INTRODUCTION 



Objective, reliable assessment of potential fire behavior, especially for land man- 

 agers not intimately familiar with fuels and fire behavior, depends upon the ability to 

 recognize and describe quantitatively certain physical properties of fuel. Development 

 and use of mathematical models for assessment of probable fire behavior on wildfires 

 and prescribed burns particularly require quantification of fuel properties. Quanti- 

 tative description of fuel properties will be a necessity as the science of fire is 

 developed and more of its principles are applied to solving practical problems. 



Like most living things, forest and range fuels are assortments of plant parts 

 and have definite bodily form depending on inherent qualities and environmental condi- 

 tions. The individual plant is composed of distinctly different parts or organs; for 

 example, flowers, stems, leaves, roots, buds, petioles, bark, and thorns. The wide 

 variety of living and dead plant parts that exist are actually fuel partiates of varying 

 sizes and shapes. A collection of fuel particles can be termed a fuel complex. 



This paper reports values for the following physical properties and their relation- 

 ships to one another for ponderosa pine (Pinus ponderosa Laws.) forest floor and 

 cheatgrass (Bromus teatorum L.) fuels: 



1. Particle size - a (ratio of particle surface area to volume) 



2. Particle density (weight per unit volume) 



3. Loading (weight per unit ground area) 



4. Porosity: 



a. Ratio of void volume in fuel complex to surface area of fuel - X 



b. Particle spacing (average distance between fuel particles) 



c. Bulk density (weight per unit volume of fuel complex) 



5. aA (a dimensionless fuel complex parameter) 



6. Particle orientation (position in space). 



Except for particle orientation, the fuel properties were chosen for study because 

 of their significant influence on ignition probability, rate of fire spread, and fire 

 intensity. For example, the ratio o is a particularly meaningful measure of fuel 

 particle size because of its relationship to rates of change in fuel temperature and in 

 moisture content. Temperature and moisture content of thin fuel particles having a 

 high surface-to-volume ratio generally fluctuate more rapidly than thick particles 

 having a low surface-to-volume ratio (Pons 1950; King and Linton 1963). Controlled 

 combustion studies have shown that ignition time varies inversely with a (Curry and 

 Pons 1938) and rate of fire spread varies in direct proportion to o (Rothermel and 

 Anderson 1966) . 



Particle density is primarily an important fuel property because of its influence 

 on themial conductivity and consequent influence on time to ignition.-^ Whether or not 

 ignition of a fuel is of the pilot-flame or spontaneous type also depends on particle 

 density . 



^D. S. Stockstad. Ignition properties of fine forest fuels. Unpublished problem 

 analysis, Intermountain Forest and Range Exp. Sta. , Ogden, Utah. 1967, 



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