Wisconsin Fire Indices
NFDRS Indices Interpretation
The following is a brief explanation of the National Fire Danger Rating (NFDRS) indices as used by Wisconsin Department of Natural Resources (Table 1). The Wisconsin DNR uses the 1978 version of NFDRS. For more details, please refer to the materials in the references.
Table 1: NFDRS for Antigo, WI on March 9, 2006
RH(%): Relative Humidity in percent
IC: The Ignition Component (IC) is an index within the 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.
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.
ERC: Energy Release Component (ERC) is a calculated field which provides a relative index of the amount of heat (Btu) per unit area (ft²) within the flaming front at the head of a fire. The ERC is derived from predictions of the rate of heat release per unit area during flaming combustion and the duration of the flaming which are a function of the fuel model, the live fuel moistures and the 1000-hour timelag fuel moisture. The ERC is very sensitive to the fuel model characteristics - loading, compaction, particle size, heat of combustion and mineral content. The condition of the larger fuels has a greater influence on the component than the finer fuels. The scale for ERC values is open ended.
Table 2: Characteristics of NFDRS
| Characteristics of NFDRS Components | |||
|---|---|---|---|
| IC | SC | ERC | |
| Fuel Model Sensitivity | Moderate | High | High |
| Memory | Low | Low | Mod/High |
| Variability | High | High | Low |
| Predictability | Low | Low | High |
| Fire Season Indicator | Low | Low | High |
| Driver | SC, 1 hr FM | Wind, 1hr FM, Live FM | 1, 10, 100, 1000 hr FM, Live FM |
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 Wisconsin Department of Natural Resources, this is the basis for the Smokey adjective and staffing. 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 3 (Deeming, 1977) can be used as a guide to relate Burning Index and fire behavior.
Table 3: Fire behavior, Controllability and Fireline Intensity Table
| Burning Index | Fireline Intensity (BTU/s/ft) | Flame Length (ft) | Narrative |
|---|---|---|---|
| 0-28 | 0-50 | 2.8 | Most prescribed burns are conducted in this range |
| 38 | 100 | 3.8 | Generally represents the limit of control for manual attack methods |
| 78 | 500 | 7.8 | The prospects for direct control by any means are poor above this intensity |
| 92 | 700 | 9.2 | The heat load on people within 30 ft of the fire is dangerous |
| 108 | 1000 | 10.8 | Above this intensity, spotting, fire whirls, and crowning should be expected |
Adjective Rating: Adjective rating describes the potential for wildland fires. Wisconsin Department of Natural Resources uses Table 4 as the determination percentile for all the danger rating. Since BI is fuel model dependent, different BI break points are derived.
Table 4: Class Day break points for Burning Index
| Class | Description | BI Percentiles | Fuel Model Q | Fuel Model G | Fuel Model E | Fuel Model R |
|---|---|---|---|---|---|---|
| 5 | Extreme | 97th | 90+ | 51+ | 49+ | 22+ |
| 4 | Very High | 90th to 96th | 79-89 | 44-50 | 42-48 | 18-21 |
| 3 | High | 61st to 89th | 40-78 | 22-43 | 22-41 | 9-17 |
| 2 | Moderate | 21st to 60th | 22-39 | 11-21 | 12-21 | 5-8 |
| 1 | Low | 0th to 20th | 0-21 | 0-10 | 0-11 | 0-4 |
Wind (mph): 10-minute averaged wind speed in mile per hour measured at 20 feet and wind direction on an 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 on an 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 5: Code for State of weather
| Code | Description |
|---|---|
| 0 | Clear, less than 1/10 cloud cover |
| 1 | Scattered clouds, 1/10-5/10 cloud cover |
| 2 | Broken clouds, 6/10-9/10 cloud cover |
| 3 | Overcast, 10/10 cloud cover |
| 4 | Fog |
| 5 | Drizzle |
| 6 | Rain |
| 7 | Snow or Sleet |
| 8 | Showers |
| 9 | Thunderstorms |
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
Vegetation Stage: Vegetation stage expresses the actual greening and curing of live herbaceous vegetation. There are 5 vegetation stage: Cured, Frozen, Green, Pre-Green, Transition.
- CURED -. It is used to signal the death of herbaceous plants due to cold temperatures when the temperature is not low enough to effect woody plants. When cured is signaled, herbaceous vegetation is affected. Woody vegetation remains unaffected.
- FROZEN - is when cold temperatures kill herbaceous plants and force woody fuels into dormancy. The criteria is 5 non-consective days of 32 degrees or less or 1 day of 25 degrees or less.
- PRE-GREEN - is used at the beginning of the season. This will alert users that the station will be active for the fire season. The fuel models react the same to Pre-Green and Frozen.
- GREEN - The herbaceous plants react to improved growing conditions. In Wisconsin this occurs once a year in the spring. The program greens up the model's living fuels at a rate set by the CLIMATE CLASS. Wisconsin is in climate class 3 and the green up process usually takes 3 weeks to occur.
- TRANSITION - . Transition occurs when the herbaceous FM falls below 120%. The station will remain in Transition until the herbaceous FM reaches 30% or cured is signaled.
In Wisconsin Department of Natural Resources, we change vegetation factors manually by the time of the year and fuel conditions. Usually in early spring, pre-green is signaled. In late spring, middle May, greenup factor is initiated and the march to 21 day green up is started. Early fall Cured is signaled when the growing processes have stopped. Frozen is signaled when the temperature criteria is met.
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 . This is used within NFDRS.
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.
References:
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. Ogden, UT: US Department of Agriculture, Forest Service, Intermountain Forest and Range Experiment Station; 1984. 44pp.
Byram, George M. Combustion of forest fuels. In: Forest fire control and use. 2nd ed. New York: McGraw-Hill Book Co.; 1959.175p.
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