Relating Fire Occurrence to Weather Conditions on the Great Basin Rangelands

Abstract

Results are presented to demonstrate the association between weather conditions and fire danger on the rangelands of the Great Basin. It was found that precipitation frequency and duration are physically related to fire occurrence in a complex manner. A climatic relationship also exists between monthly temperature and the corresponding total number of fires for a specific area. On sparsely inhabited Bureau of Land Management (BLM) rangelands in the Great Basin, fire weather stations are separated by great distances. Consequently, large areas of land are unobserved. Furthermore, there is the question of whether individual station location and exposure are representative of fire problem areas. Current observations of fire weather and danger are made in conjunction with the National Fire Danger Rating System (NFDRS). This system was developed by the National Forest Service Personnel and has been described in detail (Deeming et al. 1978). Other references concerning fire weather (Schroeder et al. 1970) and wildfire behavior (Albini 1976) are also available to relate the impact of weather on fires and fire danger. Use of the NFDRS by fire management personnel involves the input of weather data on a daily basis as a requirement for predicting an index or component of fire danger. The principal parameters which are output to field personnel are described in Table 1. Instructions concerning actual applications of these predictions are given in Deeming et al. (1978) in which the stated purpose of the system is to evaluate 'the near upper limit of the behavior of fires that might occur on a rating area during the rating period'. The purpose of the study was to examine the NFDRS indices and components with respect to actual fire occurrence and behavior in the Great Basin area, to examine other weather data in regard to fire conditions, and to indicate zones within which the fire occurrence and behavior would be homogeneous in response to existing climate characteristics. In this~ paper findings with regard to the first two objectives listed above are discussed. Methodology Fire occurrence on the rangelands of the Great Basin was examined in the light of weather data presently available for the purpose of determining efficiency of the existing fire The author is climatologist, Utah Department of Agriculture, Utah State University, UMC 48 Logan, Utah 84322. This study was supported by the Bureau of Land Management (BLM) through contract YA-512-C77-198. In addition, the contribution of data from various government agencies including the BLM and the U.S. Forest Service is acknowledged. Manuscript received March 26, 1979. weather network. Fire weather station data (Fisher et al. 1972) was obtained from the National Fire Weather Library (Furman et al. 1975). National Weather Service stations were used to supplement data from the fire weather stations. The average impact of climate, as well as the day to day influence of weather on fire occurrence, was studied. Fire reports were examined to determine the location and occurrence of individual fires. Results and Discussion Fire frequency data from individual BLM districts showed that the fire season was generally May through October (e.g., Fig. la and I b). The highest frequency of fires was usually in July for the Great Basin Fire Districts examined. In a few districts August was the month of highest fire frequency which probably reflects a different vegetative regime. Seasonal Relationship Examination of temperature data showed that a seasonal relationship exists between fire occurrence and average Table 1. Fire danger indices and components which are calculated using the National Fire Danger Rating System (NFDRS). Fire Behavior Potential Spread Component (SC)-the forward rate of fire spread. Engergy Release Component (ERC)-the potential available energy released per unit area in the fires flames. Fire Occurrence Ignition Component (IC)-related to the probability that a fire would result if a fire brand (lightning, spark, etc.) were introduced. Lightning Risk (LR)-a combination of thunderstorm prediction or observation and a local factor derived from records of lightning activity. Man-Caused Risk (MCR)-a rating assigned by the fire manager to reflect the activities of man in the area of concern. Fire Control Planning Lightning Occurrence Index (LOI)-a combination of LR and IC (see above). Man-Caused Occurrence Index (MCOI)-a combination of MCR and IC (see above). Burn Index (BI)-related to the difficulty of fire containment (from SC and ERC above). Fire Load Index (FLI)-combination of fire occurrence and behavior into a single number. 360 JOURNAL OF RANGE MANAGEMENT 33(5), September 1980 This content downloaded from 157.55.39.91 on Sat, 27 May 2017 17:57:44 UTC All use subject to http://about.jstor.org/terms