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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
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. Byram (1959) developed an equation to relate flame length
and fireline intensity, FL=0.45(I)0.46 where FL is flame length in feet and I is fireline intensity in BTU per second per feet. Table 2 (Deeming, 1977) can be used as a
guide to relate Burning Index and fire behavior Table
2: Fire behavior, Controllability and Fireline
Intensity Table
Class
Day:
Class day describes the potential for wildland
fires. Georgia Forestry Commission
uses Table 3 as the determination percentile for all the class day. Since BI is fuel model dependent, different
BI determination points are arrived. Table 3: Class Day
determination points
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 measured at 20 feet 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 measured at 20 feet 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 Table 4: Code for
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
factor is set to 0. In summer, greenup factor is set to 20. In between, greenup
factor is moved up or down 3-4 points every 30 days or so. In order to account for dry periods,
drought adjusted greenup factor based on KBDI is
adopted from Burgan (1988) (Table 5).
If greenup factor at the time of the year
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 in summer when greenup
factor should be 20, the greenup factor will
adjusted to 10 due to drought. If KBDI is 395 in winter when green up factor
is 0, the greenup factor will still stay at 0. Table
5. 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.
Gerogia Forestry Commission sets 1-hour to be the same as 10-hour. 10-hour
dead fuel moisture is measured using an electronic fuel stick manufactured by
Forestry Technology System. 100-hour and 1000-hour dead fuel moisture are
calculated X1000: Predictor value for
1000-Hour Herbaceous: Fuel moisture in live herbaceous
vegetation. Please refer to
"1-Hour" on fuel moisture computation. This value is calculated in
our system. Woody: Fuel moisture in live
woody vegetation. Please refer to
"1-Hour" on fuel moisture computation. This value is calculated in
our system. 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. References: Introduction to Nation
Fire Danger Rating System from Weather Information Management System (WIMS)
user's manual Remsoft
FireWeatherPro user's manual 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. Burgan, Robert E.; 1988
Revisions to the 1978 National Fire-Danger Rating System. Res. Pap. SE-273.
Byram, George M. Combustion of
forest fuels. In: Forest fire control
and use. 2nd ed. Deeming, John E.; Burgan,
Robert E.; Cohen, Jack D. The National Fire-Danger Rating System – 1978. 1988
Revisions to the 1978 National Fire-Danger Rating System. General Technical Report INT-39. |
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