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NFDRS Indices Interpretation
The following is a brief explanation of the National Fire Danger Rating (NFDRS) indices as used by Georgia Forestry Commission (Table 1). For more details, please refer to the materials in the references.
Table 1: NFDRS for Dallas, Ga on Oct 23, 2002
RH(%): Relative Humidity in percent
IC: The Ignition Component (IC) is an index within 1988 National Fire Danger Rating System. IC relates the probability that a fire that requires suppression action will result if a firebrand is introduced into a fine fuel complex. Theoretically, on a day when the ignition component registers a 60, approximately 60% of all fire brands that come into contact with wildland fuels will require suppression action.
BI: Dividing Burning Index (BI) by 10 gives a reasonable estimate of flame length in feet at the head of a fire. Burning Index is fuel model dependent. In Georgia Forestry Commission, this is the basis for Class Day.
Class Day: Class day describes the potential for wildland fires. Georgia Forestry Commission uses Burning Index (BI) to determine Class Day. Class Day determination points are station dependent such that it will reflect the potential of wildland fire in the local area. Table 3 shows the determination percentile for all Georgia Forestry Commission weather stations and some of our cooperator’s weather station. If a station is not found in the list, its BI is likely to be determined by the Fuel Model determination points. Definitions of the various fuel models are included in Appendix A below.
Table 2: Class Day determination points
Note: The listed values are the minimum value for each Class day. For example, when Watkinsville has BI of 19-21, it has Class 4 Day.
KBDI: Keetch-Byram Drough Index (KBDI) measures moisture in deep duffs or upper soil layers. The relative dryness of soil is important in fire suppression. KBDI varies from 0 (Wet) to 800 (Dry).
Wind (mph): 10-minute averaged wind speed in mile per hour and wind direction in 8 point compass, i.e. N, NE, E, SE, S, SW, W, NW and calm.
Mx_Wind(mph): Wind gust in mile per hour and wind direction in 8 point compass, i.e. N, NE, E, SE, S, SW, W, NW and calm. Mx_Wind has to be at least as strong as Wind and they should be in the same direction.
Rn24(inch): Rainfall amount in the last 24 hours in inches.
Dur(Hr): Duration of rainfall in hours if there is any.
Sow: State of weather
Temp(°F): Temperature in Fahrenheit
Td(°F): Dew point temperature in Fahrenheit
Tmax(°F): Maximum temperature in the last 24 hours in Fahrenheit
Tmin(°F): Minimum temperature in the last 24 hours in Fahrenheit
RHMax(%): Maximum relative humidity in the last 24 hours in percent
RHMin(%): Minimum relative humidity in the last 24 hours in percent
HrbGF: Herbaceous greenup factor (HrbGF) expresses the actual greening and curing of live herbaceous vegetation. HrbGF varies from 0 (completely cured) to 20 (completely green).
In Georgia Forestry Commission, we changed greenup factors automatically by the time of the year and KBDI. In Winter, greenup factors are set to zero. From Spring to Autumn, greenup factors will change gradually, from 0 to 20 and then from 20 to 0. Georgia Forestry Commission defined winter as from December 21 through March 20 each year. The default greenup factors by the time of a year are listed in the Table 2.
Table 2. Default Greenup factor over a year
In order to account for dry periods, drought adjusted greenup factor based on KBDI is adopted from Burgan (1988) (Table 3). If the default greenup factor exceeds the drought adjusted greenup factor, greenup factor will be reduced to the drought adjusted greenup factor. If not, green up factors are used as is. For example, if KBDI is 395 on August 4, when the default greenup factor is 20, the greenup factor will adjusted to 10 due to drought. If KBDI is 395 on December 23, when the default green up factor is 0, the greenup factor will still stay at 0.
Table 3. Drought Adjusted greenup factor based on KBDI
WdyGF: Woody greenup factor (WdyGF) expresses the actual greening and curing of live woody vegetation. WdyGF varies from 0 (completely cured) to 20 (completely green). Please refer to HrbGF for details.
1-Hour/10-Hour/100-Hour/1000-Hour: Dead fuel moisture content in fuel that takes 1 hour/10 hours/100 hour/1000 hour to lose or gain 63% of the difference between the dead fuel itself and the surrounding atmosphere. Fuel moisture percent is computed by dividing the weight of "water" in the fuel by the oven-dried weight of the fuel and then, multiplying by 100 to get to percent.
X1000: Predictor value for 1000-Hour
Herbaceous: Fuel moisture in live herbaceous vegetation. Please refer to "1-Hour" on fuel moisture computation.
Woody: Fuel moisture in live woody vegetation. Please refer to "1-Hour" on fuel moisture computation.
SC: Spread Component (SC) is the forward rate of spread at the head of the fire in feet per minute. Since 1 chain=66 feet and there are 60 minutes in an hour, so 1 feet per minute is 0.9 chain per hour. Thus, multiply the value of SC by 0.9 gives the spread rate in chain per hour.
EC: Energy Release Component (EC) is the potential available energy per square foot at the head of the fire in BTUs per square foot.
Weather Information Management System (WIMS) user's manual
KBDI Revisited: Prescribed Fire Applications by Mike Melton published in Fire Management Notes in 1996 Volumne 56, Number 4, p.8-11
KBDI: Can It Help Predict Wildland Fires? by Daniel Chan, James Paul and Alan Dozier in Fire Management Today 2004, Volumne 64, Number 2, p.39-42
KBDI: A Guide to Fire Conditions & Suppression Prodblems by Mike Melton published in Fire Management Notes in 1989 Volume 50, Number 4, p.30-34
Bradshaw, Larry S.; Deeming, John E.;Burgan, Robert E.;Cohen, Jack D., compilers. The 1978 National Fire-Danger Rating System;technical documentation. General Technical Report INT-169. Ogden, UT: US Department of Agriculture, Forest Service, Intermountain Forest and Range Experiment Station; 1984. 44pp.
Burgan, Robert E.; 1988 Revisions to the 1978 National Fire-Danger Rating System. Res. Pap. SE-273. Asheville, NC: US Department of Agriculture, Forest, Service, Southeastern Forest Experiment Station; 1988. 39pp.
FUEL MODEL DEFINITIONS
Definitions extracted 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 6 feet or more in height are 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. Situations covered by Fuel Model C are open, longleaf, slash,ponderosa,Jeffrey, and sugar pine stands. Some pinyon-juniper stands may qualify.
FUEL MODEL D
This fuel model is specifically for the palmetto-gallberry understory-pine overstory association of the southeast coastal plains. It can also be used for the so-called "low pocosins" where Fuel Model 0 might be too severe. This model should only be used in the Southeast because of a high moisture of extinction.
FUEL MODEL E
Use this model after leaf fall for hardwood and mixed hardwood-conifer types where the hardwoods dominate. The fuel is primarily hardwood leaf litter. The oakhickory types are best represented by Fuel Model E, but E is an acceptable choice for northern hardwoods and mixed forests of the Southeast. In high winds, the fire danger may be underrated because rolling and blowing leaves are not accounted for. In the summer after the trees have leafed out, Fuel Model E should be replaced by Fuel Model R
FUEL MODEL F
Fuel Model F is the only one of the 1972 NFDRS Fuel Models whose application has changed. Model F now represents mature closed chamise stands and oakbrush 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 windspeeds and where there is sparse ground fuels.
FUEL MODEL G
Fuel Model G is used for dense conifer stands where there is a heavy accumulation of litter and downed woody material. Such stands are typically overmature and may also be suffering insect, disease, 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 3 inches in diameter. The undergrowth is variable but shrubs are usually restricted to openings. Types meant to be represented by Fuel Model G are hemlock-Sitka spruce, Coast Douglas-fir, and windthrown 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 H fuels are typically slow spreading and are dangerous only in scattered areas where the downed goody material is concentrated.
FUEL MODEL I
Fuel Model I was designed for clearcut conifer slash where the total loading of materials less than 6 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 clearcut 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 clearcuts and heavily thinned conifer stands where the total loading of materials less than 6 inches in diameter is less than 25 tons/acre. Again, as the slash ages, the fire potential will be overrated
FUEL 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 clearcuts where the 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 fuel in these areas is more stable from year to year. In sagebrush areas Fuel Model T may be more appropriate.
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 reedlike. This model assumes that one-third of the aerial portion of the plants is dead fast-spreading, intense fires can occur even over standing water.
FUEL MODEL O
The O fuel model applies to dense, brushlike fuels of the Southeast. O fuels, except for a deep litter layer, are almost entirely living in contrast to B fuels. The foliage burns readily except during the active growing season. The plants are typically over 6 feet tall and are often found under an open stand 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 fuel: 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. Fuel Model P has the high moisture of extinction characteristic of the Southeast. The corresponding model for other long-needled pines is U.
FUEL MODEL Q
Upland Alaskan black spruce is represented by Fuel Model Q. The stands are dense but have frequent openings filled with usually inflammable 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 tree 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 Spread Component (SC) and BI when crowning occurs.
FUEL MODEL R
This fuel model represents the hardwood areas after the canopies leaf out in the spring. It is provided as the off-season substitute for 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 or alpine tundra on relatively well-drained sites is the S fuel. 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 bothersome 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 in the Southeast.
FUEL MODEL U
Closed stands of western long-needled pines are covered by this model. The ground fuels are primarily litter and small branchwood. Grass and shrubs are precluded by the dense canopy but occur in the occasional natural opening. Fuel Model U should be used for ponderosa, Jeffrey, sugar pine, and red pine stands of the Lake States. Fuel Model P is the corresponding model for southern pine plantations.