FIRE SEASON SEVERITY RATING

Transcription

1 CANADA Department of Northern Affairs and National Resources FORESTRY BRANCH FIRE SEASON SEVERITY RATING by D. E. Williams Forest Research Division Technical Note No

2 Published under the authority of The Honourable Alvin Hamilton, P.C., M.P., Minister of Northern Affairs and National Resources, Ottawa THE QUEEN'S:PRINTER AND CONTROLLER OF STATIONERY OTTAWA, 1959 Cat. No. R47-73

3 CONTENTS PAGE INTRODUCTION... 5 METHOD OF STUDY Rate of Spread Resistance to Control COMPUTING THE SEVERITY OF A FIRE SEASON SOME ApPLICATIONS OF THE SEVERITY RATING SUMMARy REFERENCES ! 3

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5 Fire Season Severity Rating by D. E. Williams* INTRODUCTION Canada's forest fire season normally extends from early April to the end of October. Each year, during this period, forest fires burn an average of more than one million acres of forest land. However, the number of fires starting, the area burned over, and the ensuing damage and costs, vary greatly from year to year resulting in what are known as "good" and "bad" fire sea ons. The principal reason for this variation is weather, or more specifically, those weather factors that control the inflammability of forest fuels. The amount of "dry" weather balanced against the amount of "wet" weather determines the potential severity of a fire season. The word potential is used because, under any degree of severity, the actual number of fires will be governed mainly by those ignition agents present, and the protection personnel, by their action, will largely determine what the damage and costs will be. Thus the size of the annual fire loss is controlled by three important factors, fire weather, ignition risk, and fire control activity. To date, very little can be done to alter fire weather, but every year more money is being spent on prevention work to reduce numbers of fires, and on presuppression, detection, and suppression to reduce the damage done. Because there is an economic limit to these expenditures, fire control people want to know what value they are getting for their fire control dollar and how best to spend it to increase its returns. They ask "... was the reduction in losses this year the result of our intensified effort or was this just an easier fire season than last?" To answer this question, a reliable method of measuring fire season severity is needed. Several such methods have been developed for particular areas. Lindenmuth (8), using past records, calculates the average number of fires starting per burning index unit to develop a risk factor. Countryman and Intorf (3) use both fire occurrence and area burned to develop a "severity index" to show how much a particular fire season varies from the average of a previous number of seasons. Similar work has been done by Keetch (7). There are several reasons why these systems are not particularly applicable to Canada. For example, Countryman and Intorf (3) state that one must "assume that a constant fire-starting potential exists and that the number of fires starting and the size each attains depend on weather alone". These are assumptions that could hardly be made here. In many parts of the country forest closure, cancellation of burning permits, and other steps are taken to ensure that the risk is lessened during high danger periods. Also, in the normal development of the country, additional forest areas are being opened for industry and public recreation where formerly there was no fire-starting potential save lightning. It cannot be assumed that the fire potential is low merely because these newly opened areas may have had few fires in the past. Even in accessible Forestry Officer. Forest Research Division. Forestry Branch. Ottawa. 5

6 areas where fire control has been practised for a number of years, changes in fire occurrence might be reflections of changing populations, public habits, fuel distribution, and numerous other factors. Thus past fire occurrence patterns can lead to erroneous results when used on a long-term basis. Also, when average fire size and average difficulty of control are considered along with occurrence, the amount of data available is limited since, as noted by Beall (1), much of the detailed information required does not appear on all fire report forms. METHOD OF STUDY Where a fire danger rating system based on daily fire weather measurements is in use, the problelil of rating the potential severity becomes one of evaluating the relative severity of the various danger classes indicated by the system. For example, it is necessary to have a direct measure of how much more severe "Extreme" is than "High". Once the evaluation has been made, each day of the fire sea on can be weighted according to the severity factor of the danger class into which it falls. Then, by totalling these values, a severity figure for the fire season is obtained. Where there is a standard length of fire season or where seasons are naturally of more or less equal duration, the total may be divided by the number of days in the season to obtain the severity rating of the average day in the season. This rating can be used for comparison with a similar figure for any other fire season in the same area or for comparison with the ratings for other areas where the same danger rating system is in use. The system of fire danger measurement developed by the Forestry Branch (6) is particularly well suited to this approach. The forest fire danger index is computed from daily measurements of fire weather factors and is indicated on the scale 0 to 16, with 0 representing a Nil condition. The four classes of danger recognized, Low, Moderate, High, and Extreme, contain four index units each. These unit values were established for administrative purposes and those who use them have come to associate each index with a certain level of severity. However, the 0 to 16 rating bears no numerical relationship to severity and so danger indexes cannot be totalled to indicate season severity. For example, if in two fire seasons of equal duration one had all "Moderate" days and the other had half "Nil" and half "Extreme" days, the total of the indexes would be the same in both examples, but the second would be expected La be the more severe season. It is necessary therefore to determine the true relative severity of days in each class by weighting them according to the behaviour of fires in that class. In rating severity, two important factors must be considered-rate of spread and resistance to control. The size of a fire when controlled and the amount of effort required to do the job are largely the result of their combined effect-the rate of perimeter increase times the effort required to control it. For example, if a fire perimeter is increasing at the rate of 15 chains per hour and eight manhours are needed to control one chain of line, the fire severity may be considered to be the rate of spread times the resistance to control, or, in this instance, 120. If a large number of fires in one class of fire danger were treated similarly and the average figure for all was 120, then this could be called the severity factor for that class. A similar figure for the other classes could be computed. The resulting series of figures would be indexes of the relative severity of the danger classes and it would matter little whether they were measured in man-hours or some other unit so long as the same method were used to rate each class. Ideally, rate-of-spread and resistance-to-control data from actual forest fires burning within the various classes of fire danger would give the information required to rate those classes. However, as noted in the New Brunswick study (1), there are not as yet sufficient data of this nature on a suitable number of fires. 6

7 When enough such data become available for the purpose it is suggested that they could be used as a check on the present study where other means are employed to assess rate of spread and resistance to control. Rate of Spread To obtain rate-of-spread values, use was made of the observational results of small test fires set, under a wide range of weather conditions, primarily for the purpose of preparing forest fire danger tables. Advantages over the use of wildfire data are several. First, the effect of occurrence is held constant-a desired condition recognized by Lindenmuth-because at the field sites where test fires are set an equal number are tried each day, in the absence of rain. Second, a standard method of rating test fires was established (9) and used at all Forestry Branch fire research stations across Canada, thus ensuring that the indicated relationship between fire danger classes was uniform. Since a measure of only the relative severity of the danger classes is required and since a standard test fire method was used for all fuel types, data from a single type were considered adequate for this study. Similarly, it matters little which fuel type is chosen or where the tests were made so long as extremes of both fuel type and location are avoided. However, the test fire rating system in use is better suited to some fuel types than to others and therefore, to derive a rate-of-spread index, a fuel type was selected in which results have been good and where a satisfactory distribution of tests in all danger classes was available. The jack pine fuel type filled the requirements, and data from two locations, one at a temporary fire research station in Manitoba and the other at a similar station in Saskatchewan, were chosen because of their familiarity to the investigator. Test fire observers had recorded the area burned in square feet per unit time. This was transformed to an index of perimeter increase per unit time by taking the square root of the area in square inches of each test fire. This measure of rate of spread from 1,121 test fires in the jack pine fuel type was averaged by danger classes as shown in Table 1. To obtain relative values the rate-of-spread averages were reduced to ratios. To do this the moderate class was considered as indicating normal conditions and the rate of spread for that class (7.7) was given the value 1. TABLE I.-RATE OF SPREAD IN RELATION TO FIRE DANGER CLASS Fire Danger Class No. of Test Fires Average Perimeter" Ratio Nil Low Moderate High.... Extreme.... Total , " At mid'point of Danger Class. Resistance to Control Initially, it was hoped that other measurements from the same test fires -particularly vigour and smouldering-could be used to give an index of the resistance to control. However, this was not possible because in evaluating the test fire behaviour both vigour and smouldering are given a rating in the range 0 7

8 to 5 according to an arbitrarily established schedule which could not be used as a direct index of resistance to control. Therefore the "drought index" which is readily determined from the fire danger tables was used. The drought index gauges the inflammability of deep humus and other heavy fuels such as windfallen branches and trees. In any danger class, it is the inflammability and persistence of burning in these fuels that largely determine the average resistance to control. Thus the drought index may be considered as an expression of this factor. The effect of other factors, such as soil condition and the presence of obstructions, can be expected to average out. The drought indexes associated with fire danger classes over a five-year period at two fire weather stations in widely separated parts of Canada-Saskatchewan and New Brunswick-were determined. Results show that, although the actual average drought index for each danger class varied from one area to another, the ratios of these indexes from class to class were practically identical. This is shown in Table 2. TABLE 2.-DROUGHT INDEX IN RELATION TO FIRE DANGER CLA SS Saskatchewan New Brunswick -- Fire Danger Class No. of Observations Average Drought Index' Ratio No. of o bserva tions Average Drought Index' Ratio Low Moderate.. High Extreme Totals At mid-point of Danger Class I Again, since a rating of only the relative severity of the danger classes is required, the mean of the two ratios shown above, 0.39, 1.00, 1.66, and 2.42, was uscd to represent difficulty of control in the severity rating. As stated previously, the severity of a fire may be expressed by its perimeter increase times the difficulty of controlling this perimeter. The figures established for rate of spread and resistance to control in Tables 1 and 2 are ratios which can be multiplied together to obtain a ratio of their combined effect. The results (Table 3), when rounded out where possible to the nearest whole number, are the severity factors for each class of fire danger. It is interesting to note that these factors do not differ greatly from the relative fire load ratios developed by Beall (1), and shown in the final column of Table 3. TABLE 3.-SEVERITY IN RELATION TO FIRE DANGER CLASS Fire Danger Class Perimeter Drought Severity Severity Ratio Ratio Ratio Factor Relative Fire Load Ratios N.B Low Moderate High Extreme

9 Although the severity rating was developed mainly from field data collected in the jack pine type in Manitoba and Saskatchewan, it is reasonable to assume that the severity factors 0.2, 1, 4, and 12 represent the relative severity of danger classes in any area where the Forestry Branch system of fire danger rating is in use. COMPUTING THE SEVERITY OF A FIRE SEASON The severity of a fire season is computed by multiplying the number of days falling into each class of fire danger by the appropriate severity factor, totalling the results, and dividing by the number of days in the fire season. It is, in effect, the severity rating of the average day in the season. This figure alone has some significance when it is borne in mind that if all days were moderate the season severity would be 1. It has greater significance, as is shown later, when it can be compared with severity ratings computed on the same basis for other seasons or other areas. In the example in Table 4 the severity rating is 1.1. TABLE 4.-COMPUTATION OF SEASON SEVERITY Danger Class Severity Number of Season (a)x(b) Factor (a) Days (b) Severity Rating Nil Low = 1.1 Moderate High Extreme Totals The severity rating system can be applied to any area for which the danger index has been computed. However, before calculating the severity index it is necessary to determine the number of days in the fire season falling into each fire danger class. Obviously, this is simple if only one fire-weather station exists in the area but, where there are several stations, the problem of arriving at a suitable average arises. This problem is discussed by Beall (1), and he concluded that the averaging method least open to objection is that of averaging the number of days in each danger class for a number of stations. This can be done for all stations in the area concerned or for two or more key stations having a good coverage of the major fuel types. The first way is preferable where all stations are well distributed and equally reliable, but the second is better when there are some well located stations with reliable records and others that are less dependable. Using either method, it is desirable that all stations whose records are used in the computation should have been in operation for the full fire season. Once the average number of days in each danger class has been calculated, the severity rating of each year under study may be computed as in Table 4. SOME APPLICATIONS OF THE SEVERITY RATING Applications of the severity rating may best be explained by the use of examples. For this purpose, data from two Forest Districts of New Brunswick, here to be designated as A and B, are used. Fire records and danger indexes were available for the 14 years 1938 to 1951, and fire-weather stations in both Districts were in operation for approximately the same period each year. During the 14-year period, District A experienced 334 fires and District B experienced 379. Only those fires which occurred within 25 miles of an operating weather station are included here since it has been found that the fire danger ratings are 9

10 .. Figure I Fire Season Severity 2.5 District A District >. 1.5 c (l) \ > (l) \ (f) \ \ \ \ 1.0 \ 1\ 1 \ I \ V 0.5 o.o 1938 ' 40 ' 4 2 '44 '4 6 '4 8 ' 5 0 Year Figure 2 Severity and Numbers of Fires as Per Cent of Worst District B, New Brunswick 100 Severity Actua I Fires , 1\, \ I I I \ I \ I \I V ' 40 ' 42 '44 '4 6 '4 8 Year '5 0

11 highly reliable within this radius (1). Few fires had to be excluded, however, since there are sufficient weather stations in most Districts of New Brunswick to give the desired coverage. Probably the first logical application of the rating is to compare it year by year within an area or between two areas. This is done graphically in Figure 1 for Districts A and B. Similar graphs may be prepared for any two or more areas regardless of their size or location. For a particular area a more meaningful value may be obtained by expressing the severity rating as a percentage of the rating for the worst year on record or for some other past period chosen as a standard. An example is shown in Figure 2 for District B where the year 1944 had the highest severity rating during the period under study. Another application also shown on Figure 2 is the comparison of yearly severity ratings in relation to number of fires. To simplify the comparison, the number of fires each year is also expressed as a per cent of the number experienced during the worst year. Graphs of fires and severity ratings may be used to indicate to what extent fire occurrence is related to fire weather in the area under study. The graph for District B shows, for example, a considerable decrease in fires in 1940 from 1939, whereas there was only a slight corresponding decrease in fire-weather severity. It should be possible to trace the discrepancy to some decreased risk or to the use of some new prevention regulation. The reverse situation is seen to exist between 1949 and 1950 when the number of fires increased and the severity decreased. Another application of the severity rating, similar to that suggested by Countryman (4, 5), is the recording on a chart of cumulative severity through the current fire season. For best results it should be done on a weekly basis and for individual stations as shown in Figure Figure 3 Cumulative Weekly Severity Kedgwick. New Brunswick :E'150 Q) > Q) (I) Q) > :;: 100 o ::;) E ::;) u May SPRING June 1 I July SUMMER 11 August September FALL October

12 The broken line on the graph is an average determined by assuming that the severity of each day in the year was equal to the average for the 14 years under study. It may be used as a guide in assessing the severity trend of the year in question. Although the seasonal severity rating of 1950 was near average, as shown by the proximity of the 1950 curve to that of the average at the end of the season, the curve shows that an above-average spring build-up occurred from the beginning of the season on May 1 until about mid-june. This build-up would be obvious to anyone plotting the curve from week to week. The curve for 1951 indicates that the year was one of below-average severity with a fairly regular trend throughout the fire season. Use of this technique can provide fire control officers with a picture of how the current fire season is developing as compared to past seasons known to them, and warn them of the approach of severe fire conditions. Also, any past years can be compared with regard to their severity on a monthly or seasonal basis. Such comparisons might point up the need for accelerated prevention at some particular season. The use of fire season severity ratings opens new approaches to many fire control problems. For example, using the relation between severity ratings and past fire statistics as a basis, a figure for expected number of fires for any season can be worked out. By comparing this figure with actual fire occurrence, some measure of the effectiveness of fire prevention may be obtained. Similarly, using area burned, a measure of fire control effectiveness is obtainable. The rating might also be used to point out areas or seasons of persistently high severity. The greatest value of the severity rating lies in the fact that it measures the fire potential resulting from weather alone and thus, in any calculation using it, the effect of this variable can be assessed and separated from the effects of all others. Computations of severity rating might well be included in the duties of the fire-weather observer. The use of new, simplified, 1956 Forest Fire Danger Tables (6) allows him more time for such work. SUMMARY From an analysis of a large number of small-scale test fires, a fire severity factor for each class of fire danger was developed: Nil = 0, Low = 0.2, Moderate = 1.0, High = 4, and Extreme = 12. To find the fire season severity rating for an area, the severity factor for each fire danger class is multiplied by the number of days in the fire season falling into that class; the whole is totalled, and divided by the number of days in the fire season. A few of the many practical applications of the severity rating are briefly described. It can be used to compare the potential severity of fire seasons year by year, in one area, or between different areas. When it is plotted with actual fire data, an indication of the effect of factors other than fire weather can be assessed. The severity rating can also form the basis for further and more detailed studies. 12

13 REFERENCES 1. Beall, H. W Forest fire and the danger index in New Brunswick. For. Chron., 2G (2). 2. Beall, H. W., and C. J. Lowe Forest fires in New Brunswick Dept. of Rf\ ources and Development. Forestry Branch, For. Res. Div., Research Note No Countryman. C. M., and P. H. [ntorf A fire Beason severity index for California ll:ltional f rest.s. U.S. Dept. of Agriculture, Forest Service, Cal. For. and Range Expt. Stn., Mis. paper No Countryman, C.M Fire season severity in 1955 on the Klamath national forest. U.S. Dept. of Agricult,ure, Forest Service, Cal. For. and Range Expt. Stn., For. Res. Notc Countryman, C.M. 1a57. California fire weather sevprity in U.S. Dept. of Agriculture, Forp L Servi e, Cal. For. and Range Expt. Stn., For. Res. Note Forest Fire D::mger Tables, Canada, Dept. of Northern Affairs and National Resources, Fore trv Branch. 7. Eeetch,.J.J Occnrrence rate as a measure of success in fire prevention. U.S. Dept. of Ag:riculture, Forest Service, Fire ContJ'llI Notes, 18 (J): Lindenmut,h, A. W., and J.J. Keetch A new measure of the severity of fire seasons. U.S. Dept. of Agriculture, Ft)re t Serviee, Fire Control Not-es, 11 (1): Maclf'orl, J.C. 19Ms. The effect of night weather on forest firf\ dan,rer. Canada, Dept. of Mine and ResourceB. Forestry Branch, For. Res. Div., For. Fire Res. Note No