Dead Fuel Moisture

This is the moisture content of dead organic fuels, expressed as a percentage of the oven dry weight of the sample, that is controlled entirely by exposure to environmental conditions.  The NFDRS processor models these values based on inputs such as precipitation and relative humidity.  There is modeled fuel moisture for each of the four timelag fuel classes recognized by the system.  Timelag is the time necessary for a fuel particle of a particular size to lose approximately 63% of the difference between its initial moisture content and its equilibrium moisture content in its current environment.
 
1-Hr Fuel Moisture Content
 
 The 1-hour fuel moisture content represents the modeled 
fuel moisture of dead fuels from herbaceous plants or roundwood that is less than one-quarter inch in diameter.  Also estimated is the uppermost layer of litter on the forest floor.  Due to its size, this fuel is very responsive to the current atmospheric moisture content.  It varies greatly throughout the calendar day and is principally responsible for diurnal changes in fire danger.  Values can range from 1 to 80. 
 
10-Hr Fuel Moisture Content

This is the moisture content of dead fuels consisting of roundwood in the size range of one quarter to one inch in diameter and very roughly, the layer of litter extending from just below the surface to three-quarters of an inch below the surface.  Currently, the NFDRS code models this moisture value based on length of day, cloud cover, temperature and relative humidity.  It is planned that by Summer 2002, the NFDRS code will begin modeling this moisture using solar radiation instead of cloud cover for those stations collecting solar radiation measurements.  Ten-hour fuel moisture values vary somewhat with diurnal changes but vary more so with day-to-day changes in weather.  Values can range from 1 to 60.
  

100-Hr Fuel Moisture Content

The 100-hour fuel moisture value represents the modeled moisture content of dead fuels in the 1 to 3 inch diameter class.  It can also be 
used as a very rough estimate of the average moisture content of the forest floor from three-fourths inch to four inches below the surface.  The 100-hour timelag fuel moisture is a function of length of day (as influenced by latitude and calendar date), maximum and minimum temperature and relative humidity, and precipitation duration in the previous 24 hours.  Values can range from 1 to 50 percent.  A default value based on the climate class of the first priority fuel model module in the station catalog will automatically be used if there is a break of 30 days or more in the observations entered.
 
1000-Hr Fuel Moisture Content

This value represents the modeled moisture content in the dead fuels in the 3 to 8 inch diameter class and the layer of the forest floor about four inches below the surface.  The value is based on a running 7-day average.  The 1000-hour timelag fuel moisture is a function of length of day (as influenced by latitude and calendar date), daily temperature and relative humidity extremes (maximum and minimum values) and the 24-hour precipitation duration values for a 7-day period.  Values can range from 1 to 40 percent.

-
 
-
- is derived from the 1000-hr fuel moisture value.  It is an independent variable used in the calculation of the herbaceous fuel moisture.  The X-1000 is a function of the daily change -hour timelag fuel moisture, and the average temperature.  Its purpose is to -hour -1000 value is designed to decrease at the same rate 
as the 1000-hour timelag fuel moisture, but to have a slower rate of increase than the 1000-hour timelag fuel moisture during periods of precipitation, hence limiting excessive 
 
-
, 10
-
, 100
-
, and 1000
-
Hr fuel moistures may be a 
e moisture of subsurface organic material as described 
above; however, the NFDRS processor does not include subsurface parameters in these 
calculations.  Dead fuel moistures are based solely on roundwood fuel moisture 
 
 be started 30 to 45 days prior to the onset of green
-
up to 
assure that the 1000
-
hour timelag fuel moisture and associated X
-
1000 have stabilized at 
a reasonable value for the current weather conditions.

1.07.2
 

Indices and Components
 
The following paragraphs address each component and index of the National Fire Danger Rating System.  They include a definition of each, its numeric value ranges, and the designed function of the component or index.  The numbers generated by NFDRS are relative in the sense that as the value of a component or index doubles, the activity measured by that component or index doubles.  (An Ignition Component of 60 has twice the ignition probability of an Ignition 
Component of 30.)  This helps the users of the NFDRS interpret the meaning of the numbers produced for their protection area.
  
Ignition Component - (IC)

The Ignition Component is a rating of the probability that a firebrand will cause a fire requiring suppression action.  Since it is expressed as a probability, it ranges on a scale of 0 to 100.  An IC of 100 means that every firebrand will cause a fire requiring action if it contacts a receptive fuel. Likewise an IC of 0 would mean that no firebrand would cause a fire requiring suppression action under those conditions.
Note the emphasis is on action.  The key is whether a fire will result that requires a fire manager to make a decision.  The Ignition Component is more than the probability of a fire starting; it has to have the potential to spread.  Therefore Spread Component (SC) values are entered into the calculation of IC.  If a fire will ignite and spread, some action or decision is needed. 
 
Spread Component - (SC)
  
The Spread Component is a rating of the forward rate of spread of a 
headfire.  Deeming, et al.,  (1977), states that  "the spread component is numerically equal to the theoretical ideal rate of spread expressed in feet-per-minute."  This carefully worded statement indicates both guidelines (it's theoretical) and cautions (it's ideal) that must be used when applying the Spread Component.  Wind speed, slope and fine fuel moisture are key inputs in the calculation of the spread component, thus accounting for a high variability from day-to-day.  The Spread Component is expressed on an open-ended scale; thus it has no upper limit. 
 
 
Energy Release Component - (ERC)

The Energy Release Component is a number related to the available energy (BTU) per unit area (square foot) within the flaming front at the head of a fire. Daily variations in ERC are due to changes in moisture content of the various fuels present, both live and dead.  Since this number represents the potential "heat release" per unit area in the flaming zone, it can provide guidance to several important fire activities.  It may also be considered a composite fuel moisture value as it reflects the contribution that all live and dead fuels have to potential fire intensity.  The ERC is a cumulative or "build-up" type of index.  As live fuels cure and dead fuels dry, the ERC values get higher thus providing a good reflection of drought conditions.  The scale is open-ended or unlimited and, as with other NFDRS components, is relative.  Conditions producing an ERC value of 24 represent a potential heat release twice that of conditions resulting in an ERC value of 12.
 
 As a reflection of its composite fuel moisture nature, the ERC becomes a relatively stable evaluation tool for planning decisions that might need to be made 24 to 72 hours ahead of an expected fire decision or action.  Since wind and slope do not enter into the ERC calculation, the daily variation will be relatively small.  The 1000-hr timelag fuel moisture (TLFM) is a primary entry into the ERC calculation through its effect on both living and dead fuel moisture inputs.  There may be a tendency to use the 1000-hr TLFM as a separate "index" for drought considerations.  A word of caution -any use of the 1000-hr TLFM as a separate "index" must be preceded by an analysis of historical fire weather data to identify critical levels of 1000-hr TLFM.  A better tool for measurement of drought conditions is the ERC since it considers both dead and live fuel moistures.
 
Burning Index (BI)

The Burning Index is a number related to the contribution of fire behavior to the effort of containing a fire.  The BI (difficulty of control) is derived from a combination of Spread Component (how fast it will spread) and Energy Release Component (how much energy 
will be produced).  In this way, it is related to flame length, which, in the Fire Behavior Prediction System, is based on rate of spread and heat per unit area.  However, because of differences in the calculations for BI and flame length, they are not the same.  The BI is an index that rates fire danger related to potential flame length over a fire danger rating area.  The fire behavior prediction system produces flame length predictions for a specific location (Andrews, 1986).  
 
The BI is expressed as a numeric value related to potential flame length in feet multiplied by 10. The scale is open-ended which a
llows the range of numbers to adequately define fire problems, 
even during low to moderate fire danger. 
 
A cross-reference for BI to potential flame length, fireline intensity and descriptions of expected prescribed burning and fire suppression conditions is provided in Table 1 (adapted from Deeming et al. 1977).  It is important to remember that a computed BI value is an index 
representing the near upper limit to be expected on the rating area.  In other words, if a fire occurs in the worst fuel, weather and topography conditions somewhere in the rating area, these numbers represent the potential fireline intensity and flame length.  These conditions are not expected throughout the entire fire danger rating area at any one time or under less severe conditions.  

Local relationships of fire danger outputs to fire activity are also portrayed effectively on Fire Danger PocketCards (see section 1.08.9).
 
The relationship of ERC and SC to BI is shown in the fire characteristics chart in Table 2 (Andrews and Rothermel, 1982).
  
Table 1.
 
Burning Index/Fire Behavior Cross Reference (Deeming et al. 1977)
  
BI-1978
 
Potential 
Flame          
Length (ft)
 
 
Fireline Intensity    
(BTUs/sec/ft)
 
Narrative Comments
 
0
-
30
 
0
-
3
 
0
-
55
 
Most prescribed burns are 
conducted in this
 range. 
 
30
-
40
 
3
-
4
 
55
-
110
 
Generally represent the 
limit of control for direct 
attack methods.
 
40
-
60
 
4
-
6
 
110
-
280
 
Machine methods usually 
necessary or indirect attack 
should be used.
 
60
-
80
 
6
-
8
 
280
-
520
 
The prospects for direct 
control by any means are 
poor
 above this intensity.
 
80
-
90
 
8
-
9
 
520
-
670
 
The heat load on people 
within 30 feet of the fire is 
dangerous.
 
90
-
110+
 
9+
 
670
-
1050+
 
Above this intensity, 
spotting, fire whirls, and 
crowning should be 
expected.
 

Keetch-Byram Drought Index (KBDI)

This index is not an output of the National Fire Danger Rating System itself but is often displayed by the processors used to calculateNFDRS outputs. KBDI is a stand-alone index that can be used to measure the effects of seasonal drought on fire potential. The actual numeric value of the index is an estimate of the amount of precipitation (in 100ths of inches) needed to bring the soil back to saturation (a value of 0 is complete saturation of the soil).  Since the index only deals with the top eight inches of the soil profile, the maximum KBDI value is 800 or 8.00 inches of precipitation would be needed to bring the soil back to saturation.  The Keetch-Byram Drought Index's relationship to fire danger is that as the index value increases, the vegetation is subjected to increased stress due to moisture deficiency.  At higher values, desiccation occurs and live plant material is added to the dead fuel loading on the site.  Also, an increasing portion of the duff/litter layer becomes available fuel at higher index values.
 
If you are using the 1978 version of NFDRS, KBDI values can be used in conjunction with the National Fire Danger Rating System outputs to aid decision-making.  If you are using the 1988 version of NFDRS, KBDI values are a required input to calculate daily outputs.  Refer to Section 1.05.3 for information on initializing KBDI.
  

Appendix II - Narrative Fuel Model Descriptions
  
The following descriptions of the various NFDRS fuel models are taken from Deeming et al. (1977). 
  
Fuel Model A

This fuel model represents western grasslands vegetated by annual grasses and forbs.  Brush or trees may be present but are very sparse, occupying less than one-third of the area.  Examples of types where Fuel Model A should be used are cheatgrass and medusahead.  
Open pinyon-juniper, sagebrush-grass, and desert shrub associations may appropriately be assigned this fuel model if the woody plants meet the density criteria.  The quantity and continuity of the ground fuels vary greatly with rainfall from year to year.
 
Fuel Model B 

Mature, dense fields of brush six feet or more in height is represented by this fuel model.  One-fourth or more of the aerial fuel in such stands is dead.  Foliage burns readily.  Model B fuels are potentially very dangerous, fostering intense, fast-spreading fires.  This model 
is for California mixed chaparral, generally 30 years or older.  The F model is more appropriate for pure chamise stands.  The B model may also be used for the New Jersey pine barrens.
  
Fuel Model C
 
Open pine stands typify Model C fuels.  Perennial grasses and forbs are the primary ground fuel but there is enough needle litter and branchwood present to contribute significantly to the fuel loading.  Some brush and shrubs may be present but they are of little  consequence.  Types covered by Fuel Model C are open, longleaf, slash, ponderosa, Jeffery, and sugar pine stands.  Some pinyon-juniper stands may qualify.

Fuel Model D 

This fuel model is specifically for the palmetto-gallberry understory-pine association of the southeast coastal plains.  It can also be used for the so-called "low pocosins" where Fuel Model O might be too severe.  This model should only be used in the Southeast because of the high moisture of extinction associated with it.
 
Fuel Model E
 
Use this model after fall leaf fall for hardwood and mixed hardwood-conifer types where the hardwoods dominate.  The fuel is primarily hardwood leaf litter.  Fuel Model E best represents the oak-hickory types and is an acceptable choice for northern hardwoods and mixed forests of the Southeast.  In high winds, the fire danger may beunderrated because rolling and blowing leaves are not accounted for.  In the summer after the trees have leafed out, Fuel Model R should replace Fuel Model E.
 
Fuel Model F

Fuel Model F represents mature closed chamise stands and oak brush fields of Arizona, Utah, and Colorado.  It also applies to young, closed stands and mature, open stands of California mixed chaparral.  Open stands of pinyon-juniper are represented; however, fire activity will be overrated at low wind speeds and where ground fuels are sparse. 
 
Fuel Model G
 
Fuel Model G is used for dense conifer stands where there is a heavy 
accumulation of litter and down woody material.  Such stands are typically over mature and may also be suffering insect, disease, and wind or ice damage-natural events that create a very heavy buildup of dead material on the forest floor.  The duff and litter are deep and much of the woody material is more than three inches in diameter.  The undergrowth is variable, but shrubs are usually restricted to openings.  Types to be represented by Fuel Model G are hemlock Sitka spruce, coastal Douglas fir, and wind thrown or bug-killed stands of lodgepole pine and spruce.
  
Fuel Model H

The short-needled conifers (white pines, spruces, larches, and firs) are represented by Fuel Model H.  In contrast to Model G fuels, Fuel Model H describes a healthy stand with sparse undergrowth and a thin layer of ground fuels.  Fires in the H fuels are typically slow spreading and are dangerous only in scattered areas where the downed woody material is concentrated. 
 
Fuel Model I
 
Fuel Model I was designed for clear-cut conifer slash where the total loading of materials less than six inches in diameter exceeds 25 tons/acre.  After settling and the fines (needles and twigs) fall from the branches, Fuel Model I will overrate the fire potential.  For lighter loadings of clear-cut conifer slash use Fuel Model J, and for light thinnings and partial cuts where the slash is scattered under a residual overstory, use Fuel Model K.
  
Fuel Model J
 
This model complements Fuel Model I.  It is for clear-cuts and heavily thinned conifer stands where the total loading of material less than six inches in diameter is less than 25 tons per acre.  Again as the slash ages, the fire potential will be overrated.
 
Model K
 
Slash fuels from light thinnings and partial cuts in conifer stands are represented by Fuel Model K.  Typically the slash is scattered about under an open overstory.  This model applies to hardwood slash and to southern pine clear-cuts where loading of all fuels is less than 15 tons/acre.

Fuel Model L

This fuel model is meant to represent western grasslands vegetated by perennial grasses.  The principal species are coarser and the loadings heavier than those in Model A fuels.  Otherwise the situations are very similar; shrubs and trees occupy less than one-third of the area.  The quantity of fuels in these areas is more stable from year to year.  In sagebrush areas Fuel Model T may be more appropriate.
 
Fuel Model M

There is no Fuel Model M
 
Fuel Model N
 
This fuel model was constructed specifically for the sawgrass prairies of south Florida.  It may be useful in other marsh situations where the fuel is coarse and reed like.  This model assumes that one-third of the aerial portion of the plants is dead.  Fast-spreading, intense fires can occur over standing water. 
 
Fuel Model O
 
The O fuel model applies to dense, brush like fuels of the Southeast. In contrast to B fuels, O fuels are almost entirely living except for a deep litter layer.  The foliage burns readily except during the active growing season.  The plants are typically over six feet tall and are often found under open stands of pine.  The high pocosins of the Virginia, North and South Carolina coasts are the ideal of Fuel Model O.  If the plants do not meet the 6-foot criteria in those areas, Fuel Model D should be used.
 

Fuel Model P
 
Closed, thrifty stands of long-needled southern pines are characteristic of P fuels.  A 2 to 4 inch layer of lightly compacted needle litter is the primary fuel.  Some small diameter branchwood is present but the density of the canopy precludes more than a scattering of shrubs and grass. Model P has the high moisture of extinction characteristic of the Southeast.  The corresponding model for other long-needled pines is H.
  
Fuel Model Q

Upland Alaska black spruce is represented by Fuel Model Q.  The stands are dense but have frequent openings filled with usually flammable shrub species.  The forest floor is a deep layer of moss and lichens, but there is some needle litter and small diameter branchwood.  The branches are persistent on the trees, and ground fires easily reach into the crowns.  This fuel model may be useful for jack pine stands in the Lake States.  Ground fires are typically slow  spreading, but a dangerous crowning potential exists.  Users should be alert to such events and note those levels of SC and BI when crowning occurs. 
 
Fuel Model R
 
This fuel model represents hardwood areas after the canopies leaf out in the spring.  It is provided as the off-season substitute for Fuel Model E.  It should be used during the summer in all hardwood and mixed conifer-hardwood stands where more than half of the 
overstory is deciduous.
  
Fuel Model S 

Alaskan and alpine tundra on relatively well-drained sites fit this fuel model.  Grass and low shrubs are often present, but the principal fuel is a deep layer of lichens and moss.  Fires in these fuels are not fast spreading or intense, but are difficult to extinguish.
  

Fuel Model T
 
The sagebrush-grass types of the Great Basin and the Intermountain West are characteristic of T fuels.  The shrubs burn easily and are not dense enough to shade out grass and other herbaceous plants.  The shrubs must occupy at least one-third of the site or the A or L fuel 
models should be used.  Fuel Model T might be used for immature scrub oak and desert shrub associations in the West and the scrub oak-wire grass type of the Southeast.
 
Fuel Model U
 
This fuel model represents the closed stands of western long-needled pines.  The ground fuels are primarily litter and small branchwood. Grass and shrubs are precluded by the dense canopy but may occur in the occasional natural opening.  Fuel Model U should be used for ponderosa, Jeffery, sugar pine stands of the West and red pine stands of the Lake States.  Fuel Model P is the corresponding model for southern pine plantations.