s 

0 to 

34,641 

kW/m 

Spread Direction I C 0 to 360 

deg 

0 to 360 

deg 

Aspen Fire Severity I D Low, 

Moderate+ 

Low, 

Moderate+ 

Flame Height from 

a Burning Pile

I C 0 to 100 ft 0 to 30.5 

m 

Number of 

Torching Trees

I C 0 to 30 0 to 30 

Scorch Height O I C 0 to 200 ft 0 to 61 m 

Elapsed Time I I I C 0.1 to 8.0 

h 

0.1 to 8.0 

h 

Fire Size at Report O I C 0.1 to 100 

ac 

0.0 to 40.5 

ha 

Length-to-Width 

Ratio

O I C 1 to 7 1 to 7 

14 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

 

Input variable

SURFACE SAFETY SIZE CONTAIN SPOT SCORCH MORTALITY IGNITE CROWN Variable type 

English 

range 

and 

units 

Metric 

range 

and 

units 

Map 

Map Representative 

Fraction

I I I I C 1,980 to 

1,013,760 

1,980 to 

1,013,760 

Contour Interval I C 1 to 1000 

ft 

0.3 to 

304.8 m 

Map Distance I C 0.1 to 100 

in 

0.3 to 254 

cm 

Number of Contour 

Intervals

I C 1 to 100 1 to 100 

Suppression 

Suppression Tactic I D Head, 

Rear 

Head, 

Rear 

Line Construction 

Offset

I C 0 to 100 

ch 

0 to 2012 

m 

Resource Name I D N/A N/A 

Resource Line 

Production Rate

I C 1 to 250 

ch/h 

20 to 5029 

m/h 

Resource Arrival 

Time

I C 0 to 8 h 0 to 8 h 

Resource Duration I C 0.0 to 20 h 0.0 to 20 h 

Resource Base Cost I C 0 to 

500,000 

0 to 

500,000 

Resource Hourly 

Cost

I C 0 to 

500,000 

0 to 

500,000 

Number of 

Personnel

I C 1 to 200 1 to 200 

Area per Person I C 10 to 100 

ft2

1 to 9 m2

Number of Heavy 

Equipment

I C 0 to 10 0 to 10 

Area per Heavy 

Equipment

I C 100 to 500 

ft2

9 to 46 m2

US Forest Service RMRS-GTR-213WWW. 2008. 15 
BehavePlus 4.0 Variables

Output Variable Table 

Output variables sorted according to the module that produces them are given in Table 4. 

The English and metric units of each variable are given. (BehavePlus also gives you the 

option of developing, saving, and using a custom units set.) Default output variables are 

indicated. Variables that are available as shading variables for the option “Table shading 

for acceptable fire conditions” are identified. And the variables that are affected by the 

option “Display output distances in map units” on the module selection window are 

indicated. Variable names are linked to a description of the variable. 

Table 4—Output variables for BehavePlus version 4.0. 

Output variable 

Default 

output 

variable

Table 

shading 

variable

Available 

as map 

distance 

English 

units 

Metric 

units 

SURFACE 

Basic Outputs 

Surface Rate of 

Spread

X X ch/h m/min 

Heat per Unit Area X Btu/ft2 kJ/m2

Fireline Intensity X Btu/ft/s kW/m 

Flame Length X X ft m 

Reaction Intensity Btu/ft2/min kW/m2

Direction of 

Maximum Spread

degrees degrees 

Surface Spread 

Distance

X ch m 

Slope Steepness percent percent 

Slope Elevation 

Change

ft m 

Slope Horizontal 

Distance

ft m 

Wind/Slope/Spread 

Direction Diagram

N/A N/A 

Fire Characteristics 

Chart

N/A N/A 

16 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Output variable 

Default 

output 

variable

Table 

shading 

variable

Available 

as map 

distance 

English 

units 

Metric 

units 

Wind Outputs 

Midflame Wind 

Speed

mi/h km/h 

Wind Adjustment 

Factor

fraction fraction 

Crown Ratio ratio ratio 

Crown Fill Portion percent percent 

WAF Calculation Sheltered, 

Unsheltered, 

Input 

Sheltered, 

Unsheltered, 

Input 

Effective Wind Speed mi/h km/h 

Effective Wind Speed 

Limit

mi/h km/h 

Max Effective Wind 

Exceeded?

Yes, No Yes, No 

Intermediate 

Outputs 

Characteristic Dead 

Fuel Moisture

percent percent 

Characteristic Live 

Fuel Moisture

percent percent 

Live Fuel Moisture of 

Extinction

percent percent 

Characteristic 

Surface-Area-toVolume-Ratio

ft2/ft3 m2/m3

Bulk Density lbs/ft3 kg/m3

Packing Ratio ratio ratio 

Relative Packing 

Ratio 

ratio ratio 

Dead Fuel Reaction 

Intensity

Btu/ft2/min kW/m2

Live Fuel Reaction 

Intensity

Btu/ft2/min kW/m2

Wind Factor no units no units 

Slope Factor no units no units 

Heat Source Btu/ft2/min kW/m2

Heat Sink Btu/ft3 kJ/m3

US Forest Service RMRS-GTR-213WWW. 2008. 17 
BehavePlus 4.0 Variables

Output variable 

Default 

output 

variable

Table 

shading 

variable

Available 

as map 

distance 

English 

units 

Metric 

units 

Flame Residence 

Time

min min 

 

Fuel Outputs 

Fuel Load Transfer 

Portion

percent percent 

Dead Herbaceous 

Fuel Load

ton/ac tonne/ha 

Live Herbaceous Fuel 

Load Remainder

ton/ac tonne/ha 

Total Dead Fuel Load ton/ac tonne/ha 

Total Live Fuel Load ton/ac tonne/ha 

Dead Fuel Load 

Portion

percent percent 

Aspen Outputs 

1-h Fuel Load ton/ac tonne/ha 

Live Herbaceous Fuel 

Load

ton/ac tonne/ha 

Live Woody Fuel 

Load

ton/ac tonne/ha 

1-h Surface-Area-toVolume Ratioft2/ft3 m

2

/m3

Live Woody SurfaceArea-to-Volume 

Ratio 

ft2/ft3 m

2

/m3

Probability of Aspen 

Mortality

percent percent 

P-G Fuel Outputs 

P-G Dead Fine Fuel 

Load

ton/ac tonne/ha 

P-G Dead Medium 

Fuel Load

ton/ac tonne/ha 

P-G Dead Foliage 

Fuel Load

ton/ac tonne/ha 

P-G Live Fine Fuel 

Load

ton/ac tonne/ha 

18 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Output variable 

Default 

output 

variable

Table 

shading 

variable

Available 

as map 

distance 

English 

units 

Metric 

units 

P-G Live Medium 

Fuel Load

ton/ac tonne/ha 

P-G Live Foliage 

Fuel Load

ton/ac tonne/ha 

P-G Litter Fuel Load ton/ac tonne/ha 

Fuel Bed Depth ft m 

CROWN 

Critical Surface 

Intensity

X Btu/ft/s kW/m 

Critical Surface 

Flame Length

ft m 

Transition Ratio X ratio ratio 

Transition to Crown 

Fire?

X X Yes, No Yes, No 

Crown ROS X X ch/h m/min 

Critical Crown ROS X ch/h m/min 

Active Ratio X ratio ratio 

Active Crown Fire? X X Yes, No Yes, No 

Fire Type X X Surface, 

Torching, 

Conditional 

crown, 

Crowning 

Surface, 

Torching, 

Conditional 

crown, 

Crowning 

Crown Spread 

Distance

X ch m 

Crown Fire Area ac ha 

Crown Fire Perimeter ch m 

Crown Fire Lengthto-Width Ratioratio ratio 

SAFETY 

Safety Zone 

Separation Distance

X X ft m 

Safety Zone Size X X ac ha 

Safety Zone Radius X ft m 

SIZE 

Area X X ac ha 

US Forest Service RMRS-GTR-213WWW. 2008. 19 
BehavePlus 4.0 Variables

Output variable 

Default 

output 

variable

Table 

shading 

variable

Available 

as map 

distance 

English 

units 

Metric 

units 

Perimeter X ch m 

Length-to-Width 

Ratio

ratio ratio 

Forward Spread 

Distance

X ch m 

Backing Spread 

Distance

X ch m 

Fire Length X ch m 

Maximum Fire Width X ch m 

Fire Shape Diagrams N/A N/A 

CONTAIN 

Fire Area at Initial 

Attack

ac ha 

Perimeter at Initial 

Attack

ch m 

Contain Status X X Contained, 

Escaped, 

Withdrawn 

Contained, 

Escaped, 

Withdrawn 

Time from Report X X h h 

Contained Area X X ac ha 

Fireline Constructed X ch m 

Number of Resources 

Used

integer integer 

Cost of Resources 

Used

no units no units 

Containment Diagram N/A N/A 

SPOT 

Basic Output 

Spotting Distance 

from Torching Trees

X X X mi km 

Spotting Distance 

from a Burning Pile

X X mi km 

Spotting Distance 

from a Wind-Driven 

Surface Fire

X X mi km 

20 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Output variable 

Default 

output 

variable

Table 

shading 

variable

Available 

as map 

distance 

English 

units 

Metric 

units 

Torching Tree 

Outputs 

Cover Height Applied 

for Spotting 

Distance Torching 

Trees

ft km 

Steady State Flame 

Height from 

Torching Trees

ft m 

Tree Height / Flame 

Height Ratio for 

Torching Trees

ratio ratio 

Steady State Flame 

Duration from 

Torching Trees

no units no units 

Firebrand Height 

from Torching Trees

ft m 

Flat Terrain Spotting 

Distance from 

Torching Trees

mi km 

Burning Pile 

Outputs 

Cover Height Applied 

for Spotting 

Distance from a 

Burning Pile

ft m 

Firebrand Height 

from a Burning Pile

ft m 

Flat Terrain Spotting 

Distance from a 

Burning Pile

mi km 

Surface Fire 

Outputs 

Cover Height Applied 

for Spotting 

Distance from a 

Wind-Driven

Surface Fire

ft m 

Firebrand Height 

from a Wind-Driven

Surface Fire

ft m 

US Forest Service RMRS-GTR-213WWW. 2008. 21 
BehavePlus 4.0 Variables

Output variable 

Default 

output 

variable

Table 

shading 

variable

Available 

as map 

distance 

English 

units 

Metric 

units 

Firebrand Drift 

Distance from a 

Wind-Driven

Surface Fire

mi km 

Flat Terrain Spotting 

Distance from a 

Surface Fire

mi km 

SCORCH 

Scorch Height X X ft m 

MORTALITY 

Bark Thickness in cm 

Tree Crown Length 

Scorched

ft m 

Tree Crown Volume 

Scorched

X percent percent 

Probability of 

Mortality

X X percent percent 

IGNITE 

Probability of

Ignition from a 

Firebrand

X X percent percent 

Probability of 

Ignition from 

Lightning

percent percent 

22 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Variable Descriptions 

A description is given for each of the 181 variables, which are ordered in this section by a 

grouping that differs from the organization on the Input and Output Variable Tables 3 and 

4. For example, Canopy Cover is included in the Wind section, because in BehavePlus, it 

is used only to calculate Wind Adjustment Factor. In addition, variables are assigned to 

one section although some are applicable to more than one group. For example, Elapsed 

Time is described under the Fire Size and Shape section, although it is also used to 

calculate spread distance in the SURFACE and CROWN modules. Organization of 

information about the variables is presented in several ways and the links among them 

should help you find the information that you need. 

There are links to these variable descriptions from: 

 Contents section 

 Table 2—Alphabetical List of Variables 

 Table 3—Input Variable Table 

 Table 4—Output Variable Table 

In this section, a table is included with each variable showing when it is used. When 

BehavePlus runs, it automatically produces worksheets based on user selection of 

modules, input options, and output variables. Only required input is requested. The tables 

describe relevant module and input option selection. 

For example, the following is part of the Flame Length table. 

I/O Module Condition Notes 

Input CROWN If SURFACE is not selected 

and if “Surface fire intensity 

is entered as flame length” is 

selected as an input option. 

You can read it as “Flame Length is an input to the CROWN module if the SURFACE 

module is not selected and if ‘Surface fire intensity is entered as flame length’ is selected 

as an input option.” 

The following window shows the selection of the CROWN module, not SURFACE. 

US Forest Service RMRS-GTR-213WWW. 2008. 23 
BehavePlus 4.0 Variables

The following window shows the selection of the input option for CROWN. Italics in the 

“Condition” column of the table indicate specific wording of the input option, in this 

case, “Surface fire intensity is entered as flame length.” 

24 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Surface Fire Spread and Intensity

Surface Rate of Spread 

Surface Rate of Spread is the "speed" the fire travels through the surface fuels. Surface 

fuels include the litter, grass, brush, and other dead and live vegetation within about 6 ft 

of the ground. 

A SURFACE module input option determines whether rate of spread calculations are 

done for the heading fire spread or any other direction as specified on the worksheet. 

The maximum rate of spread is the spread rate of the head fire. This value is used along 

with elapsed time to determine the size of an elliptically shaped fire in SIZE. The 

maximum spread rate is also used by CONTAIN to model fire growth as suppression 

action is occurring. 

The rate of spread prediction uses the Rothermel (1972) surface fire spread model, which 

assumes that weather, topography, and fuel are constant and uniform for the elapsed time. 

The basic model is for a head fire spreading upslope with the wind. Adaptations have 

been made to allow calculations for other direction options. 

Spread Distance = Rate of Spread × Elapsed Time 

See Fire Characteristics Chart, Figure 5. 

I/O Module Condition Notes 

Input SIZE If SURFACE is not selected. The maximum (head fire) 

spread rate is used in SIZE. 

If both SURFACE and SIZE 

are selected, the calculated 

maximum rate of spread is 

used in SIZE, even if it is not 

displayed as an output in 

SURFACE. 

CONTAIN If SURFACE is not selected. If both SURFACE and 

CONTAIN are selected, the 

calculated maximum (head 

fire) rate of spread is used in 

CONTAIN, even if it is not 

displayed as an output in 

SURFACE. 

Output SURFACE 

US Forest Service RMRS-GTR-213WWW. 2008. 25 
BehavePlus 4.0 Variables

Flame Length 

The Flame Length of a spreading surface fire within the flaming front is measured from 

midway in the active flaming combustion zone to the average tip of the flames (Figure 2). 

"Flame length is an elusive parameter that exists in the eye of the beholder. It is a poor 

quantity to use in a scientific or engineering sense, but it is so readily apparent to fireline 

personnel and so readily conveys a sense of fire intensity that it is worth featuring as a 

primary fire variable" (Rothermel 1991). 

Fireline Intensity and flame length are related to the heat felt by a person standing next to 

the flames. Flame length is calculated from Fireline Intensity, which is a function of Rate 

of Spread and Heat per Unit Area. 

Flame Length is used as a worst-case estimation of flame height for Safety Zone Size

calculations in SAFETY. The head fire flame length is always used. 

See Fireline Intensity, Figure 3. 

See Fire Characteristics Chart, Figure 5. 

I/O Module Condition Notes 

Input CROWN If SURFACE is not selected 

and if “Surface fire intensity 

is entered as flame length” is 

selected as an input option. 

SCORCH If SURFACE is not selected 

and if “Fire intensity is 

entered as flame length” is 

selected as an input option. 

If SURFACE and SCORCH 

are both selected, the flame 

length in the direction of fire 

spread is calculated in 

SURFACE and used by 

SCORCH. 

SAFETY If SURFACE is not selected. If SURFACE and SAFETY 

are both selected, the flame 

for the head fire is used to 

estimate safety zone size 

(even if another spread 

direction is specified on the 

worksheet). 

SPOT If SURFACE is not selected 

and spotting distance from a 

wind-driven surface fire is 

calculated. 

If SURFACE and SPOT are 

both selected, the flame 

length is for the head fire, in 

the direction of maximum 

spread. 

Output SURFACE 

26 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Figure 2—Flame length. 

US Forest Service RMRS-GTR-213WWW. 2008. 27 
BehavePlus 4.0 Variables

Fireline Intensity 

Fireline Intensity is the heat energy release per unit time from a 1-foot (1-meter) wide 

section of the fuel bed extending from the front to the rear of the flaming zone (Figure 3). 

Fireline Intensity is a function of Rate of Spread and Heat per Unit Area, and is directly 

related to Flame Length. 

 

Fireline Intensity and the Flame Length are related to the heat felt by a person standing 

next to the flames. 

See Surface Rate of Spread. 

See Fire Characteristics Chart, Figure 5. 

I/O Module Condition Notes 

Input CROWN If SURFACE is not selected 

and if “Fire intensity is 

entered as fireline intensity”

is selected as an input option. 

SCORCH If SURFACE is not selected 

and if “Fire intensity is 

entered as fireline intensity”

is selected as an input option. 

If SURFACE and SCORCH 

are both selected, fireline 

intensity (flame length) in the 

direction of the spread 

calculation in SURFACE is 

used by SCORCH. 

Output SURFACE 

Figure 3—Fireline intensity. 

28 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Heat per Unit Area 

Heat per Unit Area is the heat energy release per area (square foot or square meter) 

within the flaming front. Heat per Unit Area is not affected by wind, slope, or direction of 

spread. 

Additional energy is released in the burnout phase of combustion, so care should be 

exercised when using Heat per Unit Area to predict fire effects. 

See Fire Characteristics Chart, Figure 5. 

I/O Module Condition Notes 

Input None 

Output SURFACE 

Reaction Intensity 

Reaction Intensity is the rate of the energy release per area (square foot or square meter) 

within the flaming front. Reaction Intensity is not affected by wind, slope, or direction of 

spread. 

Reaction Intensity is part of the Rothermel (1972) surface fire spread model. 

I/O Module Condition Notes 

Input None 

Output SURFACE 

Dead Fuel Reaction Intensity 

Dead Fuel Reaction Intensity is the rate of the energy release per area (square foot or 

square meter) within the flaming front generated by the dead fuel. Reaction Intensity is 

not affected by wind, slope, or direction of spread. Reaction Intensity is a sum of the 

Dead and the Live Fuel Reaction Intensity. 

I/O Module Condition Notes 

Input None 

Output SURFACE Intermediate value 

US Forest Service RMRS-GTR-213WWW. 2008. 29 
BehavePlus 4.0 Variables

Live Fuel Reaction Intensity 

Live Fuel Reaction Intensity is the rate of the energy release per area (square foot or 

square meter) within the flaming front generated by the live fuel. Reaction Intensity is not 

affected by wind, slope, or direction of spread. Reaction Intensity is a sum of the Live 

and Dead Fuel Reaction Intensity. 

I/O Module Condition Notes 

Input none 

Output SURFACE Intermediate value 

Direction of Maximum Spread 

The Direction of Maximum Spread is calculated based on the influence of slope and 

wind and their relative directions (Figure 4). When the wind is not blowing directly 

uphill, the calculated Direction of Maximum Spread will be between the uphill direction 

and the direction in which the wind is blowing (direction of the wind vector). The 

Direction of Maximum Spread is expressed as degrees from upslope or as degrees from 

north, depending on the input option that was selected. 

I/O Module Condition Notes 

Input none 

Output SURFACE 

Figure 4—Direction of maximum spread is determined from directions of wind and slope. 

30 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Surface Spread Distance 

Surface Spread Distance is the distance a surface fire travels during a specified elapsed 

time. It is the product of the Surface Rate of Spread in the specified direction and Elapsed 

Time. 

I/O Module Condition Notes 

Input none 

Output SURFACE Can also be output as a map 

distance if “Display output 

distances in map units” is 

selected. 

Spread Direction 

Spread Direction is the direction in which the surface fire Rate of Spread is calculated. 

Spread Direction can be specified with respect to upslope or to north, depending on the 

selected input option. 

I/O Module Condition Notes 

Input SURFACE If “Surface fire spread 

direction is calculated in 

directions specified on the 

worksheet” is selected as an 

input option. 

Output none 

Wind/Slope/Spread Direction Diagrams 

Wind/Slope/Spread Direction Diagrams are diagrams of the relative directions of 

slope, wind, and fire spread. This illustrates the relative effect of wind and slope on head 

fire direction. 

I/O Module Condition Notes 

Input none 

Output SURFACE 

US Forest Service RMRS-GTR-213WWW. 2008. 31 
BehavePlus 4.0 Variables

Fire Characteristics Chart 

A Fire Characteristics chart displays four basic fire characteristics—Rate of Spread, 

Heat per Unit Area, Flame Length, and Fireline Intensity—as a single point on the chart 

(Figure 5) (Andrews and Rothermel 1982, Rothermel 1983). 

Each of the calculated values in table output is plotted on a simple fire characteristics 

chart. Each calculation result produces a numbered point on the chart. A simple Run 

produces a single point labeled 1. A Run with one range variable of three values (for 

example, wind speed of 0, 5, and 10 mi/h) plots three points labeled 1, 2, and 3. If a 

second range variable exists (for example, for fuel models 1 and 2), six points are plotted. 

The labels follow the same row and column order as the output table. If the fuel models 

are the table row variables and the wind speeds are the table column variables, points 1, 

2, and 3 are for the first row (fuel model 1) columns 1 through 3 (0, 5, and 10 mi/h wind), 

and points 4, 5, and 6 are for the second row (fuel model 2) columns 1 through 3. 

I/O Module Condition Notes 

Input none 

Output SURFACE Table output must be 

requested. 

Figure 5—The fire characteristics chart shows the relationship among rate of spread, 

heat per unit area, fireline intensity, and flame length. 

32 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Fire characteristics 

Flame 

length 

(ft) 

Fireline 

intensity 

(Btu/ft/s) 

Interpretations 

Under 4 Under 100 • Fires can generally be attacked at the head or flanks by 

persons using hand tools. 

• A hand line should hold the fire. 

4 to 8 100 to 500 • Fires are too intense for direct attack on the head by 

persons using hand tools. 

• A hand line cannot be relied on to hold the fire. 

• Equipment such as dozers, pumpers, and retardant aircraft 

can be effective. 

8 to 11 500 to 1000 • Fires may present serious control problems -- torching out, 

crowning, and spotting. 

• Control efforts at the fire head will probably be ineffective. 

Over 11 Over 1000 • Crowning, spotting, and major fire runs are probable. 

• Control efforts at head of fire are ineffective. 

Wind Factor 

The Wind Factor is an intermediate value in Rothermel’s (1972) surface fire spread 

model. Rate of Spread is calculated for zero-wind and zero-slope (R0) and then increased 

by the Wind Factor (Φw) and the Slope Factor (Φs). 

ROS = R0 ( 1 + Φw + Φs) 

I/O Module Condition Notes 

Input none 

Output SURFACE Intermediate value 

Slope Factor 

The Slope Factor is an intermediate value in Rothermel’s (1972) surface fire spread 

model. Rate of Spread is calculated for zero-wind and zero-slope (R0) and then increased 

by the Wind Factor (Φw) and the Slope Factor (Φs). 

ROS = R0 ( 1 + Φw + Φs) 

I/O Module Condition Notes 

Input None 

Output SURFACE Intermediate value 

US Forest Service RMRS-GTR-213WWW. 2008. 33 
BehavePlus 4.0 Variables

Heat Source 

The Heat Source is an intermediate value in Rothermel’s surface fire spread model 

(Rothermel 1972, p. 6, eq. 9). The Rate of Spread is calculated from the ratio of the heat 

flux received from the source (Heat Source, the numerator) and the heat required for 

ignition by the potential fuel (Heat Sink, the denominator). The propagating heat flux is 

increased by Wind and Slope Factors due to flame tilting over the potential fuel to get the 

total propagating heat flux or Heat Source. 

I/O Module Condition Notes 

Input none 

Output SURFACE Intermediate value 

Heat Sink 

The Heat Sink is an intermediate value in Rothermel’s (1972) surface fire spread model. 

The Rate of Spread is calculated from the ratio of the heat flux received from the source 

(Heat Source, the numerator) and the heat required for ignition by the potential fuel (Heat 

Sink, the denominator). The heat required for ignition is dependent upon ignition 

temperature, moisture content of the fuel, and the amount of fuel involved in the ignition 

process. 

I/O Module Condition Notes 

Input none 

Output SURFACE Intermediate value 

Flame Residence Time 

Flame Residence Time is a function of the Characteristic Surface-Area-to-Volume Ratio

of the fuel bed as calculated by Rothermel’s (1972) surface fire spread model. It is used 

to calculate flame depth and Heat per Unit Area. Fireline Intensity is calculated as the 

product of Reaction Intensity times flame depth or Heat per Unit Area times Rate of 

Spread. 

Flame Residence Time only applies to the fine fuels that carry fire spread. Flames are 

also a result of the burning of fuels behind the fire front. Care should be taken in 

interpreting and using Flame Residence Time. 

I/O Module Condition Notes 

Input none 

Output SURFACE Intermediate value 

34 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Surface Fuel

Fuel Model 

A Fuel Model is a set of numerical values that describe the fuel inputs for Rothermel’s 

mathematical model that predicts surface fire spread. 

There are 53 standard fire behavior fuel models including the original 13 described by 

Anderson (1982), plus the 40 defined by Scott and Burgan (2005). Custom fuel models 

can be developed and saved for later use. Standard and custom fuel models can be used 

with one of the three options for calculating a weighted Rate of Spread through two fuel 

models. BehavePlus also offers two special case fuel models that require different input: 

palmetto-gallberry (Hough and Albini 1978) and western aspen (Brown and Simmerman 

1986). 

I/O Module Condition Notes 

Input SURFACE If “Fuel is entered as fuel 

models or two fuel models” is 

selected as an input option. 

If “Fuel is entered as fuel 

parameters” is selected as the 

input option, the assigned 

parameters can be saved as a 

custom fuel model. 

Output none 

First Fuel Model 

The First Fuel Model is specified for the two fuel model option. It does not have to be 

the model that covers the most area. 

I/O Module Condition Notes 

Input SURFACE If “Fuel is entered as two fuel 

models” is selected as an 

input option. 

Output none 

First Fuel Model Coverage 

The First Fuel Model Coverage is the percent of the area represented by the first fuel 

model for the two fuel model option. The area represented by the Second Fuel Model is 

the remaining area. 

I/O Module Condition Notes 

Input SURFACE If “Fuel is entered as two fuel 

models” is selected as an 

input option. 

Output none 

US Forest Service RMRS-GTR-213WWW. 2008. 35 
BehavePlus 4.0 Variables

Second Fuel Model 

The Second Fuel Model is specified for the two fuel model option. It does not have to be 

the model that covers the least area. 

I/O Module Condition Notes 

Input SURFACE If “Fuel is entered as two fuel 

models” is selected as an 

input option. 

Output none 

Fuel Model Type 

Fuel Model Type is static (S) or dynamic (D) for standard or custom fuel models. 

Dynamic fuel models have a live herbaceous fuel component, a portion of which is 

transferred into the Dead Herbaceous Fuel Load depending upon the live herbaceous fuel 

moisture content—the lower the moisture content, the greater the load transfer. The Fuel 

Load Transfer Portion can also be entered directly. 

I/O Module Condition Notes 

Input SURFACE If “Fuel is entered as fuel 

parameters” is selected as an 

input option. 

If “Fuel is entered as fuel 

models” then the value stored 

for the standard or custom 

fuel model is used in the 

calculations. 

Output none 

36 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Fuel Bed Depth 

Fuel Bed Depth represents the depth of the surface fuel and is a Fuel Model parameter. 

Fuel Bed Depth is sometimes called fuel bed bulk depth. A single Fuel Bed Depth value 

must be used for a Fuel Model (Figure 6). 

The Rothermel fire spread model calculations are highly sensitive to the Fuel Bed Depth. 

Fuel Bed Depth is assumed to be uniform for Rothermel’s fire spread model. It is 

necessary to estimate a characteristic value for the area. The depth of the fuel bed 

determined as an output of the planar intercept method (Brown 1974) is called the 

"average high particle depth." Albini and Brown (1978) showed that for logging slash 

fuel beds, the Fuel Bed Depth is approximately 63.3 percent of the average high particle 

depth. For the special case fuel model, palmetto-gallberry (P-G), Fuel Bed Depth is 

calculated as 2/3 of the P-G Height of Understory variable entered on the worksheet. The 

option of using two fuel models to describe an area is also available. 

 

I/O Module Condition Notes 

Input SURFACE If “Fuel is entered as fuel 

parameters” is selected as an 

input option. 

If “Fuel is entered as fuel 

models” or the “special case 

fuel model, western aspen” is 

selected as an input option, 

then the appropriate value is 

used. 

Output SURFACE If “Fuel is entered as special 

case fuel model, palmettogallberry” is selected as an 

input option. 

Figure 6—Fuel bed depth is the average for an area. 

US Forest Service RMRS-GTR-213WWW. 2008. 37 
BehavePlus 4.0 Variables

Dead Fuel Moisture of Extinction 

Dead Fuel Moisture of Extinction is the Characteristic Dead Fuel Moisture above 

which a predictable steady state rate of fire spread is not attainable. Dead Fuel Moisture 

of Extinction is a Fuel Model parameter. 

I/O Module Condition Notes 

Input SURFACE If “Fuel is entered as fuel 

parameters” is selected as an 

input option. 

If “Fuel is entered as fuel 

models” or a “special case 

fuel model,” then the 

appropriate stored value is 

used. 

Output none 

Live Fuel Moisture of Extinction 

Live Fuel Moisture of Extinction is the Characteristic Live Fuel Moisture above which 

the live fuel is a Heat Sink. Below that value, the live fuel is a Heat Source and 

contributes to the surface fire spread. Live Fuel Moisture of Extinction is calculated from 

Dead and Live Fuel Moisture, load, size, and Dead Fuel Moisture of Extinction. 

I/O Module Condition Notes 

Input None 

Output SURFACE 

Dead Fuel Heat Content 

Dead Fuel Heat Content is the amount of heat energy contained in a unit of fuels, and is 

a Fuel Model parameter. The heat content for the standard fire behavior fuel models is 

8,000 Btu/lb (18,622 kJ/kg). 

I/O Module Condition Notes 

Input SURFACE If “Fuel is entered as fuel 

parameters” is selected as an 

input option. 

If “Fuel is entered as fuel 

models” or a “special case 

fuel model,” then the 

appropriate stored value is 

used. 

Output none 

38 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Live Fuel Heat Content 

Live Fuel Heat Content is the amount of heat energy contained in a unit of fuels, and is 

a Fuel Model parameter. The heat content for the standard fire behavior fuel models is 

8,000 Btu/lb (18,622 kJ/kg). 

I/O Module Condition Notes 

Input SURFACE If “Fuel is entered as fuel 

parameters” is selected as an 

input option. 

If “Fuel is entered as fuel 

models” or a “special case 

fuel model,” then the 

appropriate stored value is 

used. 

Output none 

1-h Fuel Load 

The 1-h Fuel Load is the weight of the 1-h dead fuel per unit area, and is a Fuel Model

parameter. The one-hour (1-h) timelag dead fuel category includes dead fuel from 0 to 

0.25 inches (0.64 cm) in diameter. This includes needles, leaves, cured herbaceous plants 

and fine dead stems of plants. This is an important fuel parameter for Rothermel’s fire 

spread model. 

I/O Module Condition Notes 

Input SURFACE If “Fuel is entered as fuel 

parameters” is selected as an 

input option. 

If “Fuel is entered as fuel 

models” or a “special case 

fuel model,” then the 

appropriate stored value is 

used. 

Output SURFACE If “Fuel model is entered as 

the aspen special case fuel 

model” is selected as an input 

option. 

US Forest Service RMRS-GTR-213WWW. 2008. 39 
BehavePlus 4.0 Variables

10-h Fuel Load 

The 10-h Fuel Load is the weight of the 10-h dead fuel per unit area, and is a Fuel Model

parameter. The ten-hour (10-h) timelag dead fuel category includes fuels from 0.25 to 1.0 

inch (0.64 to 2.54 cm) in diameter. 

I/O Module Condition Notes 

Input SURFACE If “Fuel is entered as fuel 

parameters” is selected as an 

input option. 

If “Fuel is entered as fuel 

models” or a “special case 

fuel model aspen,” then the 

appropriate stored value is 

used. 

Output none 

100-h Fuel Load 

The 100-h Fuel Load is the weight of the 100-h dead fuel per unit area, and is a Fuel 

Model parameter. The hundred-hour (100-h) timelag fuel category includes fuels from 1 

to 3 inches (2.54 to 7.62 cm) in diameter. Fuels larger than this category (for example, 

1000-h fuel) do not contribute to the Rate of Spread as calculated by Rothermel’s surface 

fire spread model. 

I/O Module Condition Notes 

Input SURFACE If “Fuel is entered as fuel 

parameters” is selected as an 

input option. 

If “Fuel is entered as fuel 

models” or a “special case 

fuel model,” then the 

appropriate stored value is 

used. 

Output none 

40 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Fuel Load Transfer Portion 

Fuel Load Transfer Portion is the percent or fraction of Live Herbaceous Fuel Load

that is transferred to the Dead Herbaceous Fuel Load. This variable is meaningful only 

for dynamic fuel models (standard or custom). The load transfer portion can be calculated 

from Live Fuel Moisture as described by Scott and Burgan (2005) or entered directly. 

I/O Module Condition Notes 

Input SURFACE If “Dynamic curing load 

transfer is input directly” is 

selected as an input option. 

Output SURFACE If “Dynamic curing load 

transfer is calculated from 

live herbaceous fuel 

moisture” is selected as an 

input option. 

Dead Herbaceous Fuel Load 

Dead Herbaceous Fuel Load is the weight of that class of fuel per unit area and is a 

meaningful variable only for dynamic fuel models (standard and custom). The initial 

value is zero. A portion of the Live Herbaceous Fuel Load is transferred into the Dead 

Herbaceous Fuel Load category. The portion of the load transferred is either specified by 

the user or calculated from Live Herbaceous Fuel Moisture Content. The moisture 

content for this fuel category is the value assigned to the 1-h fuel. The surface-area-tovolume ratio is specified for the live herbaceous fuel. 

I/O Module Condition Notes 

Input None 

Output SURFACE If a fuel model is “dynamic.” 

US Forest Service RMRS-GTR-213WWW. 2008. 41 
BehavePlus 4.0 Variables

Live Herbaceous Fuel Load 

Live Herbaceous Fuel Load is the weight of that class of fuel per area and is a fuel 

model parameter. Live herbaceous fuels are living grasses and forbs, either annual or 

perennial. If the Fuel Model is dynamic, the initial Live Herbaceous Fuel Load is stored 

with the fuel model. A portion of the Live Herbaceous Fuel Load is transferred to Dead 

Herbaceous Fuel Load. The portion of the load transferred is either specified by the user 

or calculated from Live Herbaceous Fuel Moisture Content. 

I/O Module Condition Notes 

Input SURFACE If “Fuel is entered as fuel 

parameters” is selected as an 

input option. 

If “Fuel is entered as fuel 

models” or a “special case 

fuel model,” then the 

appropriate stored value is 

used. 

If the fuel model is 

“dynamic,” the stored value is 

the highest load. 

Output SURFACE If the standard or custom fuel 

model is “dynamic.” 

If “Fuel model is entered as 

the aspen special case fuel 

model” is selected as an input 

option. 

If the fuel model is 

“dynamic,” the value used in 

the calculations is the initial 

value reduced according to 

the Fuel Load Transfer

Portion. 

Live Herbaceous Fuel Load Remainder 

Live Herbaceous Fuel Load is the weight of that class of fuel that remains after some of 

the load is transferred to the dead herbaceous fuel class. This is a meaningful variable 

only for dynamic models (standard and custom fuel). 

I/O Module Condition Notes 

Input none 

Output SURFACE If a fuel model is “dynamic.” 

42 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Live Woody Fuel Load 

Live Woody Fuel Load is the weight of that class of fuel per unit area. Live Woody Fuel 

Load is a fuel model parameter. Live woody fuels include the foliage and very fine stems 

of living shrubs. 

I/O Module Condition Notes 

Input SURFACE If “Fuel is entered as fuel 

parameters” is selected as an 

input option. 

If “Fuel is entered as fuel 

models” or a “special case 

fuel model,” then the 

appropriate stored value is 

used. 

Output SURFACE If “Fuel model is entered as 

the aspen special case fuel 

model” is selected as an input 

option. 

Total Dead Fuel Load 

Total Dead Fuel Load is the sum of the load of all dead fuel classes. For standard and 

custom fuel models this includes 1-h, 10-h, 100-h, and Dead Herbaceous Fuel Load. The 

calculation is done after the load transfer is done for dynamic fuel models. This variable 

is most meaningful in an examination of dynamic fuel models. 

I/O Module Condition Notes 

Input 

Output SURFACE Of primary interest for 

“dynamic” fuel models. 

Total Live Fuel Load 

Total Live Fuel Load is the sum of the load of all live fuel classes. For standard and 

custom fuel models this includes Live Woody and Live Herbaceous Fuel Load. The 

calculation is done after the load transfer is done for dynamic fuel models. This variable 

is most meaningful in an examination of dynamic fuel models. 

I/O Module Condition Notes 

Input 

Output SURFACE Of primary interest for 

“dynamic” fuel models. 

US Forest Service RMRS-GTR-213WWW. 2008. 43 
BehavePlus 4.0 Variables

Dead Fuel Load Portion 

Dead Fuel Load Portion is the percent or fraction of the total fuel load that is dead fuel. 

The calculation is done after the load transfer is done for dynamic fuel models. This 

variable is most meaningful in an examination of dynamic fuel models. 

I/O Module Condition Notes 

Input 

Output SURFACE Of primary interest for 

“dynamic” fuel models. 

1-h Surface-Area-to-Volume Ratio 

1-h Surface-Area-to-Volume Ratio (SA/V) is the amount of area on the outside of the 

fuel (surface area) divided by the volume of the fuel. 1-h fuel is dead fuel 0 to 0.25 in 

(cm) diameter. The 1-h SA/V is a Fuel Model parameter. 

I/O Module Condition Notes 

Input SURFACE If “Fuel is entered as fuel 

parameters” is selected as an 

input option. 

If “Fuel is entered as fuel 

models” then the value stored 

for the standard or custom 

fuel model is used. 

Output SURFACE If “Fuel model is entered as 

the aspen special case fuel 

model” is selected as an input 

option. 

Live Herbaceous Surface-Area-to-Volume Ratio 

Live Herbaceous Surface-Area-to-Volume Ratio (SA/V) is the total surface area of an 

herbaceous plant leaf divided by the volume occupied by the plant leaf. Live Herbaceous 

Surface-Area-to-Volume Ratio is a Fuel Model parameter. 

I/O Module Condition Notes 

Input SURFACE If “Fuel is entered as fuel 

parameters” is selected as an 

input option. 

If “Fuel is entered as fuel 

models,” then the value 

stored for the standard or 

custom fuel model is used. 

Output none 

44 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Live Woody Surface-Area-to-Volume Ratio 

Live Woody Surface-Area-to-Volume Ratio (SA/V) is the total surface area of a shrub 

plant leaf divided by the volume occupied by the plant leaf. Live Woody Surface-Areato-Volume Ratio is a Fuel Model parameter. 

I/O Module Condition Notes 

Input SURFACE If “Fuel is entered as fuel 

parameters” is selected as an 

input option. 

If “Fuel is entered as fuel 

models,” then the value 

stored for the standard or 

custom fuel model is used. 

Output SURFACE If “Fuel model is entered as 

the aspen special case fuel 

model” is selected as an input 

option. 

Characteristic Surface-Area-to-Volume Ratio 

The surface-area-to-volume ratio is the area on the outside of the fuel (surface area) 

divided by the volume of the fuel. The fuel bed Characteristic Surface-Area-to-Volume 

Ratio (σ, ft3/ft2) is a weighted value for all dead and live fuel classes. This is an 

intermediate value in Rothermel’s (1972) surface fire spread model. 

I/O Module Condition Notes 

Input none 

Output SURFACE Intermediate value 

Relative Packing Ratio 

Relative Packing Ratio is the ratio of the fuel bed Packing Ratio to the optimum packing 

ratio (β / βop). This is an intermediate value in Rothermel’s (1972) surface fire spread 

model. It is a factor in determining the optimum reaction velocity. 

I/O Module Condition Notes 

Input None 

Output SURFACE Intermediate value 

US Forest Service RMRS-GTR-213WWW. 2008. 45 
BehavePlus 4.0 Variables

Bulk Density 

The surface fuel bed Bulk Density (lb/ft3) is the total oven-dry fuel loading per cubic 

volume of fuel bed (fuel loading divided by Fuel Bed Depth). This is an intermediate 

value in Rothermel’s (1972) surface fire spread model. 

I/O Module Condition Notes 

Input none 

Output SURFACE Intermediate value 

Packing Ratio 

The Packing Ratio (β) is the ratio of the fuel bed Bulk Density (lb/ft3) to the fuel particle 

density (lb/ft3

). This is an intermediate value in Rothermel’s (1972) surface fire spread 

model. 

I/O Module Condition Notes 

Input none 

Output SURFACE Intermediate value 

46 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Aspen Fuel Model 

There are five special case Aspen Fuel Models (Brown and Simmerman 1986). Their use 

requires different user input than is required for standard or custom fuel models. 

Aspen Fuel model Fuel bed depth, ft 10-h Fuel Load, tons/ac 

• Aspen/shrub 0.65 0.975 

• Aspen/tall forb 0.30 0.475 

• Aspen/low forb 0.18 1.035 

• Mixed/shrub 0.50 1.340 

• Mixed forb 0.18 1.115 

For all aspen fuel models, the following are constants: 

• Heat content = 8,000 Btu/lb 

• Moisture of extinction = 25 percent 

• 1-h Surface-Area-to-Volume Ratio = 109 ft2/ft3

• Live Herbaceous Surface-Area-to-Volume Ratio = 2,800 ft2/ft3

Aspen Curing Level is used to calculate 1-h Fuel Load, Live Herbaceous Fuel Load, Live 

Woody Fuel Load, 1-h SA/V, and Live Woody SA/V. 

I/O Module Condition Notes 

Input SURFACE If “Fuel model is entered as 

the aspen special case fuel 

model” is selected as an input 

option. 

Output none 

Aspen Curing Level 

Aspen Curing Level is the percent (or fraction) of herbaceous biomass that is dead. This 

value is used only for the special case aspen fuel models (Brown and Simmerman 1986). 

It is used to calculate 1-h Load, Live Herbaceous Fuel Load, Live Woody Fuel Load, 1-h 

SA/V, and Live Woody SA/V. Aspen Curing Level is different from the Fuel Load 

Transfer Portion used to represent curing for dynamic fuel models (standard or custom) 

(Scott and Burgan 2005). 

I/O Module Condition Notes 

Input SURFACE If “Fuel model is entered as 

the aspen special case fuel 

model” is selected as an input 

option. 

Output none 

US Forest Service RMRS-GTR-213WWW. 2008. 47 
BehavePlus 4.0 Variables

Aspen Fire Severity 

Aspen Fire Severity is used for predicting mortality of aspen (Brown and Simmerman 

1986, Brown and Debyle 1987). 

• Low fire severity in this case refers to fires that char, but do not completely consume 

leaf litter, and have patches of unburned vegetation and litter. 

• Moderate+ severity fires consume litter and some duff. 

I/O Module Condition Notes 

Input SURFACE If “Fuel model is entered as 

the aspen special case fuel 

model” is selected as an input 

option. 

Required for Probability of 

Aspen Mortality. 

Output none 

P-G Age of Rough 

Palmetto-Gallberry Age of Rough (understory vegetation) is applicable only for the 

special case Palmetto-Gallberry Fuel Model. Age of Rough is used in determining the 

load of all the fuel classes. The age of understory rough is usually the number of years 

since the last fire. It is an important factor in the calculation of fuel load. 

I/O Module Condition Notes 

Input SURFACE If “Fuel is entered as the 

palmetto-gallberry special 

case fuel model” is selected. 

Output none 

P-G Height of Understory 

Height of the palmetto-gallberry understory vegetation (P-G Height of Understory) is 

used in determining the load of all the live fuel classes and the load of the dead 0.0- to 

0.25-inch fuel class. Fuel bed depth is calculated as 2/3 P-G Height of Understory. 

In a very dense understory, the average height is to the top of the foliage of the palmetto 

and gallberry plants. In a patchy or scattered palmetto stand, however, it is necessary to 

measure the heights of the low shrubs or grasses that occur in the openings as well as the 

palmetto and gallberry foliage. The average height is between these two measurements 

and should be weighted by the amount of coverage in each category. 

I/O Module Condition Notes 

Input SURFACE If “Fuel is entered as the 

palmetto-gallberry special 

case fuel model” is selected. 

Output none 

48 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

P-G Palmetto Coverage 

The P-G Palmetto Coverage is the percentage of the ground area covered by palmetto 

fronds. This variable is used to determine the load of the live and dead foliage and load of 

the dead 0.25- to 1.0-inch fuel class. 

According to Hough and Albini (1978), “percent palmetto coverage can be estimated by 

eye.” The simplest method is to locate a number of 100-ft transects at different places in 

the stand. The number of feet of transect covered by palmetto fronds equals the palmetto 

coverage. If transects are not 100 ft long, the percent coverage must be calculated by 

dividing the feet of palmetto frond coverage by the length of the transect. 

I/O Module Condition Notes 

Input SURFACE If “Fuel is entered as the 

palmetto-gallberry special 

case fuel model” is selected. 

Output none 

P-G Overstory Basal Area 

Basal area of the overstory above the palmetto-gallberry rough (P-G Overstory Basal 

Area) is used to determine litter fuel load for the palmetto-gallberry special case fuel 

model. No other variables are affected by Overstory Basal Area. 

Basal area is the cross-sectional area (at breast height; 4.5 ft above ground) of trees, per 

unit of horizontal ground area. In a fixed-radius plot, basal area is the sum of crosssectional area of all trees at breast height (ft2

) divided by the plot area (acres). 

I/O Module Condition Notes 

Input SURFACE If “Fuel is entered as the 

palmetto-gallberry special 

case fuel model” is selected. 

Output none 

P-G Dead Fine Fuel Load 

The P-G Dead Fine Fuel Load for the palmetto-gallberry Fuel Model is the fuel load of 

dead palmetto-gallberry stems with 0- to 0.25-inch diameters. It is calculated from Age of 

Rough and Height of Understory. 

I/O Module Condition Notes 

Input none 

Output SURFACE If “Fuel is entered as the 

palmetto-gallberry special 

case fuel model” is selected. 

US Forest Service RMRS-GTR-213WWW. 2008. 49 
BehavePlus 4.0 Variables

P-G Dead Medium Fuel Load 

The P-G Dead Medium Fuel Load for the palmetto-gallberry Fuel Model is the fuel 

load of dead palmetto-gallberry stems with 0.25- to 1.0-inch diameters. 

I/O Module Condition Notes 

Input none 

Output SURFACE If “Fuel is entered as the 

palmetto-gallberry special 

case fuel model” is selected. 

P-G Dead Foliage Fuel Load 

The P-G Dead Foliage Fuel Load for the palmetto-gallberry Fuel Model is the fuel load 

of dead palmetto-gallberry foliage that is still on the stem. This variable is calculated 

from Age of Rough and Height of Understory. 

I/O Module Condition Notes 

Input none 

Output SURFACE If “Fuel is entered as the 

palmetto-gallberry special 

case fuel model” is selected. 

P-G Live Fine Fuel Load 

The P-G Live Fine Fuel Load for the palmetto-gallberry Fuel Model is the fuel load of 

live palmetto-gallberry stems with 0- to 0.25-inch diameters. This variable is calculated 

from Age of Rough and Height of Understory. 

I/O Module Condition Notes 

Input none 

Output SURFACE If “Fuel is entered as the 

palmetto-gallberry special 

case fuel model” is selected. 

50 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

P-G Live Medium Fuel Load 

The P-G Live Medium Fuel Load for the palmetto-gallberry Fuel Model is the load of 

live palmetto-gallberry stems with 0.25- to 1.0-inch diameters. This variable is calculated 

from Age of Rough and Height of Understory. 

I/O Module Condition Notes 

Input none 

Output SURFACE If “Fuel is entered as the 

palmetto-gallberry special 

case fuel model “is selected. 

P-G Live Foliage Fuel Load 

The P-G Live Foliage Fuel Load for the palmetto-gallberry Fuel Model is the load of 

live palmetto-gallberry foliage still on the stem. This variable is calculated from Age of 

Rough and Height of Understory. 

I/O Module Condition Notes 

Input none 

Output SURFACE If “Fuel is entered as the 

palmetto-gallberry special 

case fuel model” is selected. 

P-G Litter Fuel Load 

The P-G Litter Fuel Load for the palmetto-gallberry Fuel Model is the load of the litter 

layer. This variable is calculated from Age of Rough and Height of Understory. 

I/O Module Condition Notes 

Input none 

Output SURFACE If “Fuel is entered as the 

palmetto-gallberry special 

case fuel model” is selected. 

US Forest Service RMRS-GTR-213WWW. 2008. 51 
BehavePlus 4.0 Variables

Surface Fuel Moisture

1-h Moisture 

1-h Fuel Moisture content is the portion (percent or fraction) of the 1-h timelag dead 

fuel that is water, calculated on a dry weight basis. This value is also used for Dead 

Herbaceous Fuel Moisture Content for dynamic fuel models. 

I/O Module Condition Notes 

Input SURFACE 

IGNITE If “Probability of Ignition 

from a Firebrand” is selected 

as output. 

Output None 

10-h Moisture 

The 10-h Fuel Moisture content is the portion of the 10-h timelag dead fuel that is water, 

calculated on a dry weight basis. The 10-hr Fuel Moisture has less influence on the fire 

spread calculations than does the 1-h Fuel Moisture. The fuel moisture obtained for the 

"fuel moisture sticks" at a National Fire Danger Rating System (NFDRS) weather station 

is an estimate of the 10-h Fuel Moisture. 

I/O Module Condition Notes 

Input SURFACE If there is 10-h fuel in the fuel 

model. 

Output none 

100-h Moisture 

The 100-h Fuel Moisture content is the portion of the 100-h timelag dead fuel that is 

water, calculated on a dry weight basis. The 100-hr fuel moisture content has less 

influence on the fire spread calculations than does the 1-h or 10-h Fuel Moisture content. 

I/O Module Condition Notes 

Input SURFACE If there is 100-h fuel in the 

fuel model. 

IGNITE If “Probability of Ignition 

from Lightning” is selected 

as output. 

Output None 

52 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Dead Fuel Moisture 

The Dead Fuel Moisture value is used for 1-h, 10-h, and 100-h fuels. In most cases this 

gives the same result as 10-h = 1-h + 1 percent and 100-h = 1-h + 2 percent. 

I/O Module Condition Notes 

Input SURFACE If “Moisture is entered by 

dead and live category” is 

selected as an input option. 

Output none 

Live Herbaceous Moisture 

Live Herbaceous Fuel Moisture content is the portion of the live grasses and forbs that 

is water, calculated on a dry weight basis. This can be a very important value for dynamic 

fuels if it is used to determine the amount of fuel that is transferred from the Live

Herbaceous Fuel category to Dead Herbaceous Fuel. (Alternatively, the user can directly 

specify the portion of fuel load that is transferred.) 

. 

I/O Module Condition Notes 

Input SURFACE If live herbaceous fuels are in 

the fuel model. 

If the fuel model is 

“dynamic” and if “Dynamic 

curing load transfer is 

calculated from live 

herbaceous fuel moisture” is 

selected, the value is used to 

determine live and dead 

herbaceous load values. 

If the “special case palmettogallberry fuel model” is 

selected, the value is used for 

P-G live foliage moisture. 

Output none 

US Forest Service RMRS-GTR-213WWW. 2008. 53 
BehavePlus 4.0 Variables

Live Woody Moisture 

Live Woody Fuel Moisture content is the portion of the shrub foliage and very fine 

stems that is water, calculated on a dry weight basis. 

I/O Module Condition Notes 

Input SURFACE If live woody fuels are in the 

fuel model. 

If the “special case palmettogallberry fuel model” is 

selected, the value is used for 

P-G live stemwood moisture. 

Output none 

Live Fuel Moisture 

The Live Fuel Moisture value is used for live herbaceous and live woody fuels. 

I/O Module Condition Notes 

Input SURFACE If “Moisture is entered by 

dead and live category” is 

selected as an input option. 

Output none 

Moisture Scenario 

A fuel Moisture Scenario is a set of fuel moisture values representing moisture 

condition of the surface fuel: 1-h, 10-h, 100-h, Live Herbaceous, and Live Woody Fuel

Moisture. Fuel moisture scenarios are analogous to Fuel Models—they package a number 

of parameters into a labeled set. The user can define and save a Moisture Scenario for 

later use. 

Fuel Moisture Scenarios are useful for testing custom Fuel Models or developing fire 

prescriptions. For example, a moisture Scenario might be developed to represent a 

location's 90th-, 95th-, and 97th-percentile fire weather conditions. 

I/O Module Condition Notes 

Input SURFACE If “Moisture is entered by 

moisture scenario” is 

selected as an input option. 

Values entered when 

“Moisture is entered by 

individual size class” is 

selected can be “Saved as 

Moisture Scenario.” 

Output none 

54 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Characteristic Dead Fuel Moisture 

The Characteristic Dead Fuel Moisture is the calculated Dead Fuel Moisture for the 

whole fuel bed. The calculation involves weighting by dead fuel loading and surfacearea-to-volume ratio for each dead class. 

I/O Module Condition Notes 

Input none 

Output SURFACE Intermediate value 

Characteristic Live Fuel Moisture 

The Characteristic Live Fuel Moisture is the calculated live fuel moisture for the whole 

fuel bed. The calculation involves weighting by live fuel loading and surface-area-tovolume ratio for each live class. 

I/O Module Condition Notes 

Input none 

Output SURFACE Intermediate value 

US Forest Service RMRS-GTR-213WWW. 2008. 55 
BehavePlus 4.0 Variables

Wind

20-Ft Wind Speed 

20-Ft Wind Speed is the wind value at 20 ft above the vegetation. For fire spread and 

intensity calculations in SURFACE, the 20-Ft Wind Speed is reduced to the Midflame 

Wind Speed. In SPOT the 20-Ft Wind Speed is the wind that carries the embers. 

If Wind speed is entered as midflame height is selected as an input option for SURFACE, 

and if either SPOT or CROWN is also selected, the initial version of the worksheet will 

request both Midflame Wind Speed and 20-ft Wind Speed. The program then offers a 

choice of how to resolve the conflict. 

I/O Module Condition Notes 

Input SURFACE If “Wind speed is entered as 

20-ft wind and input or

calculated wind adjustment 

factor” is selected as an input 

option. 

SPOT 

CROWN 

SCORCH If SURFACE is selected and 

“Wind speed is entered as 20-

ft wind and input or

calculated wind adjustment 

factor” is selected as an input 

option. 

Output None 

56 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

10-Meter Wind Speed 

10-Meter Wind Speed is the wind 10 meters above the vegetation. The 10-m Wind 

Speed is reduced to 20-Ft Wind Speed by dividing by 1.15. For fire spread and intensity 

calculations in SURFACE, the 20-Ft Wind Speed is further reduced to Midflame Wind 

Speed according to the Wind Adjustment Factor. 

I/O Module Condition Notes 

Input SURFACE If “Wind speed is entered as 

10-m wind and input or

calculated wind adjustment 

factor” is selected as an input 

option. 

CROWN If “Wind speed is entered as 

10-m wind and input or

calculated wind adjustment 

factor” is selected as an input 

option in SURFACE. 

Otherwise, 20-ft wind speed 

is requested. 

SPOT If “Wind speed is entered as 

10-m wind and input or

calculated wind adjustment 

factor” is selected as an input 

option in SURFACE. 

Otherwise, 20-ft wind speed 

is requested. 

Output None 

US Forest Service RMRS-GTR-213WWW. 2008. 57 
BehavePlus 4.0 Variables

Midflame Wind Speed 

Midflame Wind Speed is the wind speed that exists at midflame height above the fuel 

bed and affects surface fire spread. Midflame wind is often called eye-level wind. 

Technically, Midflame Wind Speed is the average wind speed from the top of the fuel 

bed to the height of the flame above the fuel (Albini and Baughman 1979). Calculation of 

the height as a function of the flames is not feasible. The variability in the Fuel Bed 

Depth, the flames, and the wind overwhelm other considerations. 

Wind speed affects Scorch Height in two ways: in the calculation of Flame Length in 

SURFACE and flame tilt in SCORCH. 

I/O Module Condition Notes 

Input SURFACE If “Wind speed is entered as 

midflame height” is selected 

as an input option. 

SCORCH If “Wind speed is entered as 

midflame height” is selected 

as an input option. 

Output SURFACE If “Wind speed is entered as 

20-ft wind (or 10-m wind) 

and Input (or calculated) 

wind adjustment factor” is 

selected as an input option. 

Effective Wind Speed 

Effective Wind Speed is the combined effect of Midflame Wind Speed and Slope 

Steepness in the direction of maximum spread (head fire). Effective Wind Speed is used 

to determine the shape (length-to-width ratio) of a point source fire. Calculation of 

Effective Wind Speed is based on the Wind and Slope Factors that are part of 

Rothermel’s fire spread model. 

I/O Module Condition Notes 

Input SIZE If SURFACE is not also 

selected. 

If both SURFACE and SIZE 

are selected, the effective 

wind speed value for the 

direction of maximum spread 

calculated in SURFACE is 

used in SIZE. 

Output SURFACE 

58 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Maximum Effective Wind Exceeded? 

Maximum Effective Wind Exceeded? has a value of “Yes” or “No” to indicate whether 

the Effective Wind Speed Limit, as calculated in the surface fire spread model, has been 

exceeded. 

The user has the option of Impose maximum reliable effective wind speed? (Yes or No). 

If “Yes,” the maximum reliable effective wind speed is used in the calculations. If “No,” 

the input values are used. 

I/O Module Condition Notes 

Input none 

Output SURFACE An output value, even if the 

wind limit is not imposed. 

Effective Wind Speed Limit 

The Effective Wind Speed Limit is a function of Reaction Intensity. At high wind 

speeds, fire spread rate can become difficult to predict. In fact, studies have shown that at 

high wind speeds, the spread rate may decrease when the wind speed increases. The data 

are being reanalyzed to assess the role of fuel moisture in that relationship. Other studies 

have not shown a wind limit. The user has the option of “Impose maximum reliable 

effective wind speed?” (Yes or No). If “Yes,” the maximum reliable Effective Wind 

Speed from Rothermel (1972) is used in the calculations. If “No,” the input wind and 

slope values are used. If the Effective Wind Speed Limit is not applied, it might be 

advisable to view the output variable Maximum Effective Wind Exceeded?

I/O Module Condition Notes 

Input none 

Output SURFACE 

Canopy Cover 

Canopy Cover is the percent of the ground covered by overstory canopy. It is used to 

calculate Wind Adjustment Factor. 

I/O Module If Notes 

Input SURFACE If “Wind speed is entered as 

20-ft (or 10-m) wind and 

Calculated wind adjustment 

factor” is selected as an input 

option. 

Output none 

US Forest Service RMRS-GTR-213WWW. 2008. 59 
BehavePlus 4.0 Variables

Wind Adjustment Factor 

Wind Adjustment Factor (WAF) is a value between 0 and 1 used to adjust the wind 

speed at 20-ft above the vegetation to Midflame Wind Speed. 20-Ft Wind Speed * WAF 

= Midflame Wind Speed. Wind Adjustment Factor depends on sheltering of fuels from 

the wind. If fuels are not sheltered from the wind, WAF is a function of Fuel Bed Depth. 

If fuels are sheltered from the wind, WAF is not affected by the surface fuel model. 

Sheltering is determined by canopy cover and position on the slope. If WAF is calculated, 

then the user should be cautious of terrain and canopy features that are not input. In some 

cases, the preferred method might be to use human judgment to enter WAF directly. 

I/O Module Condition Notes 

Input SURFACE If “Wind speed is entered as 

20-ft (or 10-m) wind and 

Input wind adjustment factor” 

is selected as an input option. 

Output SURFACE If “Wind speed is entered as 

20-ft (or 10-m) wind and 

Calculated wind adjustment 

factor” is selected as an input 

option. 

Crown Fill Portion 

Crown Fill Portion (percent or fraction) is an intermediate value in calculating Wind 

Adjustment Factor. It is calculated from Canopy Cover, Canopy Height, and Crown 

Ratio. If Crown Fill Portion is less than 5 percent, the calculation for unsheltered WAF is 

used; otherwise, the calculation is for sheltered WAF. If WAF is input, the value for 

Crown Fill Portion is not meaningful and is set to zero. 

I/O Module Condition Notes 

Input none . 

Output SURFACE If “Wind speed is entered as 

20-ft (or 10-m) wind and 

Calculated wind adjustment 

factor” is selected as an input 

option. 

60 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

WAF Calculation 

WAF (Wind Adjustment Factor) Calculation indicates the method of calculating Wind 

Adjustment Factor (WAF). If the Crown Fill Portion is less than 5 percent, the 

unsheltered model is used; otherwise, the sheltered model is used. If WAF is input rather 

than calculated, the variable is set to “Input.” 

The variable WAF Calculation can have three values: 

 Sheltered 

 Unsheltered 

 Input 

I/O Module Condition Notes 

Input none . 

If “Wind speed is entered as 

20-ft (or 10-m) wind and 

Calculated wind adjustment 

factor” is selected as an input 

option. 

Output SURFACE 

US Forest Service RMRS-GTR-213WWW. 2008. 61 
BehavePlus 4.0 Variables

Direction of Wind Vector (from Upslope) 

Direction of the Wind Vector measured in degrees from Upslope is the direction that 

the wind is pushing the fire (Figure 7). This is useful in examining the vectoring approach 

to wind and slope influences. 

Alternatively, the user can select the option to specify wind direction as the direction 

from which the wind is blowing. It is obviously critical that the user be certain of the 

method of specification of wind direction. 

I/O Module Condition Notes 

Input SURFACE If “Wind is in specified 

directions” 

and if “Wind and spread 

directions are degrees 

clockwise from upslope 

(direction the wind is pushing 

the fire)” are selected as 

input options. 

Output none 

Figure 7—Direction of the wind vector is the direction the wind is pushing the fire. 

62 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Wind Direction (from North) 

Wind Direction measured in degrees from North is the direction from which the wind is 

blowing (Figure 8). This is the usual way of specifying wind direction. This requires 

additional information on aspect to determine the direction of upslope in order to 

consider the combined effect of wind and slope on fire spread. 

Alternatively the user can select the option to specify direction of the wind vector, the 

direction the wind is pushing the fire. It is obviously critical that the user be certain of the 

method of specification of wind direction. 

I/O Module Condition Notes 

Input SURFACE If “Wind direction is specified 

on the worksheet” 

and if “Wind and spread 

directions are degrees 

clockwise from north 

(direction from which the 

wind is blowing)” are 

selected as input options. 

Output none 

Figure 8—Wind direction is the direction from which the wind is blowing. 

US Forest Service RMRS-GTR-213WWW. 2008. 63 
BehavePlus 4.0 Variables

Slope

Aspect 

Aspect is the direction a slope faces (Figure 9). It is used to determine the upslope 

direction in order to consider the combined effect of wind and slope on fire spread. 

I/O Module Condition Notes 

Input SURFACE If “Wind and spread 

directions are degrees 

clockwise from north 

(direction from which the 

wind is blowing)” are 

selected as an input option. 

Output none 

Figure 9—Aspect is the direction the slope is facing. 

64 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Number of Contour Intervals 

The Number of Contour Intervals is a count of the number of contour intervals between 

two points on a map. It is used in the calculation of Slope Steepness. 

When determining the Slope Steepness of a hillside, the user should select two points, 

one at the bottom of the slope and the other at the top. The line connecting these two 

points should be, as much as possible, perpendicular to the contour lines. 

 

I/O Module Condition Notes 

Input SURFACE If “Slope steepness is 

calculated from map 

measurements” is selected as 

an input option. 

Output none 

Contour Interval 

The Contour Interval is the difference in elevation between adjacent topographic 

contours on a topographic map. It is used in the calculation of Slope Steepness. 

I/O Module Condition Notes 

Input SURFACE If “Slope steepness is 

calculated from map 

measurements” is selected as 

an input option. 

Output none 

Map Distance 

The Map Distance is the distance between two points on a map generally expressed in 

inches or centimeters. It is used in the calculation of Slope Steepness. 

I/O Module Condition Notes 

Input SURFACE If “Slope steepness is 

calculated from map 

measurements” is selected as 

an input option. 

Output none 

US Forest Service RMRS-GTR-213WWW. 2008. 65 
BehavePlus 4.0 Variables

Map Representative Fraction 

The Map Representative Fraction is the ratio of the number of units on the map 

between two points to the distance on the ground between these points. For example, if 

the Map Representative Fraction is 1:24,000, then each inch on the map represents 

24,000 inches on the ground. 

I/O Module Condition Notes 

Input SURFACE If “Display output distances 

in map units” is checked on 

the module selection page 

and if distance variables are 

selected as output variables. 

If “Slope steepness is 

calculated from map 

measurements” is selected as 

an input option. 

See the output variable table 

for the variables for which 

map distances are available. 

CROWN 

SIZE 

SPOT 

If “Display output distances 

in map units” is checked on 

the module selection page 

and if distance variables are 

selected as output variables. 

See the output variable table 

for the variables for which 

map distances are available. 

Output none 

Slope Horizontal Distance 

The Slope Horizontal Distance is the ground distance between two locations on the 

ground. This is an intermediate value in the calculation of Slope Steepness from map 

measurements. 

I/O Module Condition Notes 

Input none 

Output SURFACE If “Slope steepness is 

calculated from map 

measurements” is selected as 

an input option. 

66 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Slope Elevation Change 

The Slope Elevation Change is the difference in elevation between the two points used 

to find the map distance. This is an intermediate value in the calculation of Slope 

Steepness from map measurements. 

I/O Module Condition Notes 

Input none 

Output SURFACE If “Slope steepness is 

calculated from map 

measurements” is selected as 

an input option. 

Slope Steepness 

Slope Steepness is the maximum steepness of the slope. It is expressed as either percent 

or degrees depending on the input option that is selected. This is not necessarily the Slope 

Steepness in the direction of spread. 

I/O Module Condition Notes 

Input SURFACE If “Slope steepness is 

specified on the worksheet” is 

selected as an input option. 

Output SURFACE If “Slope steepness is 

calculated from map 

measurements” is selected as 

an input option. 

Crown Fire

Foliar Moisture 

Foliar Moisture content is the moisture content of the conifer overstory foliage (needles 

only). It is used along with surface fire intensity and crown base height to determine the 

threshold for transition to crown fire. 

I/O Module Condition Notes 

Input CROWN 

Output none 

US Forest Service RMRS-GTR-213WWW. 2008. 67 
BehavePlus 4.0 Variables

Canopy Base Height 

The Canopy Base Height for an individual tree is the height at which sufficient fuel 

density exists for sustained canopy ignition. For a stand of trees, Canopy Base Height 

considers both the main canopy layer and ladder fuels in the understory. Canopy Base 

Height and Foliar Moisture Content are used to calculate critical fireline intensity for 

transition to crown fire. 

I/O Module Condition Notes 

Input CROWN 

Output None 

Canopy Bulk Density 

Canopy Bulk Density is a stand description of weight of canopy fuels per unit of canopy 

volume (lb/ft3 or kg/m3). It is used to calculate Critical Crown Fire Rate of Spread to 

determine the threshold for achieving active crown fire (Van Wagner 1977). 

I/O Module Condition Notes 

Input CROWN 

Output None 

Critical Surface Intensity 

The Critical Surface Intensity is the Fireline Intensity of the surface fire that is required 

for a transition from a surface fire to a crown fire. It is calculated from Foliar Moisture

content and Canopy Base Height. 

I/O Module Condition Notes 

Input none 

Output CROWN 

Critical Surface Flame Length 

The Critical Surface Flame Length is associated with the Critical Surface Fireline 

Intensity required for transition to crown fire. 

I/O Module Condition Notes 

Input None 

Output CROWN 

68 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Transition Ratio 

The Transition Ratio is the surface Fireline Intensity divided by the Critical Surface

Fireline Intensity. If the Transition Ratio is greater than or equal to 1, then the surface 

Fireline Intensity is sufficient for a transition to crown fire. Viewing Transition Ratio 

provides more information than the “Yes” or “No” for the Transition to Crown Fire?

variable. It is instructive to know whether the Transition Ratio is close to 1 or 

significantly higher than or lower than 1. 

I/O Module Condition Notes 

Input None 

Output CROWN 

Transition to Crown Fire? 

Transition to Crown Fire? is either “Yes” or “No” to indicate whether conditions 

indicate that transition from surface to crown fire is likely. Calculation depends on the 

Transition Ratio. If the Transition Ratio is greater than or equal to 1, then Transition to 

Crown Fire is “Yes.” If the Transition Ratio is less than 1, then Transition to Crown Fire 

is “No.” 

I/O Module Condition Notes 

Input None 

Output CROWN 

Crown ROS 

The Crown Fire Rate of Spread (ROS) is the forward spread rate of a crown fire 

estimated using Rothermel's (1991) crown fire spread model. It is the overall spread for a 

sustained run over several hours. The spread rate includes the effects of spotting. It is 

calculated from 20-Ft Wind Speed and surface fuel moisture values. The crown fire 

spread model does not include a description of the overstory. 

I/O Module Condition Notes 

Input None 

Output CROWN 

US Forest Service RMRS-GTR-213WWW. 2008. 69 
BehavePlus 4.0 Variables

Critical Crown ROS 

The Critical Crown Fire Rate of Spread (ROS) is the rate at which a crown fire must 

spread to maintain itself as an active crown fire (Van Wagner 1977). It is calculated from 

the Canopy Bulk Density. 

I/O Module Condition Notes 

Input none 

Output CROWN 

Active Ratio 

The Active Ratio is Crown Fire Rate of Spread divided by the Critical Crown Fire Rate 

of Spread. If the Active Ratio is greater than or equal to 1, then the fire may be an active 

crown fire. Viewing Active Ratio provides more information than the “Yes” or “No” for 

the active crown fire variable. It is instructive to know whether the Active Ratio is close 

to 1 or significantly higher than or lower than 1. 

I/O Module Condition Notes 

Input none 

Output CROWN 

Active Crown Fire? 

Active Crown Fire? is either “Yes” or “No” to indicate whether conditions indicate that 

a fire can maintain itself as an active crown fire. Calculation depends on the Active Ratio. 

If the Active Ratio is greater than or equal to 1, then Active Crown Fire is “Yes.” If the 

Active Ratio is less than 1, then Active Crown Fire is “No.” 

I/O Module Condition Notes 

Input None 

Output CROWN 

70 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Fire Type 

Fire Type is one of the following four types: 

 Surface (understory fire) 

 Torching (passive crown fire, surface fire with occasional torching trees) 

 Conditional crown (active crown fire possible if the fire transitions to the overstory) 

 Crowning (active crown fire, fire spreading through the overstory crowns) 

Assignment of Fire Type depends on the variables Transition to Crown and Active 

Crown Fire. 

Transition to Crown? Active Crown fire? Fire Type 

No No Surface 

Yes No Torching 

No Yes Conditional crown 

Yes Yes Crowning 

I/O Module Condition Notes 

Input none 

Output CROWN 

Crown Spread Distance 

The Crown Fire Spread Distance is the forward spread distance of a crown fire 

calculated from Crown Fire Rate of Spread and Elapsed Time. It is the overall spread 

distance for a sustained run over several hours. The Spread Distance includes the effects 

of spotting. 

I/O Module Condition Notes 

Input none 

Output CROWN 

Crown Fire Area 

Crown Fire Area assumes an elliptically shaped fire. The model is in Rothermel (1991, 

p.16). 

I/O Module Condition Notes 

Input None 

Output CROWN 

US Forest Service RMRS-GTR-213WWW. 2008. 71 
BehavePlus 4.0 Variables

Crown Fire Perimeter 

Crown Fire Perimeter assumes an elliptically shaped fire. The model is in Rothermel 

(1991, p.16). 

I/O Module Condition Notes 

Input none 

Output CROWN 

Crown Fire Length-to-Width Ratio 

The Crown Fire Length-to-Width Ratio is based on 20-Ft Wind Speed as described by 

Rothermel (1991, p.16). 

I/O Module Condition Notes 

Input none 

Output CROWN 

Safety Zone

Number of Personnel 

Number of Personnel to be located within the safety zone includes all firefighters, 

overhead team members, and any other personnel to be protected in the safety zone. 

I/O Module Condition Notes 

Input SAFETY 

Output None 

Area per Person 

The Area per Person is the average area occupied by each person to be located within 

the safety zone. A reasonable figure is 50 ft2

 per person, which is enough to allow shelter 

deployment in the event it becomes necessary. 

I/O Module Condition Notes 

Input SAFETY 

Output None 

72 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Number of Heavy Equipment 

The Number of Heavy Equipment is the number of pieces of heavy equipment to be 

sheltered within the safety zone. Each piece is assumed to occupy the area specified by 

the Area per Heavy Equipment variable. 

In the calculations, space is allocated in the safety zone for equipment brought in or 

already present. Equipment is a liability in a safety zone and often carries flammable 

liquids. In no other situation is heavy equipment allowed to operate in such close 

proximity to personnel. Heavy equipment or engines should not be brought to a safety 

zone if such equipment compromises the safety of personnel in the safety zone. 

I/O Module Condition Notes 

Input SAFETY 

Output None 

Area per Heavy Equipment 

The Area per Heavy Equipment is the average area needed for each piece of equipment 

to be located within the safety zone. For general planning, when the equipment types are 

not known, use a mean area of 300 ft2. 

I/O Module Condition Notes 

Input SAFETY 

Output None 

Safety Zone Separation Distance 

The Safety Zone Separation Distance is the minimum distance a firefighter in 

protective clothing must be separated from flames to prevent radiant heat injury. 

Separation Distance is calculated as 4 x flame height. Flame Length is used as a worst 

case estimator of flame height. If SURFACE is used to calculate Flame Length, the 

maximum Flame Length is used for SAFETY even if SURFACE calculations are for a 

direction other than for the direction of maximum spread. 

To estimate Separation Distance from a crown fire, the user must estimate flame height 

since there is not a Flame Length calculation in CROWN. 

Separation Distance is calculated to prevent only radiant heat injury. Exposure to 

convective heat is not accounted for in this model. 

I/O Module Condition Notes 

Input none 

Output SAFETY 

US Forest Service RMRS-GTR-213WWW. 2008. 73 
BehavePlus 4.0 Variables

Safety Zone Radius 

Safety Zone Radius is the radius of a circular safety zone just large enough to protect the 

specified number of personnel and heavy equipment from radiant burn injury. Calculation 

is based on Separation Distance plus the radius of a circle that fits all personnel and 

equipment. This core area represents the only safe area of the safety zone. Outside the 

core area persons may receive burns. 

 

I/O Module Condition Notes 

Input None 

Output SAFETY 

Safety Zone Size 

Safety Zone Size is the area of a circular safety zone just large enough to protect the 

specified number of personnel and heavy equipment from radiant burn injury. Calculation 

is based on the Safety Zone Radius, which is estimated from flame height. Flame Length

can be calculated for a surface fire in SURFACE and used as a worst-case estimate of 

flame height. Flame height must be estimated by the user for a crown fire. 

 

I/O Module Condition Notes 

Input none 

Output SAFETY 

74 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Fire Size and Shape

Elapsed Time 

Elapsed Time is the time for which the fire is spreading at an assumed constant rate. 

Rate of spread is multiplied by Elapsed Time to calculate the spread distance of a surface 

fire or a crown fire. In SIZE it is the time from when a fire begins spreading (which may 

be different from the time of ignition) until the time that the size calculation is done. 

For surface fire, the fuel, fuel moisture, wind, and slope are assumed to be constant for 

the entire elapsed time. Because wildland fires burn under non-uniform conditions, the 

Elapsed Time and choice of input values must be considered carefully to achieve useful 

projections. 

The crown fire model applies to spread times on the order of hours. During that time, 

conditions can vary considerably. Crown fire rate of spread includes the effect of 

spotting. 

I/O Module Condition Notes 

Input SURFACE 

CROWN 

SIZE 

Output none 

US Forest Service RMRS-GTR-213WWW. 2008. 75 
BehavePlus 4.0 Variables

Area or Fire Size at Report 

The area of a fire started from a point source is based on the elliptical fire shape model 

(Figure 10). To avoid confusion, the variable is called Area when it is an output of the 

SIZE module. It is called Fire Size at Report when it is an input to the CONTAIN 

module. Calculation of Area is based on maximum rate of spread, Effective Wind Speed, 

and Elapsed Time. This variable applies to surface fire. Crown fire area can also be 

calculated. 

I/O Module Condition Notes 

Input CONTAIN If SIZE is not also selected. Labeled “Fire Size at 

Report.” 

Output SIZE Labeled “Area.” 

Figure 10—A point source fire shape and size is assumed to be elliptically shaped. 

Perimeter 

The Perimeter of a point source fire without suppression action is based on the elliptical 

fire shape model. Perimeter is the distance around the outside of the ellipse. This variable 

applies to surface fire. Crown fire perimeter can also be calculated. 

See Area, Figure 10. 

I/O Module Condition Notes 

Input none 

Output SIZE 

76 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Length-to-Width Ratio 

The Length-to-Width Ratio of an elliptically shaped fire is sometimes called length-tobreadth ratio. The Length-to-Width Ratio is a function of the Effective Wind Speed. Fire 

burning under a high wind speed will be long and narrow with a large Length-to-Width 

Ratio (Figure 11). This variable applies to surface fire. Crown fire Length-to-Width Ratio 

can also be calculated. 

I/O Module Condition Notes 

Input CONTAIN If neither SURFACE nor 

SIZE is selected. 

If SURFACE and CONTAIN 

are both selected, the lengthto-width ratio is calculated in 

SURFACE and made 

available to CONTAIN even 

if SIZE is not selected. 

Output SIZE 

Figure 11—Fire shape depends on effective wind speed. Higher winds result in a higher 

length-to-width ratio. 

US Forest Service RMRS-GTR-213WWW. 2008. 77 
BehavePlus 4.0 Variables

Forward Spread Distance 

The Forward Spread Distance is based on the elliptical fire shape model and is the 

distance the head fire travels in a specified period of time. This is the same as Surface 

Spread Distance for a heading fire. 

See Area, Figure 10. 

I/O Module Condition Notes 

Input None 

Output SIZE Can also be output as a map 

distance if “Display output 

distances in map units” is 

checked. 

Backing Spread Distance 

The Backing Spread Distance is based on the elliptical fire shape model and is the 

distance the fire travels in a specified period of time in the direction of 180 degrees from 

the Direction of Maximum Spread. 

See Area, Figure 10. 

I/O Module Condition Notes 

Input None 

Output SIZE Can also be output as a map 

distance if “Display output 

distances in map units is 

checked.” 

Fire Length 

Fire Length is the distance from the head of a fire to the back end of the fire calculated 

for an elliptically shaped fire. Fire Length is the sum of the Forward Spread Distance and 

the Backing Spread Distance. 

See Area, Figure 10. 

I/O Module Condition Notes 

Input none 

Output SIZE Can also be output as a map 

distance if “Display output 

distances in map units is 

checked.” 

78 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Maximum Fire Width 

The Maximum Fire Width is the maximum width of a fire and is based on the elliptical 

fire shape model and is maximum width of the ellipse. 

See Area, Figure 10. 

I/O Module Condition Notes 

Input none 

Output SIZE Can also be output as a map 

distance if “Display output 

distances in map units” is 

checked. 

Fire Shape Diagrams 

Fire Shape Diagrams of the elliptical shape of a point source surface fire with no 

suppression action are produced for each result in the table output. Rate of Spread and 

Elapsed Time determine the Forward Spread Distance. Effective Wind Speed determines 

the Length-to-Width Ratio. 

I/O Module Condition Notes 

Input none 

Output SIZE A diagram is produced for 

each result in the table output.

Containment

Suppression Tactic 

There are two possible Suppression Tactics: 

 Head attack, where fireline is constructed starting at the head of the fire and proceeds 

at an equal rate down both flanks. 

 Rear attack, where fireline is constructed starting at the rear of the fire and proceeds at 

an equal rate up both flanks. 

Because the attack proceeds along both flanks, the Resource Line Production Rate along 

each flank is half of the specified rate for the resource. Head attack is more risky than 

rear attack, but head attack results in smaller fires and quicker containment. 

I/O Module Condition Notes 

Input CONTAIN Head or Rear attack can be 

specified. 

Output none 

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BehavePlus 4.0 Variables

Line Construction Offset 

Line Construction Offset is the distance from the fire at which the suppression line is 

constructed. Line Construction Offset distance of zero indicates a direct attack, while any 

other value indicates a parallel attack. 

I/O Module Condition Notes 

Input CROWN 

Output none 

Resource Name 

Resource Name is a simple, one-word identifier for a containment resource. The 

Resource Name is used to associate the resource with its Resource Arrival Time, 

Resource Line Production Rate, and Resource Duration. The Resource Name must be a 

single word with no embedded blanks or tabs. Examples include: Engine1, Type1Crew1, 

Type_2_Crew_2, and Bulldozer1. 

The number of Resource Name entries on the worksheet must match the number of 

Resource Arrival Time, Resource Line Production Rate, and Resource Duration entries. 

I/O Module Condition Notes 

Input CONTAIN If “Suppression input entered 

for multiple resources” is 

selected. 

Output none 

Resource Line Production Rate 

The Resource Line Production Rate is the average, sustained rate at which the resource 

can build and hold fire line. Enter the total rate for the resource, which is assumed to be 

split evenly between the two flanks. Thus, a crew (or single piece of equipment) that 

constructs fireline at 20 ch/h is assumed to be constructing 10 ch/h on the left flank and 

another 10 ch/h on the right flank. Guidelines are given in the Fireline Handbook. 

If “Suppression is entered for multiple resources” is selected as an input option, there 

must be exactly one Resource Line Production Rate entry in the worksheet for every 

Resource Name entry. 

I/O Module Condition Notes 

Input CONTAIN 

Output None 

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BehavePlus 4.0 Variables 

Resource Arrival Time 

The Resource Arrival Time is the elapsed time from the time the fire was reported until 

the containment resource arrives at the fire and begins line construction. All firefighting 

activities that occur before line construction begins should be accounted in the Resource 

Arrival Time (for example, travel time to fire, scouting and size up, safety zone 

construction, and so forth). 

If “Suppression input entered for multiple resources” is selected as an input option, there 

must be exactly one Resource Arrival Time entry in the worksheet for every Resource 

Name entry. Each resource will be dispatched to the fire according to its Resource 

Arrival Time. 

I/O Module Condition Notes 

Input CONTAIN 

Output None 

Resource Base Cost 

Resource Base Cost is the fixed cost of mobilizing a resource on a fire. If “Suppression 

input entered for multiple resources” is selected as an input option, there must be exactly 

one Resource Base Cost entry in the worksheet for every Resource Name entry. If a 

resource is dispatched to a fire, its total cost is the Resource Base Cost plus its Resource 

Hourly Cost for the time spent on the fire until the fire is contained or the Resource 

Duration expires. If a resource is not dispatched to a fire, nothing is added to the total 

cost for that resource. Units for cost are not assigned. Costs can be calculated as, for 

example, euros, dollars, or hundreds of dollars. 

I/O Module Condition Notes 

Input CONTAIN 

Output none 

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BehavePlus 4.0 Variables

Resource Hourly Cost 

The Resource Hourly Cost is the cost per hour of using the resource on a fire. If 

“Suppression input entered for multiple resources” is selected as an input option, there 

must be exactly one Resource Base Cost entry in the worksheet for every Resource Name

entry. If a resource is dispatched to a fire, its total cost is the Resource Base Cost plus its 

Resource Hourly Cost for the time spent on the fire until the fire is contained or the 

Resource Duration expires. If a resource is not dispatched to a fire, nothing is added to 

the total cost for that resource. Units for cost are not assigned. Costs can be calculated as, 

for example, euros, dollars, or hundreds of dollars. 

I/O Module Condition Notes 

Input CONTAIN 

Output None 

Resource Duration 

Resource Duration is the amount of time during which the containment resource is able 

to sustain its Resource Line Production Rate. The resource begins line construction at its 

designated Resource Arrival Time, performs line construction at its specified Resource 

Line Production Rate, and stops line construction after its Resource Duration has expired. 

If “Suppression input entered for multiple resources” is selected as an input option, there 

must be exactly one Resource Duration entry in the worksheet for every Resource Name

entry. All resources will be dispatched to the fire according to their arrival times until the 

fire is either contained or escapes. 

I/O Module Condition Notes 

Input CONTAIN 

Output none 

Fire Area at Initial Attack 

Fire Area at Initial Attack is the fire size when the first resource begins suppression 

action on the fire. It is calculated from the Fire Size at Report and the Resource Arrival 

Time. 

I/O Module Condition Notes 

Input None 

Output CONTAIN 

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BehavePlus 4.0 Variables 

Perimeter at Initial Attack 

Perimeter at Initial Attack is the perimeter of the elliptical shaped fire when the first 

resource begins suppression action. It is calculated from the Fire Size at Report and the 

Resource Arrival Time. 

I/O Module Condition Notes 

Input None 

Output CONTAIN 

Contain Status 

The Contain Status is the final status of the initial attack effort. This output variable has 

three possible values: 

 Contained, which indicates that the fire was successfully contained using one or more 

of the specified resources. 

 Withdrawn, which indicates that the fire escaped containment after resources arrived 

and constructed some containment line but were withdrawn because the specified 

resource duration was exceeded. (The resources are unable to work long enough to 

contain the fire even though they might have been gaining on it.) 

 Escaped, which indicates that the specified available resources are insufficient to 

enclose the fire, so no containment line was constructed. 

 

I/O Module Condition Notes 

Input None 

Output CONTAIN 

Time from Report 

The Time from Report is the elapsed time from the fire report time until it was either 

successfully contained or it escaped the initial attack. 

I/O Module Condition Notes 

Input none 

Output CONTAIN 

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BehavePlus 4.0 Variables

Contained Area 

Contained Area is the size of the fire enclosed by the containment line. The Contained 

Area will be larger than the fire area at the time of containment when the Line 

Construction Offset is greater than zero. If the fire escapes because resource productivity 

is insufficient or all specified resources have arrived at the fire and their duration times 

have expired, the Contained Area is given as “-1.” 

I/O Module Condition Notes 

Input None 

Output CONTAIN 

Fireline Constructed 

Fireline Constructed is the total length of fireline constructed by all resources at the 

time the fire was contained or determined to have escaped. If the fire is not successfully 

contained, Fireline Constructed is the total length of fire line constructed by all resources 

at the time the last available resource duration expires. If the fire is successfully 

contained, Fireline Constructed is the total length of line constructed by all resources at 

the time of containment. 

 

I/O Module Condition Notes 

Input none 

Output CONTAIN 

Number of Resources Used 

The Number of Resources Used indicates those actually used to contain the fire. It may 

be less than the number of resources specified on the worksheet if the fire is contained 

before all resources arrive. 

I/O Module Condition Notes 

Input none 

Output CONTAIN If “Suppression input entered 

for multiple resources” is 

selected. 

84 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Cost of Resources Used 

Cost of Resources Used is the total cost of all resources dispatched to the fire. If a 

resource is dispatched to a fire, its cost is the Resource Base Cost plus its Resource 

Hourly Cost for the time spent on the fire until the fire is contained or its Resource 

Duration expires. Units for cost are not assigned. Costs can be calculated as, for example, 

euros, dollars, or hundreds of dollars. 

I/O Module Condition Notes 

Input None 

Output CONTAIN 

Containment Diagram 

The Containment Diagram is a diagram of the final fire shape, size, and constructed 

fireline at the time it was successfully contained, escaped initial attack, or reached the 

specified Spread Distance Limit. 

I/O Module Condition Notes 

Input None 

Output CONTAIN 

US Forest Service RMRS-GTR-213WWW. 2008. 85 
BehavePlus 4.0 Variables

Spotting Distance

Ridge-to-Valley Elevation Difference 

The Ridge-to-Valley Elevation Difference is the vertical elevation difference in feet or 

meters between ridge top and valley bottom in the direction of terrain flow. A stylized 

terrain shape is used by the model to calculate spotting distance (Figure 12). 

I/O Module Condition Notes 

Input SPOT 

Output none 

Figure 12—Terrain is described as a simplified sine shape for the spotting distance 

model. 

Ridge-to-Valley Horizontal Distance 

The Ridge-to-Valley Horizontal Distance is the horizontal distance in miles or 

kilometers between the ridge top and the valley bottom in the direction of the wind flow. 

A stylized terrain shape is used by the model to calculate spotting distance. 

If winds are blowing directly up or down a valley, the Ridge-to-Valley Horizontal 

Distance will often be set to zero. 

I/O Module Condition Notes 

Input SPOT 

Output none 

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BehavePlus 4.0 Variables 

Spotting Source Location 

 The Spotting Source Location refers to the origin of the firebrands that can result in 

spotting. The possible locations are: 

 ridge top (RT) 

 midslope, windward side (MW) 

 valley bottom (VB) 

 midslope, leeward side (ML) 

I/O Module Condition Notes 

Input SPOT 

Output none 

Canopy Height 

Canopy Height describes the stand as it affects calculation of the Wind Adjustment 

Factor in SURFACE. If canopy height is less than 6 ft, then the unsheltered WAF 

calculation is used. 

In SPOT, Canopy Height is the mean cover height in the downwind direction as it affects 

the distance a firebrand is carried. If the forest cover is open, a value of half the tree 

height should be entered. 

I/O Module Condition Notes 

Input SURFACE If wind adjustment factor is 

calculated. 

SPOT 

Output none 

US Forest Service RMRS-GTR-213WWW. 2008. 87 
BehavePlus 4.0 Variables

Flame Height from a Burning Pile 

Flame Height from a Burning Pile is the average height of the flames measured from 

the base of a burning pile (Figure 13). This is not the same as Flame Length from a 

spreading surface fire. There is not a model to calculate flame height from a burning pile, 

it is an input determined by expert opinion. 

I/O Module Condition Notes 

Input SPOT If “Spotting distance from a 

burning pile” is selected as 

an output variable. 

Output none 

Figure 13—Continuous flame height is a user input for the model for spotting distance 

from a burning pile. 

Number of Torching Trees 

Number of Torching Trees is the number of trees that torch simultaneously. It is used to 

calculate the height to which firebrands are lofted. 

I/O Module Condition Notes 

Input SPOT If “Spotting distance from 

torching trees” is selected as 

an output variable. 

Output none 

88 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Spot Tree Species 

Spot Tree Species is used in estimating the maximum spotting distance from torching 

trees. Fourteen tree species are available for SPOT. 

Tree species is also an input to MORTALITY. There are 206 species available for 

MORTALITY and 14 for SPOT. To avoid confusion, there are two tree species variables: 

Spot Tree Species and Mortality Tree Species. The same tree species codes are used for 

both variables. 

I/O Module Condition Notes 

Input SPOT 

Output none 

Cover Height Applied for Spotting Distance Torching Trees 

Cover Height Applied for Spotting Distance from Torching Trees is the cover height 

actually used in the computation of spotting distance. This is an intermediate value in the 

calculations that might be of interest to someone examining the spotting distance model. 

A critical cover height is first determined from the maximum firebrand height. The 

applied cover height is the greater of the critical cover height or the input canopy height. 

I/O Module Condition Notes 

Input none 

Output SPOT Intermediate value 

Steady State Flame Height from Torching Trees 

The Steady State Flame Height from Torching Trees is used to calculate the height to 

which a firebrand is lofted. This is an intermediate value in the calculations that might be 

of interest to someone examining the spotting distance model. 

I/O Module Condition Notes 

Input none 

Output SPOT Intermediate value 

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BehavePlus 4.0 Variables

Tree Height/Flame Height Ratio for Torching Trees 

The ratio of the torching tree height to the steady state flame height (Tree Height/Flame 

Height Ratio for Torching Trees) is used to determine the appropriate equation for 

firebrand height. This is an intermediate value in the calculations that might be of interest 

to someone examining the spotting distance model. 

I/O Module Condition Notes 

Input none 

Output SPOT Intermediate value 

Steady State Flame Duration from Torching Trees 

Steady State Flame Duration from Torching Trees is a dimensionless value (no units) 

used to calculate firebrand height. This is an intermediate value in the calculations that 

might be of interest to someone examining the spotting distance model. 

I/O Module Condition Notes 

Input none 

Output SPOT Intermediate value 

Firebrand Height from Torching Trees 

The Firebrand Height from Torching Trees is the maximum height to which a 

firebrand is lifted and is still viable for starting a fire when it lands. This is an 

intermediate value in the calculations that might be of interest to someone examining the 

spotting distance model. 

I/O Module Condition Notes 

Input none 

Output SPOT Intermediate value 

Flat Terrain Spotting Distance from Torching Trees 

The Flat Terrain Spotting Distance from Torching Trees is the maximum spotting 

distance over flat terrain. This value is then adjusted according to the terrain descriptors. 

This is an intermediate value in the calculations that might be of interest to someone 

examining the spotting distance model. 

I/O Module Condition Notes 

Input none 

Output SPOT Intermediate value 

90 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Cover Height Applied for Spotting Distance from a Burning Pile 

Cover Height Applied for Spotting Distance from a Burning Pile is the cover height 

actually used in the computation of spotting distance. A critical cover height is first 

determined from the maximum firebrand height. The applied cover height is the greater 

of the critical cover height or the input canopy height. This is an intermediate value in the 

calculations that might be of interest to someone examining the spotting distance model. 

I/O Module Condition Notes 

Input None 

Output SPOT Intermediate value 

Firebrand Height from a Burning Pile 

Firebrand Height from a Burning Pile is the maximum height to which a firebrand is 

lifted and is still viable for starting a fire when it lands. This is an intermediate value in 

the calculations that might be of interest to someone examining the spotting distance 

model. 

I/O Module Condition Notes 

Input none 

Output SPOT Intermediate value 

Flat Terrain Spotting Distance from a Burning Pile 

The Flat Terrain Spotting Distance from a Burning Pile is the maximum spotting 

distance over flat terrain. This value is then adjusted according to the terrain descriptors. 

This is an intermediate value in the calculations that might be of interest to someone 

examining the spotting distance model. 

I/O Module Condition Notes 

Input none 

Output SPOT Intermediate value 

US Forest Service RMRS-GTR-213WWW. 2008. 91 
BehavePlus 4.0 Variables

Cover Height Applied for Spotting Distance from a Wind-Driven 

Surface Fire 

Cover Height Applied for Spotting Distance from a Wind-Driven Surface Fire is the 

cover height actually used in the computation of spotting distance. A critical cover height 

is first determined from the maximum firebrand height. The applied cover height is the 

greater of the critical cover height and the input canopy height. This is an intermediate 

value in the calculations that might be of interest to someone examining the spotting 

distance model. 

I/O Module Condition Notes 

Input none 

Output SPOT Intermediate value 

Firebrand Height from a Wind-Driven Surface Fire 

Firebrand Height from a Wind-Driven Surface Fire is the maximum height to which a 

firebrand is lifted and is still viable for starting a fire when it lands. This is an 

intermediate value in the calculations that might be of interest to someone examining the 

spotting distance model. 

I/O Module Condition Notes 

Input None 

Output SPOT Intermediate value 

Firebrand Drift Distance from a Wind-Driven Surface Fire 

Firebrand Drift Distance from a Wind-Driven Surface Fire is the distance that the 

firebrand is carried as it is being lifted. This is an intermediate value in the calculations 

that might be of interest to someone examining the spotting distance model. 

I/O Module Condition Notes 

Input none 

Output SPOT Intermediate value 

Flat Terrain Spotting Distance from a Surface Fire 

The Flat Terrain Spotting Distance from a Surface Fire is the maximum spotting 

distance of a wind-driven surface fire over flat terrain. This value is then adjusted 

according to the terrain descriptors. This is an intermediate value in the calculations that 

might be of interest to someone examining the spotting distance model. 

I/O Module Condition Notes 

Input none 

Output SPOT Intermediate value 

92 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Spotting Distance from Torching Trees 

Spotting Distance from Torching Trees is the maximum distance that one can expect 

potential spot fires resulting from firebrands from torching trees (Figure 14). 

The spotting model is applicable under conditions of intermediate fire severity in which 

spotting distance up to a mile or two might be expected. This model does not apply to 

those extreme situations where spotting may occur up to tens of miles from the fire front. 

The model is for spotting from torching trees, not from a spreading crown fire. 

I/O Module Condition Notes 

Input none 

Output SPOT Spot distance can also be 

output as a map distance if 

“Display output distances in 

map units” is checked. 

Figure 14—Maximum spotting distance from torching trees. 

US Forest Service RMRS-GTR-213WWW. 2008. 93 
BehavePlus 4.0 Variables

Spotting Distance from a Burning Pile 

Spotting Distance from a Burning Pile is the maximum distance that one can expect 

potential spot fires resulting from firebrands from the burning pile. Flame height from a 

burning pile is an input, which is used to calculate the lofting strength from the burning 

pile. 

I/O Module Condition Notes 

Input none 

Output SPOT Spot distance can also be 

output as a map distance if 

“Display output distances in 

map units” is checked. 

Spotting Distance from a Wind-Driven Surface Fire 

Spotting Distance from a Wind-Driven Surface Fire is the maximum distance that one 

can expect potential spot fires based on firebrands from a spreading wind-driven surface 

fire. The model is applicable only if the fire is truly wind-driven through surface fuels 

that are not sheltered from the wind by overstory. 

If both SURFACE and SPOT are selected, calculated Flame Length in the direction of 

maximum spread is used in the spotting distance calculations. Because the spotting model 

is only valid for wind-driven head fires, the SURFACE calculation must be for the 

direction of maximum spread. The program provides guidance on resolving a conflict. 

I/O Module Condition Notes 

Input none 

Output SPOT Spot distance can also be 

output as a map distance if 

“Display output distances in 

map units” is checked. 

94 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Scorch Height

Air Temperature 

Air Temperature is the ambient dry bulb temperature measured in the shade. It is used 

to calculate Scorch Height and Probability of Ignition from a Firebrand. Note that Air 

Temperature is not included in the SURFACE Rate of Spread and Intensity calculations. 

I/O Module Condition Notes 

Input SCORCH 

IGNITE If “Probability of ignition 

from a firebrand” is 

calculated. 

Output none 

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BehavePlus 4.0 Variables

Scorch Height 

Scorch Height is the height above the ground that the temperature in the convection 

column reaches the lethal temperature to kill live crown foliage (Figure 15). This 

temperature is assumed to be 140° Fahrenheit (60° Celsius). 

If both SURFACE and SCORCH modules are selected, Midflame Wind Speed and the 

Flame Length in the direction of the spread calculation are used as input to SCORCH. 

I/O Module Condition Notes 

Input MORTALITY If SCORCH is not selected. If both MORTALITY and 

SCORCH are selected, then 

the scorch height calculated 

in scorch is used in 

MORTALITY. 

Output SCORCH 

Figure 15—Tree crown length scorched is found from tree height, tree crown length, and 

scorch height. 

96 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Tree Mortality

Tree Height 

The Tree Height is the distance from the ground to the top of a tree. It is the height of a 

torching tree in the SPOT module, and is used to calculate percent Crown Volume 

Scorched in MORTALITY. 

See Scorch Height, Figure 15. 

I/O Module Condition Notes 

Input MORTALITY 

SPOT 

Output none 

Crown Ratio 

Crown Ratio is the proportion of the total vertical tree height that is occupied by the 

vertical length of the tree crown (Figure 16). Crown Ratio is used to calculate the percent 

crown scorch in the MORTALITY module, and Wind Adjustment Factor in the 

SURFACE module. 

I/O Module Condition Notes 

Input MORTALITY 

SURFACE If wind adjustment factor is 

calculated. 

Output SURFACE If wind adjustment factor is 

calculated and canopy base 

height is entered for 

CROWN. 

Figure 16—Crown ratio examples. 

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BehavePlus 4.0 Variables

D.B.H. 

D.B.H. is the abbreviation for diameter at breast high. It is the measurement of the 

diameter of a tree measured at about 4.5 ft (1.4 m) from the ground level. It is used to 

calculate Bark Thickness in MORTALITY and is part of the torching tree description in 

SPOT. 

I/O Module Condition Notes 

Input MORTALITY 

SPOT 

Output none 

Bark Thickness 

Bark Thickness is used in the tree mortality calculations. Bark Thickness can either be 

entered directly or it can be calculated from tree species and D.B.H. 

I/O Module Condition Notes 

Input MORTALITY If “Bark thickness is specified 

on the worksheet” is selected 

as an input option. 

Output MORTALITY If “Bark thickness is 

estimated from species and 

d.b.h.” is selected as an input 

option. 

Mortality Equation 

Two equations used in the tree mortality model, one for spruce species and one for all 

other species. The 206 species available for the variable Mortality Tree Species are used 

only for the Bark Thickness calculation. If Bark Thickness is input directly rather than 

calculated, the user is not asked to select from 206 species, but rather is asked to specify 

the Mortality Equation as “Nonspruce” or “Spruce.” 

I/O Module Condition Notes 

Input MORTALITY If “Bark thickness is specified 

on the worksheet” is selected 

as an input option. 

Output none 

98 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Mortality Tree Species 

Mortality Tree Species is used to estimate Bark Thickness for the tree mortality model 

in the MORTALITY module. Two hundred and six species are available for the 

MORTALITY bark thickness model. There are only two equations for the mortality 

model itself, one for spruce species and one for all other species. 

Tree species is also an input to SPOT. Because 14 species are used in the SPOT module, 

to avoid confusion, there are two tree species variables: Spot Tree Species and Mortality 

Tree Species. The same tree species codes are used for both variables. 

I/O Module Condition Notes 

Input MORTALITY If “Bark thickness is 

estimated from species and 

d.b.h.” is selected as an input 

option. 

Output none 

Tree Crown Length Scorched 

Tree Crown Length Scorched is calculated as Scorch Height - (Tree Height - Live 

Crown Length). If Scorch Height is greater than the Tree Height, then Tree Crown 

Length Scorched is set equal to the Tree Crown Length. Tree Crown Length Scorched is 

used to calculate percent Crown Volume Scorched, which is used to calculate tree 

mortality. 

See Scorch Height, Figure 15. 

I/O Module Condition Notes 

Input None 

Output MORTALITY 

Tree Crown Volume Scorched 

Tree Crown Volume Scorched is the percentage (or fraction) of the tree crown volume 

that is scorched, and is used to calculate tree mortality. It is calculated from the Length of 

Tree Crown Scorched and the total crown length. 

I/O Module Condition Notes 

Input None 

Output MORTALITY 

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BehavePlus 4.0 Variables

Probability of Mortality 

Probability of Mortality is the likelihood that a tree will be killed by a fire. Probability 

of Mortality is based on Bark Thickness and Percent Crown Volume Scorched, which are 

derived from Scorch Height, Tree Height, Crown Ratio, Species, and tree diameter. 

Probability of Mortality can be interpreted as the probability of an individual tree dying 

or it can be multiplied by the number of trees on a site to estimate the number of trees 

that will die. 

Probability of Aspen Mortality is calculated by a different model and is available by 

selecting the western aspen special case fuel model in SURFACE. 

I/O Module Condition Notes 

Input None 

Output MORTALITY 

Probability of Aspen Mortality 

Probability of Mortality is the likelihood that a tree will be killed by a fire. Probability of 

Aspen Mortality is based on Flame Length, D.B.H., and fire severity of either low or 

moderate+ (Brown and Debyle 1987). This calculation is done as part of the special case 

aspen fuel model that can be selected in SURFACE. It is not included as part of the 

MORTALITY module. 

Probability of Aspen Mortality can be interpreted as the probability of an individual tree 

dying or it can be multiplied by the number of trees on a site to estimate the number of 

trees that will die. 

I/O Module Condition Notes 

Input none 

Output SURFACE If “Fuel model is entered as 

the aspen special case fuel 

model” is selected as an input 

option. 

Aspen mortality is a special 

case and is not available in 

the MORTALITY module. 

100 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Probability of Ignition

Fuel Shading from the Sun 

Fuel Shading from the Sun is the portion of the fuel that is not receiving direct sunlight 

due to cloud cover and overstory. It is used to calculate Probability of Ignition by a 

Firebrand. 

Less than 50 percent shading is sometimes referred to as being in an unshaded condition; 

Fifty percent or more shading is referred to as shaded condition. 

I/O Module Condition Notes 

Input IGNITE 

Output none 

Probability of Ignition from a Firebrand 

Probability of Ignition from a Firebrand is an indication of the chance that a firebrand 

will cause an ignition. The number of firebrands, their size, and the fuel on which they 

land is generally unknown. Therefore, there is no specific interpretation of the probability 

of ignition of a fire. 

The calculation of probability of ignition is based on shading of fuels from the sun, fine 

dead fuel moisture, and the dry bulb temperature. 

I/O Module Condition Notes 

Input None 

Output IGNITE 

Lightning Ignition Fuel Type 

Lightning Ignition Fuel Type is used to calculate probability of ignition from a 

lightning strike. There are eight fuel types, which are only used for this ignition model. 

Depending on fuel type, ignition from lightning is a function of either 100-Hr Moisture 

Content or Lightning Duff and Litter Depth. 

I/O Module Condition Notes 

Input IGNITE For probability of ignition by 

lightning. 

Output none 

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BehavePlus 4.0 Variables

Lightning Duff and Litter Depth 

Lightning Duff and Litter Depth is the average depth of the litter and duff layers at the 

point of the lightning strike. It is used to calculate the probability of ignition from a 

lightning strike. The word "lightning" is included in the variable name to emphasize that 

this duff and litter depth is not used in any other module. 

I/O Module Condition Notes 

Input IGNITE For probability of ignition by 

lightning. 

Output none 

Lightning Strike Type 

Lightning Strike Type is the type of lightning discharge (polarity). It can be 

 negative (-) 

 positive (+) 

 unknown (?) 

The characteristic of lightning responsible for ignition of ground fuel is the presence and 

duration of a continuing current, which varies by Lightning Strike Type. The presence or 

duration of a continuing current is not detected by the current operational lightning 

detection network. 

I/O Module Condition Notes 

Input IGNITE For probability of ignition by 

lightning. 

Output none 

Probability of Ignition from Lightning 

The Probability of Ignition from Lightning is the chance that a lightning strike will 

ignite ground fuel of the type specified. It is calculated from fuel type, duff and litter 

depth, fuel moisture, and type of lightning discharge (negative, positive, or unknown). 

This model predicts ignition of duff and similar ground fuels. No model is available to 

determine the probability of surface fire ignition from the ground fire. Not all ground fuel 

ignitions from lightning will result in a spreading surface fire. 

I/O Module Condition Notes 

Input none 

Output IGNITE 

102 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

References 

Albini, Frank A. 1976a. Computer-based models of wildland fire behavior: a user's 

manual. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain 

Forest and Range Experiment Station. 68 p. 

Albini, Frank A. 1976b. Estimating wildfire behavior and effects. Gen. Tech. Rep. 

INT-30. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain 

Forest and Range Experiment Station. 92 p. 

Albini, Frank A. 1979. Spot fire distance from burning trees – a predictive model. Gen. 

Tech. Rep. INT-56. Ogden, UT: U.S. Department of Agriculture, Forest Service, 

Intermountain Forest and Range Experiment Station. 73 p. 

Albini, Frank A. 1981. Spot fire distance from isolated sources – extensions of a 

predictive model. Res. Note INT-309. Ogden, UT: U.S. Department of Agriculture, 

Forest Service, Intermountain Forest and Range Experiment Station. 9 p. 

Albini, Frank A.; Baughman, Robert G. 1979. Estimating windspeeds for predicting 

wildland fire behavior. Res. Pap. INT-221. Ogden, UT: U.S. Department of 

Agriculture, Forest Service, Intermountain Forest and Range Experiment Station. 

12 p. 

Albini, Frank A.; Brown, James K. 1978. Predicting slash depth for fire modeling. Res. 

Pap. INT-206. Ogden, UT: U.S. Department of Agriculture, Forest Service, 

Intermountain Forest and Range Experiment Station. 22 p. 

Albini, F. A.; Korovin, G. N.; Gorovaya, E. H. 1978. Mathematical analysis of forest fire 

suppression. Res. Pap. INT-207. Ogden, UT: U.S. Department of Agriculture, Forest 

Service, Intermountain Forest and Range Experiment Station. 19 p. 

Anderson, Hal E. 1969. Heat transfer and fire spread. Res. Pap. INT-69. U.S. 

Department of Agriculture, Forest Service, Intermountain Forest and Range 

Experiment Station. 20 p. 

Anderson, Hal E. 1982. Aids to determining fuel models for estimating fire behavior. 

Gen. Tech. Rep. INT-122. Ogden, UT: U.S. Department of Agriculture, Forest 

Service, Intermountain Forest and Range Experiment Station. 22 p. 

Anderson, Hal E. 1983. Predicting wind-driven wild land fire size and shape. Gen. Tech. 

Rep. INT-305. Ogden, UT: U.S. Department of Agriculture, Forest Service, 

Intermountain Forest and Range Experiment Station. 26 p. 

Andrews, Patricia L. 1986. BEHAVE: fire behavior prediction and fuel modeling system 

– BURN subsystem, part 1. Gen. Tech. Rep. INT-194. Ogden, UT: U.S. Department 

of Agriculture, Forest Service, Intermountain Research Station. 130 p. 

Andrews, Patricia L. 2007. BehavePlus fire modeling system: past, present, and future, 

In: Proceedings of 7th Symposium on Fire and Forest Meteorology, 2007 October 22-

26; Bar Harbor, ME: 13 p. http://ams.confex.com/ams/pdfpapers/126669.pdf. 

Andrews, Patricia L.; Bevins, Collin D.; Seli, Robert C. 2008. BehavePlus fire modeling 

system, version 4.0: user's guide. Gen. Tech. Rep. RMRS-GTR-106WWW revised. 

Ogden, UT: U.S. Department of Agriculture, Forest Service, Rocky Mountain 

Research Station. 116 p. 

Andrews, Patricia L.; Rothermel, Richard C. 1982. Charts for interpreting wildland fire 

behavior characteristics. Gen. Tech. Rep. INT-131. Ogden, UT: U.S. Department 

of Agriculture, Forest Service, Intermountain Forest and Range Experiment 

Station. 21 p. 

US Forest Service RMRS-GTR-213WWW. 2008. 103 
BehavePlus 4.0 Variables

Brown, James K. 1974. Handbook for inventorying downed woody material. Gen. Tech. 

Rep. INT-16. Ogden, UT: U.S. Department of Agriculture, Forest Service, 

Intermountain Forest and Range Experiment Station. 24 p. 

Brown, James K.; Debyle, Norbert V. 1987. Fire damage, mortality, and suckering in 

aspen. Canadian Journal of Forest Research, 17: 1100-1109. 

Brown, James K.; Simmerman, Dennis G. 1986. Appraising fuels and flammability in 

western aspen: a prescribed fire guide. Gen. Tech. Rep. INT-205. Ogden, UT: U.S. 

Department of Agriculture, Forest Service, Intermountain Research Station. 48 p. 

Burgan, Robert E. 1979. Estimating live fuel moisture for the 1978 National Fire Danger 

Rating System. Res. Pap. INT-226. Ogden, UT: U.S. Department of Agriculture, 

Forest Service, Intermountain Forest and Range Experiment Station. 17 p. 

Burgan, Robert E. 1987. Concepts and interpreted examples in advanced fuel modeling. 

Gen. Tech. Rep. INT-238. Ogden, UT: U.S. Department of Agriculture, Forest 

Service, Intermountain Research Station. 40 p. 

Burgan, Robert E.; Rothermel, Richard C. 1984. BEHAVE: fire behavior prediction and 

fuel modeling system—FUEL subsystem. Gen. Tech. Rep. INT-167. Ogden, UT: 

U.S. Department of Agriculture, Forest Service, Intermountain Forest and Range 

Experiment Station. 126 p. 

Byram, George M. 1959. Combustion of forest fuels. In: Davis, Kenneth P., ed. Forest 

fire: control and use. New York, NY: McGraw Hill: 61-123. 

Chase, Carolyn H. 1981. Spot fire distance equations for pocket calculators. Res. Note 

INT-310. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain 

Forest and Range Experiment Station. 21 p. 

Finney, Mark A. 2003. Calculation of fire spread rates across random landscapes. 

International Journal of Wildland Fire. 12(2): 167-174. 

Finney, Mark A. 1998. FARSITE: fire area simulator--model development and 

evaluation. Res. Pap. RP-RMRS-4. Ogden, UT: U.S. Department of Agriculture, 

Forest Service, Rocky Mountain Research Station. 47 p. 

Fried, Jeremy S.; Fried, Burton D. 1996. Simulating wildfire containment with realistic 

tactics. Forest Science. 42(3): 267-281. 

Fujioka, Francis M. 1985. Estimating wildland fire rate of spread in a spatially 

nonuniform environment. Forest Science. 31(1): 21-29. 

Hough, Walter A.; Albini, Frank A. 1978. Predicting fire behavior in palmetto-gallberry 

fuel complexes. Res. Pap. SE-174. Asheville, NC: U.S. Department of Agriculture, 

Forest Service, Southeastern Forest Experiment Station. 44 p. 

Latham, Don J.; Schlieter, Joyce A. 1989. Ignition probabilities of wildland fuels based 

on simulated lightning discharges. Res. Pap. INT-411. Ogden, UT: U.S. Department 

of Agriculture, Forest Service, Intermountain Research Station. 16 p. 

Reinhardt, Elizabeth D.; Crookston, Nicholas L. 2003. The Fire and Fuels Extension to 

the Forest Vegetation Simulator. Gen. Tech. Rep. RMRS-GTR-116. Ogden, UT: U.S. 

Department of Agriculture, Forest Service, Rocky Mountain Research Station. 209 p. 

Rothermel, Richard C. 1972. A mathematical model for predicting fire spread in wildland 

fuels. Res. Pap. INT-115. Ogden, UT: U.S. Department of Agriculture, Forest 

Service, Intermountain Forest and Range Experiment Station. 40 p. 

104 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Rothermel, Richard C. 1983. How to predict the spread and intensity of forest and range 

fires. Gen. Tech. Rep. INT-143. Ogden, UT: U.S. Department of Agriculture, Forest 

Service, Intermountain Forest and Range Experiment Station. 161 p. 

Rothermel, Richard C. 1991. Predicting behavior and size of crown fires in the Northern 

Rocky Mountains. Res. Pap. INT-438. Ogden, UT: U.S. Department of Agriculture, 

Forest Service, Intermountain Research Station. 46 p. 

Ryan, Kevin C.; Reinhardt, Elizabeth D. 1988. Predicting post fire mortality of seven 

western conifers. Canadian Journal of Forest Research. 18: 1291-1297. 

Schroeder, Mark J. 1969. Ignition probability. Unpublished report on file at: U.S. 

Department of Agriculture, Forest Service, Rocky Mountain Forest and Range 

Experiment Station, Fort Collins, CO. 26 p. 

Scott, Joe H.; Burgan, Robert E. 2005. Standard fire behavior fuel models: a 

comprehensive set for use with Rothermel's surface fire spread model. Gen. Tech. 

Rep. RMRS-GTR-153. Fort Collins, CO: U.S. Department of Agriculture, Forest 

Service, Rocky Mountain Research Station. 72 p. 

Scott, Joe H.; Reinhardt, Elizabeth D. 2001. Assessing crown fire potential by linking 

models of surface and crown fire behavior. Res. Pap. RMRS-RP-29. Fort Collins, 

CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research 

Station. 59 p. 

Turner, J.A.; Lawson, B.D. 1978. Weather in the Canadian Forest Fire Danger Rating 

System: a user guide to national standards and practices. Inf. Rep. BC-X-177. 

Canadian Forest Service, Pacific Forestry Centre, Victoria, BC, Canada. 40 p. 

Van Wagner, C. E. 1973. Height of crown scorch in forest fires. Canadian Journal of 

Forest Research. 3(3): 373-378. 

Van Wagner, C. E. 1977. Conditions for the start and spread of crown fire. Canadian 

Journal of Forest Research. 7(1): 23-34. 

Van Wagner, C. E. 1993. Prediction of crown fire behavior in two stands of jack pine. 

Canadian Journal of Forest Research. 23(3): 442-449. 

US Forest Service RMRS-GTR-213WWW. 2008. 105 
BehavePlus 4.0 Variables

Appendix A—Mathematical models in BehavePlus 

Table A1—Models that are included in each of the BehavePlus modules with citations 

and notes. 

BehavePlus 

Module 

Model Reference 

and notes

SURFACE Surface head fire rate of spread 

Reaction intensity, heat per unit area 

Intermediate values: characteristic dead fuel moisture, 

live fuel moisture of extinction, relative packing ratio, 

etc. 

(Rothermel 1972) 

(Albini 1976a) minor adjustments 

Fireline intensity 

Flame length 

(Byram 1959) with adjustments to work with 

Rothermel’s surface fire spread model by 

(Albini 1976b) 

Surface fire flame residence time 

(used to calculate fireline intensity) 

(Anderson 1969) 

Direction of maximum spread (Rothermel 1983) using manual vectoring 

(Rothermel 1983) (Finney 1998) 

calculations based on Rothermel’s wind 

and slope factors 

Fire characteristics chart, relationship among rate of 

spread, heat per unit area, fireline intensity, and flame 

length 

(Finney 1998) 

Spread in direction from ignition point from a point 

source fire 

(Andrews 1986) 

Effective wind speed (Albini 1976b) 

Wind adjustment factor (Albini and Baughman 1979, Baughman 

and Albini 1980, Rothermel 1983) 

Wind speed at 10 m adjusted to 20 ft (Turner and Lawson 1978) 

13 standard fire behavior fuel models (Rothermel 1972) 11 fuel models) 

(Albini 1976b) slight revision of the 11 plus 

two more fuel models 

(Anderson 1982) fuel model selection guide 

40 standard fire behavior fuel models (Scott and Burgan 2005) 

Custom fire behavior fuel models (Burgan and Rothermel 1984, Burgan 

1987) 

Dynamic fuel load transfer (Burgan 1979) 

(Burgan and Rothermel 1984, Andrews 

1986) as used in BEHAVE 

(Scott and Burgan 2005) as used in the 

2005 standard fire behavior fuel models 

Two fuel models weighted rate of spread (Rothermel 1983) 

Two fuel models, harmonic mean (Fujioka 1985) 

Two fuel models, 2-dimensional expected spread (Finney 2003) 

Palmetto gallberry special case fire behavior fuel 

model 

(Hough and Albini 1978) 

106 US Forest Service RMRS-GTR-213WWW. 2008.
BehavePlus 4.0 Variables 

Table A1 (continued) 

BehavePlus 

Module 

Model Reference 

and Notes

CROWN Critical surface intensity needed for transition from 

surface to crown fire 

(Van Wagner 1977) 

Transition to crown fire, relationship of surface fire 

intensity and critical surface fire intensity 

(Finney 1998) 

(Scott and Reinhardt 2001) 

Crown fire rate of spread, area, and perimeter (Rothermel 1991) 

Critical crown fire rate of spread, needed for an active 

crown fire

(Van Wagner 1993)

Active crown fire, relationship of crown fire rate of 

spread and critical crown fire rate of spread

(Finney 1998)

(Scott and Reinhardt 2001)

Fire type: surface, torching, conditional crown, 

crowning

(Finney 1998)

(Scott and Reinhardt 2001)

SAFETY Safety zone size, separation distance, radius (Butler and Cohen 1996; 1998b, a) 

SIZE Elliptical fire size and shape, area, perimeter, lengthto-width ratio 

(Anderson 1983) double ellipse 

(Andrews 1986) simplified to simple ellipse 

CONTAIN Fire containment (Albini and others 1978) in the old 

BEHAVE 

(Fried and Fried 1996) in BehavePlus 

SPOT Spotting distance from torching trees (Albini 1979, Chase 1981) 

Spotting distance from a burning pile (Albini 1981) 

Spotting distance from a wind-driven surface fire (Albini 1983a, Albini 1983b, Chase 1984) 

SCORCH Crown scorch height (Van Wagner 1973) 

MORTALITY Tree mortality (Ryan and Reinhardt 1988, Reinhardt and 

Crookston 2003) 

Bark thickness (Ryan and Reinhardt 1988, Reinhardt and 

Crookston 2003) 

IGNITE Probability of ignition from firebrand (Schroeder 1969) 

Probability of ignition from lightning (Latham and Schlieter 1989) 

Fine dead 

fuel moisture 

Tool 

Fine dead fuel moisture tables (Rothermel 1983) 

US Forest Service RMRS-GTR-213WWW. 2008. 107 
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