.+ Government of Canada · objective is to promote the wise management and use of Canada's forest...

.+ Government

of Canada

'CanadianForestryService

Gouvernementdu Canada

Servicecanadien desfor~ts

THE CANADIAN FORESTRY SERVICEGOVERNMENT OF CANADA

The Canadian Forestry Service is the principal source of federal expertise in forestry. Its generalobjective is to promote the wise management and use of Canada's forest resources for theeconomic, social, and environmental benefit of Canadians.

The following are the main functions of the CFS:

1 Coordination of federal policies, for the promotion of better resource management and forestindustry development.

2.

Provision of scientific and technological leadership in forestry through research anddevelopment.

3.4.

5.

Provision and analysis of national and international statistics and information as a basis forpolicy formulation.

Development and certification of codes and standards for wood product performance.

Protection of Canada's forests from foreign pests.

6. Fostering the potential use of the forest resource for energy.

7.

Contributing to the environmental objectives of the government of Canada.

A number of federal agencies are involved in forestry programs and a Federal Forestry SectorStrategy Committee has been established to coordinate federal forestry activities. The CanadianForestry Service has been designated the lead agency role.

The Canadian Forestry Service is comprised of a headquarters unit, six forestry centres and twonational institutes. The forestry centres are responsive to regional priorities and maintain closeliaison with the respective provincial government forestry departments and other clients. They alsoparticipate in, and frequently lead, national programs. The national institutes provide the focus forprograms of national scope.

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@ Minister of Supply and Services Canada 1987

Catalog No. FO64-35/1987EISBN 0-662-15198-4

Additional copies of this publication areavailable at no charge from:

Canadian Forestry ServicePlace Vincent Massey, 3rd FloorOttawa, OntarioK1A 1G5

A microfiche edition of this publicationmay be purchased from:

Micromedia Ltd.158 Pearl StreetToronto, OntarioM5H 1L3

Cette publication est aussi disponible en fran<1aissous Ie titreElaboration et structure de I'lndice Foret-Meteode la methode canadienne.

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THE FIRE WEATHER INDEX. 23

The Duff Moisture Function 23

The FWI Equation 24

25The Daily Severity Rating THE FIRE WEATHER INDEX SYSTEM IN USE.

26

Calculation 26

Interpretation. . 26

Calibration 28

Danger Classes 29

Latitude 29

30Conclusion

REFERENCES 32

APPENDIX 36

36Symbols

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PREFACE

The original Canadian Forest Fire Weather Index System issued in 1970 was the result of fouryears' effort, in which many fire research staff of the Canadian Forestry Service took part. Theprincipal work was done by S.J. Muraro and J.A. Turner of the Pacific Forest Research Centre (nowthe Pacific Forestry Centre), A.J. Simard of the Forest Fire Research Institute (later amalgamatedwith the Petawawa National Forestry Institute), and C.E. Van Wagner of the Petawawa ForestExperiment Station (now the Petawawa National Forestry Institute). D.E. Williams, Director of theForest Fire Research Institute, acted as coordinator of the project, which involved voluminouscorrespondence and several full-scale meetings. Others who contributed in various ways and gaveadvice and opinion as the system developed included R.C. Henderson, B.D. Lawson, and R.N.Russell of the Pacific Forest Research Centre, A.D. Kiil, D. Quintilio, and J.E. Grigel of the NorthernForest Research Centre (now Northern Forestry Centre), B.J. Stocks and J.D. Walker of the GreatLakes Forest Research Centre (now Great Lakes Forestry Centre), E.W. Howard and D.B.Bradshaw of the Newfoundland Forest Research Centre (now Newfoundland Forestry Centre),and D.G. Fraser, L.B. MacHattie, P.M. Paul, L. Pouliot, and Gy. Pech of the Forest Fire ResearchInstitute. Finally, J.C. Macleod, Program Coordinator for Forest Fire Research, provided beneficialinfluence and support throughout.

Subsequent maintenance and further development of the Fire Weather Index System has beenin the hands of the Candian Forestry Service's Fire Danger Working Group, currently comprisingM.E. Alexander of the Northern Forestry Centre, B.D. Lawson of the Pacific Forestry Centre, B.J.Stocks of the Great Lakes Forestry Centre, and C.E. Van Wagner of the Petawawa NationalForestry Institute.

The present publication is a revision of Departmental Publication 1333, "Structure of theCanadian Forest Fire Weather Index" issued in 1974 by the Canadian Forestry Service, Depart-ment of Environment.

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RESUME

L'lndice Foret-Meteo (IFM) est un indice des conditions meteorologiques propices aux incen-dies de forets. II a ete publie en 1970 apres plusieurs annees de travaux par des agents derecherche du Service canadien des forets. Les meilleures caracteristiques de I'ancien indice dedanger de feu ont ete retenues dans Ie nouvel indice et un lien a ete conserve entre I'ancien et Ienouveau. L'IFM est fonde sur la teneur en humidite de trois classes de combustibles forestiers,plus I'effet du vent sur Ie comportement du feu. II consiste en six composantes: trois sous-indicesprimaires representant I'humidite du combustible, deux sous-indices intermediaires representantIe taux de propagation du feu et la consommation des combustibles, et un indice final representantI'intensite de feu sous forme de rendement energetique par unite de longueur. L'IFM se referefondamentalement a une foret standard de pin, mais sert utilement comme indice general dudanger d'incendies de forets au Canada. C'est par des observations journalieres des conditionsmeteorologiques, a midi, qu'on Ie redige: temperature, humidite relative, vitesse du vent etpluviosite (si elle existe). Ce document decrit Ie developpement de I'IFM, leg concepts qui ontpreside a sa preparation, et sa structure mathematique. De nouvelles versions de I'IFM ont etepubliees en 1976 et 1984.

Correct citation

Van Wagner, C.E. 1987. Development and structure of the Canadian Forest Fire Weather IndexSystem. Can. For. Serv., Ottawa, Onto For. Tech. Rep. 35.

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NOI.l:JnOOt:J.lNI

THE BASIC STRUCTURE OF THE FIRE WEATHERINDEX SYSTEM

The Fire Weather Index (FWI) System comprises six standard components. The first three arefuel moisture codes that follow daily changes in the moisture contents of three classes of forest fuelwith different drying rates. The last three components are fire behavior indexes representing rate ofspread, fuel weight consumed, and fire intensity. The system depends solely on weather readingstaken each day at noon local standard time (LST): temperature, relative humidity, wind speed, andrain (if any) during the previous 24 hours. The current month must also be specified. In basic form,the FWI System is a set of equations (Van Wagner and Pickett 1985) that can be readily processedby computer; this is in contrast to previous systems, in which the basic form was always a set ofcurves that were converted directly to tables. The FWI can also be worked out from the publishedset of nine tables derived directly from these equations (Canadian Forestry Service 1984).

It should be understood that although the FWI is calculated from noon weather readings, it reallyrepresents fire danger at its midafternoon peak, generally specified as 1600 hours. This comesabout because, in the original work, the noon weather variables were correlated with fine fuelmoisture data and test-fire results taken later in the afternoon. In this respect, the FWI System issimilar to all previous Canadian systems used since 1938.

For each of the three fuels embodied in the FWI System a subsidiary index was developed withtwo phases, one for wetting by rain and one for drying. These subindexes, called fuel moisturecodes, are in fact bookkeeping systems that add moisture after rain and subtract some for eachday's drying. They are arranged in code form with values rising as moisture content decreases forthe best psychological effect. The three moisture codes and their corresponding fuels are:

1. Fine Fuel Moisture Code (FFMC), which represents the moisture content of litter and othercured fine fuels in a forest stand, in a layer of dry weight about 0.25 kg/m2;

2.

Duff Moisture Code (DMC) , which represents the moisture content of loosely compacted,decomposing organic matter weighing about 5 kg/m2 when dry;

Drought Code (DC), which represents a deep layer of compact organic matter weighingperhaps 25 kg/m2 when dry.

3.

Although these descriptions are useful for explanatory purposes, comparison of the moisturecodes is best made in terms of water capacity and drying speed. Each fuel is considered to dryexponentially, so that its instantaneous drying rate is proportional to its current free moisturecontent. The proper measure of drying speed is either the timelag (i.e. time to lose 1-1/e [about 2/3]of the free moisture above equilibrium), or the slope of the exponential curve of free moisturecontent over time, here called the log drying rate. Table 1 gives for each moisture code the timelagin terms of normal days (with noon temperature 21.1 °C (70°F) and relative humidity 45 percent inJuly), as well as its water capacity, fuel properties, and the daily weather parameters required forcalculation.

The two slow-reacting moisture codes, the DMC and DC, respond to changing daylength as theseason progresses. This feature is necessary because the amount of moisture lost daily by slow-drying fuels is as much dependent on the time available as on the noon atmospheric conditions.The midafternoon moisture content of fast-drying fuels, represented by the FFMC, is less depen-dent on day length.

Table 1. Properties of the three fuel moisture codes.Water Nominal Nominal

Timelag capacity Required fuel depth fuel loadCode days mm parameters1 cm kg/m2-

FFMC 2/3 0.6 T, H, W, r 1.2 0.25

DMC 12 15 T, H, r, mo 7 5

DC 52 1 00 T, r, mo 18 25

1T -temperature, H -humidity; W -wind, r -rain, mo -month.

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THE FINE FUEL MOISTURE CODE

BACKGROUND

The ancestor of the Fine Fuel Moisture Code (FFMC) is the Tracer Index for litter that firstappeared as the principal component of an early fire danger rating system for eastern pine forests(Wright 1937). The Tracer Index was developed from concurrent weather and fuel-moisture dataobtained in pine stands at Petawawa by multiple correlation of present moisture content withcurrent weather and the previous day's moisture content. The fuel represented was a layer of pineneedles or other surface litter, probably about 0.25 kg/m2. This fuel can be called relatively fast-drying, but the substantial effect of the previous day's value meant that drying time was by nomeans instantaneous. In subsequent years, this Tracer Index was found by various fire re-searchers to correlate well with litter moisture contents in many parts of Canada; it was retained,therefore, as an integral part of all subsequent fire danger tables issued by the federal forestresearch organization. The particular version of the Tracer Index chosen for this work appears inBeall's (1948) Forest Fire Danger Tables. The special needs of the new system required analysisand modification of the original Tracer Index.

The following description of the Fine Fuel Moisture Code is fairly complete because there is noother substantial reference to it.

SCALE

The original Tracer Index was presented in the simple code form of 150 minus moisture content.Because the highest and lowest tracer values were 144 and 40, the scale length was 104. For thetabular version of the new system, a two-digit code was desired with maximum value of 99, a scalelength 5 points less than the original. The minimum moisture content (m) was set at 2 percent,resulting in a scale equation

(1)F = 101 -m

where F is FFMC. This scale was the basis of the FFMC in the first three editions of the FWISystem, but was changed in the 1984 version on the following grounds:

1) The real moisture content of pine litter ranges up to about 250 percent.

A realistic moisture scale was desired to render the standard FFMC compatible with futuredevelopments in fine fuel moisture prediction.

2)

A new scale, the FF scale, was devised, described by the following equations

(2a)= 59.5 (25Q-m)/(147.2 + m)

(2b)m = 147.2 (101-F)/(59.5 + F)

This scale (see Figure 2) permits realistic conversions from code to moisture content, allows theinternal operations of the FFMC to be carried out on moisture content value, and retains thetraditional code scale for quoting the FFMC itself. Furthermore, the former artificial 2 percentminimum limit on moisture content is no longer deemed necessary in computing practice. The

potential code scale length is now 101.

DRYING PHASE

The first step in analyzing the Tracer Index was to list a series of day-to-day drying sequences,each at constant relative humidity (H), wind (W), and temperature (T), by starting at saturation andrepeatedly entering the Tracer Index tables until no further change occurred. The resultingequilibrium moisture content (E) for each drying sequence was subtracted from the day-to-daymoisture contents (m) and the free moisture contents were plotted as log (m-E) against time indays. In this way, a set of six semilog curves at different wind speeds was obtained for each of eighthumidity classes, all for the original temperature class 60 -80°F (15.6 -26.7°C). The classmidpoint 70°F (21.1°C) was then taken to be the normal temperature for the whole analysis.

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Fourth, the effect of temperature on drying and wetting rates was assigned from laboratoryevidence. The temperature effect in the original Tracer Index was deemed too weak, providing onlya 15 percent increase in drying rate for a20°F (11.1°C) rise in temperature, whereas laboratory testson pine needles indicated a 50 percent increase. The function to accomplish this temperatureeffect is, for temperature in degrees Celsius,

k = ko x 0.581eo.O365T (6)

where k is either log drying rate (kd) or log wetting rate (kw) in units of log moisture content per day.In the published account of these laboratory tests (Van Wagner 1979), the temperature effectbecomes a straight line when graphed as log k versus the reciprocal of absolute temperature. Therelation in Equation 6 above is essentially equivalent and is illustrated in Figure 3.

In the first edition of tables of the FWI system (Canadian Forestry Service 1970 and 1972), theFFMC was presented as one main table in terms of Hand W, with a subsidiary table fortemperature correction. Although expedient then, the results were not quite true to the mathe-matics. In later editions, separate tables in Hand W have been given for each range of T, thusexpressing the equations in proper fashion.

5 10 15 20 25 30

T r'c)35 40

20 600 10 30 40W (km/h)

50

Variation of log drying rate k with 7; H, and Win the FFMC:a) k vs T at constant Hand W; b) k vs H at constant Tand W; c) k vs W at constant T and H. Constant values areT = 21.1°C, H = 45%, W = 12 km/h.

Figure 3.

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If mo turns out to be less than Ew, m is found from-k

m = Ew- (Ew -mo) x 10 w (10)

where kd or kw is the appropriate log drying or wetting rate obtained from Equation (5). If mo liesbetween Ed and Ew, no change is assumed, and m = mo.

RAINFALL PHASE

Next, the effect of rainfall on the Tracer Index was examined. The original rainfall table wasanalyzed for increase in moisture content per unit of rain. It was observed that this quantitydecreased a) with increasing rainfall amount, and b) with increasing initial moisture content mo. Inother words, as the amount of rainfall increased, a smaller proportion of it was held by the litter;also, the higher the litter's initial moisture, the less rain it could absorb. For earlier versions of theFWI system, J.A. Turner supplied a set of three equations, for different ranges of rain amount, thatmatched the original rainfall table well. These operated on the code value. For the new version, arainfall routine was required that operated on moisture content itself, rather than on code value. Anew equation was therefore developed in the form

6m/r = C e-b/(2SD-mO) (1-e-a/r) (11)

where 6m is increase in m due to rain r, and a, b, and c are constants. The 250 represents themaximum value of moisture content in the previously described FF scale. The rainfall effectiveness6m/r is decreased by the first bracketed term with increasing mo, and by the second bracketedterm with increasing r.

The constant c was first set as the limit of L:.m/r as mo and r approach zero. For the standardFFMC fuel load of 0.25 kg/m2, the first millimetre of rain should, therefore, cause a rise of 400percent in m. This value was reduced to 42.5 to match the original Tracer Index rainfall table andTurner's equations, implying that the maximum effectiveness of rain in wetting litter is only about 10percent and decreases from there.

Trials with other coefficients produced the following equation

6m/rf = 42.5 e-100/(251-"'o) (1-e-6.93/rf) (12)

The 251 appears in place of 250 in order to prevent division by zero should mo ever equal 250; r f isnet rainfall, described below in Equation 14. When converted to code values, this equation's resultsmatch the output of the previous equations within one or two points over a wide range of rainamount and initial moisture content, with a slight exception explained below.

Because the behavior of Equation 12 is slightly anomalous when both rain amount and initialmoisture content are large, a corrective term was added for use at high initial moisture content. Thiscorrection equals

0.0015 {mO-150)2rfO.5 (13)

It is added to Equation 12 only when mo exceeds 150, and comes into play when the resultingFFMC code value is about 5 or less.

Finally, the amount of rain to be used in Equations 12 and 13 was reduced to allow for loss in theoverhead canopy. Thus

(14)rf=ro-O.5

where rf is net rain amount and ro is observed rainfall in the open, in millimetres. In other wordsrainfalls of up to 0.5 mm are to be ignored for purposes of the FFMC. In dry weather, the rainfallequations are not used. The increase in moisture content due to rain is assumed to occur alwaysbefore the day's drying begins.

8

6

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THE DUFF MOISTURE CODE

BACKGROUND

In Beall's (1939) supplement to the second generation of Canadian forest fire danger indexes,there appeared a subsidiary index called the Drought Index. While the Tracer Index representedusually only several days of past weather and its effect on fine fuel moisture content, the DroughtIndex took account of up to 25 days of weather history. Its purpose was to stratify the day-to-day fireweather according to the number of days since appreciable rain; in physical concept it could belikened to a simple reservoir in which rain accumulated additively, and which lost a constantamount for each day without rain. Although the Drought Index, unlike a real fuel, was linear in bothits drying and wetting phases, it performed a useful function in the next two systems of danger

rating.

When work began around 1970 on the first version of the FWI System, it was generally agreedthat a subsidiary index was required that represented the moisture content of some real slow-drying forest fuel. The fuel chosen because of its universal and continuous presence in Canadianforests was the duff layer, roughly equivalent to the F-layer of soil science. An index filling this needappears in the new system as the Duff Moisture Code (DMC). It was developed after 4 years of fieldwork, mainly in red pine and jack pine stands. The basic method was to transfer rectangles oforganic matter to trays 60 x 40 cm in area set in the forest floor, and to weigh them daily. The DMCrepresents duff layers about 7 cm deep and 5 kg/m2 in dry weight. Because a published descrip-tion already exists (Van Wagner 1970a), only an outline of its structure is presented here.

SCALE

A scale for the DMC was designed after the manner of the U.S. Buildup Index (U.S. ForestService 1966) on a logarithmic function of the actual moisture content that rises with dryness. Thetype expression is

p = c [log (Mmax-E) -log (M -E)] (15)

where c is a scale constant, P is code value, M is moisture content, and E is the equilibriummoisture content. Mmax was set equal to 300, and E to 20. The actual scale equation in naturallogarithms for the chosen moisture limits of 300 and 20 percent was then

(16)p = 244.72 -43.43 In (M -20)

This scale is illustrated in Figure 5. The main advantage of such a scale is that it permits theaddition of daily drying increments that are independent of the current value of the code.

RAINFAll PHASE

In the wetting phase of the DMC, two principles form the basis of the equations:

The increase in moisture content per unit of rain is inversely proportional to the amount of a

rainfall.1)

The wetting effect of a rainfall decreases with increasing initial moisture content.2)

10

300

250

~

~~ 200I-ZUJI-Z0u 150.

UJ~=>l-V><5 100.~

--~; ~ ~~ -~:::::::::::: =50

00 50 100

CODE VALUE (P)

150200

Figure 5. Graph of scale linking DMC to duff moisture content.

In other words, the duff layer retains a greater fraction of a light rainfall than a heavy one, and thewetter the duff the less rain it can absorb. Although the effect of rainfall duration was demonstratedin the original work, it was decided to omit this complication in the practical moisture code. In theDMC, the effective rain r e is given as a function of the total rain r 0 by

re = 0.92 ro -1.27 (17)ro >1.5

where r e is net rain amount and r 0 is observed rainfall in the open. In any event, rainfalls of up to1.5 mm are ignored for purposes of the DMC, and Equations 17 and 18 (below) are not used unlessro exceeds that amount. The moisture content after rain (Mr) is given by

Mr = Mo + 1000 re/(48.77 + bre) (18)

The coefficient b was first expressed as a plotted curve, which was then rendered in terms of theinitial code Po by a set of three empirical equations for different ranges of Po:

b = 100/(0.5 + 0.3 Po) , Po .:S 33 (19a)

, 33 < Po .:S 65b = 14 -1.3 In Po

b = 6.2 In Po -17.2

(1gb)

I Po> 65 (19c)

11

DRYING PHASE

The drying phase of the DMC consists of two equations and a short table based on the following

points:

1) Day-to-day drying in constant weather is exponential.

2) The duff layer has, for practical purposes, a constant equilibrium moisture content E of20 percent.

3)

4)

The log drying rate K is proportional to temperature, becoming negligible at about -1°C.

The log drying rate K is proportional to the deficit in relative humidity.

5) The daylength, varying with season, has an effect roughly proportional to three less than thenumber of hours between sunrise and sunset.

The log drying rate K is given by

K = 1.894 (T + 1.1)(100 -H) Le x 10-6

where Le is an empirical daylength factor listed by month in Table 2. The present day's DMC isthen found from

P = Po (or Pr) + 100K

where K is multiplied by 100 to yield values matching the chosen scale.

Table 2. Daylength factor L" in Duff Moisture Code

January

February

March

April

May

June

6.5

7.5

9.0

12.8

13.9

13.9

12.4

10.9

9.4

8.0

7.0

6.0

July

August

September

October

November

December

12

THE

DROUGHT CODE

BACKGROUND

The Fine Fuel Moisture Code covers only the thin surface layer of fast-drying material, whereasthe Duff Moisture Code applies to organic layers of moderate depth and dry weight. Real firedanger,

however, is also affected by the state of the deeper organic layers common in many parts ofCanada, by concentrations of large downed wood, and even by the availability of water in small

streams and swamps. It was decided, therefore, to include a third, very slow-drying moisture indexin the FWI System and to call it the Drought Code.

The Drought Code (DC) was first developed by Turner (1966) to serve as an index of the waterstored in the soil rather than to follow the moisture state of a particular slow-drying forest fuel.Because soil loses moisture exponentially, such an index is quite suited to represent certain heavyfuels.

Muraro and Lawson (1970) identified one such material; they established that the DC followsreasonably well the moisture variations in deep, compact duff layers on Vancouver Island. Theselayers are, on the average, about 18 cm deep and 25 kg/m2 in dry weight. Deep duff layers of asimilar nature under balsam fir forest at Petawawa and upland black spruce in northern Quebechave also been found to match the DC well in drying rate (Van Wagner 1974). One function of aslow-acting moisture code such as the DC is to warn of occasions when the lower layers of deepduff may be drier than the upper. Both Muraro and Lawson (1970) and Kiil1 report field studies ofthis phenomenon, which results in persistent deep smoldering even though surface fire behaviormay not be severe.

In its initial form, the DC was called the 8tored Moisture Index (8MI) and was expressed directlyin 100ths of an inch of water up to a maximum of 800 (i.e., 8 inches of water). To match the style ofthe DMC, the 8MI was converted to a logarithmic scale rising with dryness. A brief mathematicaldescription of the DC follows, and Turner (1972) gives a complete description.

800

~

a;: 600zW-l

~S-ow 400w~::)l-V)

a~ 200 \

'"

'""""

01000

CODE VALUE (D)0 500

15002000

Figure 6. Graph of scale linking DC to its moisture equivalent Q.

1Kiil, A.D. 1970. Distribution of moisture in spruce-fir duff and its relevance to fire danger rating. Can. Forest. Serv., NorthernForest Res. Centre Internal Rep. A-34.

13

SCALE

The scale of the 8MI (the original form of the DC) was 800 units in length, 800 representingsaturation and zero representing the driest condition normally encountered. The required equationfor the new logarithmic scale was analogous to the one forming the basis of the DMC. As anexponential expression of the moisture equivalent Q, the chosen scale equation is

D = 400 In (BOO/a) (22)

where D is the current DC. The constant 400 represents the maximum theoretical moisture contentof the fuel represented by the DC. As in the DMC, this type of scale permits the addition of dailydrying increments that are independent of the current value of the DC (Figure 6).

RAINFALL PHASE

Rainfall in the DC is first reduced to an effective rainfall rd and then simply added to the existingmoisture equivalent 00 to give 0 r' the moisture equivalent after rain. The two equations are

rd = O.~3 ro -1.27 , ro > 2.8 (23)

and Or = O~ + 3.937rd (24)

Rainfalls of up to 2.8 mm are ignored for purposes of the DC; Equations 23 and 24 are not usedunless r 0 exceeds that amount.

DRYING PHASE

The daily dry-weather additives to the DC actually represent potential evapotranspiration (V),which is given by an empirical equation dependent on noon temperature and season:

v = 0.36 (T + 2.8) + Lf(25)

The seasonal daylength adjustment Lf is listed by month in Table 3. Because the scale length ofthe DC is in a sense one-half that of the original SMI (see Equation 22), the value of V is halved andadded to the initial code Do (or, in case of rain, to Dr). The present day's DC is thus found from

D = Do (or Dr) + 0.5 V

OVERWINTERING

(26)

The timelag of the DC, 52 days in standard weather, is long enough that the amount ofprecipitation during winter may affect the season's starting value. In other words, the assumptionthat melting snow always saturates the DC fuel layer cannot be counted on. Following theprocedure of Turner and Lawson (1978), the DC starting value is computed from

as = a af + b (3.94rw)(27)

where Of is final fall moisture equivalent (from final D by Equation 22),r w is winter precipitation in millimetres,Os is starting spring moisture equivalent, anda and b are constants.

14

The value of D corresponding to as may then be found by Equation 22. This procedure nowconstitutes an integral aspect of the DC and its computation. The values of a and b are setaccording to the judgment of each agency and its advisors. Experience so far is that an overwinteradjustment is almost never necessary in eastern Canada, but will be needed occasionally incertain western regions as described, for example, by Alexander (1982).

Table 3. Daylength adjustment Lf in Drought Code

Month Lf

-1.+0.

3.

5.

6.

5.

2.

O.

November to March

April

May

June

July

August

September

October

15

69

88

4

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~NIl"~ A11~3A3S All"a 3Hl

THE FIRE WEATHER INDEX SYSTEM IN USE

CALCULATION

The codes and indexes of the FWI System may be determined either by computer or fromtables. The basic form of the system is the standard set of equations. Most of these have beenused in the present publication in explaining and illustrating the system's structure. The completeset is listed in Van Wagner and Pickett (1985), and includes several others designed to improveinternal consistency in minor ways. The published set of tables (Canadian Forestry Service 1984)was derived from these equations.

Several historical computer programs have figured in the development of the FWI System and itsuse. The first was by Nikleva and Parent (1971). Others were designed by Simard (1970b), Engischand Walker (1971), Kean (1975), and Kourtz (1980). The present official program (included in VanWagner and Pickett 1985) processes the present equations to give the purest possible mathe-matical output, with no artificial constraints other than those required by the empirical nature ofsome of the equations. Although it is arranged to handle one station's weather data one season at atime, its mathematical routines serve as a standard of comparison, and may be incorporated asblocks into operational daily programs serving any number of stations.

A perennial problem in computation deserves mention. The traditional means of working out firedanger indexes in Canada in years past was by a set of manual tables; these by themselvesformerly constituted the standard. The FWI System, however, is based on equations that can onlybe processed conveniently by computer. It is, of course, impossible to design a set of tables thatwill match the pure mathematical computer output all of the time. Great pains have been taken tomake the tables as accurate as possible and to minimize bias in the average table output, but day-to-day error will occur to some degree in all codes and indexes. Most agencies now compute theFWI System output centrally and redistribute it to the individual stations. However, the tables arestill in widespread use locally for quick answers as soon as weather observations are available. Anagency then has two choices. It can. adopt the mathematical output as standard, instructing anystation that keeps a table record to adjust it occasiona)ly to match the computer record. Or it canplace the tables themselves in the computer, and adopt the table output as standard. Of course,this choice will be a matter of policy. Nevertheless, it is understood that whenever criticalcomparison or development is required the equation output is the true standard.

Since the FWI System depends only on weather readings and the date (by month), its output canjust as easily be calculated from forecast weather to yield a forecast of fire danger. In cooperationwith the Atmospheric Environment Service, most forest fire agencies now issue such forecasts upto 24 hours in advance of the standard noon determinations. Paul (1970), Pouliot (1974), andNikleva (1975) have described operational procedures.

INTERPRETATION

The FWI was originally conceived to represent frontal fire intensity as defined by Byram (1959) inthe equation

I = HWR (42)

where, in compatible units, H is heat of combustion, W is weight of fuel consumed per unit area, Ris rate of advance, and I is energy output rate per unit length of fire front.

26

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Consider first the FFMC. Some average June hourly weather data from five stations at latitudesfrom 48° to 66°N were processed by the hourly FFMC computation described by Van Wagner(1977). The 1600 h FFMCs obtained by this method were compared with the standard FFMCworked from the noon observations; this latter presumably predicts the 1600 h actual value. Thediscrepancies, computed as 1600-hour value minus standard value, ranged from 1.1 at 48°N to 2.5at 66°N, on base FFMCs in the high 80s. The correction, if it were desired, would be to progres-sively delay the standard observation time from 1200 h at lower latitudes to about 1400 h at higherlatitudes. This is because the time of the maximum temperature and minimum RH in high summeris progressively later as latitude increases. However, the discrepancies were not judged seriousenough to warrant officially recommending such revised procedures. The FFMC, in other words,measures the peak flammability condition reasonably well at all latitudes. Its diurnal range,however, does decrease markedly with latitude in high summer, meaning that fine fuel remainsflammable for a longer proportion of the day as one proceeds north.

To test the DMC, theoretical daylength factors were computed for three latitudes 45°, 55°, and65°N, and used to compute modified DMCs for the weather in a fairly dry season (actually 1967 atLac la Biche). In comparison with the standard DMC, the output is tabulated below, based on sixmonths, May to October.

45°N 55°N 65°N StandardSeason average 34.5 32.4 30.0 31.9

Maximum value 85 83 78 88

The standard version does not fit quite smoothly with the three special versions because itsdaylength factors were worked out empirically. Nevertheless, it represents the average casereasonably well. Several such tests were carried out, and the differences were again not judgedserious enough to warrant supplying special DMCs for different latitudes.

The DC was tested in an analogous manner using the same season's weather data. The additivenature of the daylength adjustment In the DC produces higher differences than obtained with theDMC, especially toward the end of the season. The output of the three theoretical versions is )tabulated with the standard DC below, again based on the six months from May to October. )'

45°N 55°N 65°N StandardSeason average 312 302 274 293

Maximum value 523 497 436 495

Again, the standard version was representative, and these variations were not judged greatenough to warrant special DCs for different latitudes.

The three tests described above were computational only. True definitive testing for latitudeeffect would need field data, which are not yet available in sufficient quantity. In any event, becauseall three moisture codes are consistent with weather and its effect on regional fuels at any particularlatitude, the problem of comparison among widely different latitudes will probably never becomeserious.

CONCLUSION

Because it was built up to represent fire behavior in a generalized standard fuel type, the FWISystem will obviously have different meanings in different forest or fuel types. The initial interestwas mainly in logging slash of different types (Muraro 1971, Quintilio 1972, Stocks 1972), but datalinking spread rate with the FWI System in various forest and vegetation types has accumulatedsteadily. In fact the FWI System is just part of the larger Canadian Forest Fire Danger RatingSystem (CFFDRS), which now includes the Canadian Forest Fire Behavior Prediction (FBP)System, designed to provide quantitative estimates of fire behavior in particular fuel types. Aninterim version of this latter (Alexander et al. 1984) provides equations for predicting rate of spreadin 14 vegetation and slash fuel types throughout Canada, as well as topographical adjustmentsand fire growth. It is on this line of work that further research and development will be concentratedin future.

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S3:JN3}::13::J3}::1

Kean, W.A. 1975. A PDP-8L program for calculating the Fire Weather Index. Environ. Can., Can.For. Serv., Petawawa Forest Exp. Sta., Info Rep. PS-X-57. 12 p.

Kiil, A.D.; MacTavish, J.S. 1962. Forest fire danger tables: District of Mackenzie, NorthwestTerritories. Can. Dep. For., Forest Res. Br. 12 p.

Kiil, A.D.; Miyagawa, R.S.; Quintilio, D. 1977. Calibration and performance of the Canadian FireWeather Index in Alberta. Can. For. Serv., Northern Forest Res. Centre. Info Rep. NOR-X-173.45 p.

Kourtz, P.H. 1980. A calculator program tor the Canadian Forest Fire Weather Index (magneticcard version). Environ. Can., Can. For. Serv., Petawawa Nat. For. Inst., Int. Rep. PI-X-3. 10 p.

Lawson, B.D. 1972. An interpretive guide to the Canadian Forest Fire Behavior System. Can. For.Serv., Pacific Forest Res. Centre, Info Rep. BCP-3-72. 17 p.

Lawson, B.D. 1977. Fire Weather Index: the basis for fire danger rating in British Columbia.Fisheries and Environ. Can., Can. For. Serv., Pacific Forest Res. Centre, Info Rep. BC-P-17.

24 p.

MacTavish, J.S. 1965. Forest fire danger tables: British Columbia Coast. Can. Dep. For. Publication

1099. 20 p.

McArthur, A.G. 1967. Fire behavior in eucalypt forests. Australia Forest. Timber Bureau, Forest

Res. Inst. Canberra. Leaf!. 107. 36 p.

Muraro, S.J. 1968. A modular approach to a revised national fire danger rating system. InContributions on the development of a national fire danger rating system. Can. For. Serv., Pacific

Forest Res. Centre. Info Rep. BG-X-37. 9 p.

Muraro, S.J. 1971. A burning index for spruce-fir logging slash. Can. For. Serv., Pacific Forest Res.Centre. Suppl. BG-3 to Canadian Forest Fire Behavior System. 16 p.

Muraro, S.J.; Russell, R.N.; Lawson, B.D. 1969. Development of diurnal adjustments table for theFine Fuel Moisture Code. Can. For. Serv., Pacific Forest Res. Centre. Info Rep. BC-X-35. 24 p.

Muraro, S.J.; Lawson, B.D. 1970. Prediction of duff moisture distribution for prescribed burning.Can. For. Serv., Pacific Forest Res. Centre. Info Rep. BG-X-46. 23 p.

Nelson, R.M. 1964. The National Fire Danger Rating System: Derivation of Spread Index foreastern and southern states. U.S. Forest Serv., Southeastern Forest Exp. Sta., Asheville, NC.

Res. Pap. SE-13. 44 p.

Nikleva, S. 1975. Atmospheric Environment Service Fire Weather Program for British Columbia.Can. Dep. Environ., Atmos. Environ. Servo (Pacific Region). 30 p.

Nikleva, S.; Parent, L.E. 1971. Calculation of fire danger indices by computer. Can. Dep. Environ

Atmos. Environ. Servo Tech. Mem. TEC757. 7 p.

Paul, P.M. 1969. Field practices in forest fire danger rating. Can. For. Serv., Forest Fire Res. Inst. Info

Rep. FF-X-20. 27 p.

33

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DROUGHT CODE (DC)

Q

Qo

Qr

Qf

Qs

V

Lf

Do

D.

moisture equivalent of DC, units of 0.01 inch water

moisture equivalent of previous day's DC

moisture equivalent after rain

final fall moisture equivalent

starting spring moisture equivalent

potential evapotranspiration, units of 0.01 inch water per day

day length adjustment in DC

previous day's DC

DC after rain

present day's DCD

INITIAL SPREAD INDEX (ISI)

f(W)

f(F)

R

wind function

fine fuel moisture function

present day's ISI

BUILDUP INDEX BUI

'esent day's BL

FIRE WEATHER INDEX (FWI)

1(0)B

S

duff moisture f~nction

FWI (intermediate form)

present day's FWI

SEVERITY RATING

Daily Severity Rating

Seasonal Severity Rating

DSR

SSR

37

.+ Government of Canada · objective is to promote the wise management and use of Canada's forest...

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Transcript of .+ Government of Canada · objective is to promote the wise management and use of Canada's forest...