Fuel Loads and Fuel Type Mapping

Chapter 5

Fuel Loads and Fuel Type Mapping

Emilio Chuvieco Department of Geography, University of Alcald Colegios, 2, 28801, Alcald de Henares, Spain E-mail: [email protected] David Riafto Department of Geography, University of Alcald Colegios, 2, 28801, Alcald de Henares, Spain E-mail: [email protected] Jan Van Wagtendok USGS Western Ecological Research Center, Yosemite Field Station El Portal, CA 95318-0700 E-mail: [email protected] Felix Morsdof Remote Sensing Laboratories, Department of Geography, University of Zurich, Winterthurerstr. 190, CH-8057 Zurich, Switzerland E-mail: [email protected] Correct description of fuel properties is critical to improve fire danger assessment and fire behaviour modeling, since they guide both fire ignition and fire propagation. This chapter deals with properties of fuel that can be considered static in short periods of time: biomass loads, plant geometry, compactness, etc. Mapping these properties require a detail knowledge of vegetation vertical and horizontal structure. Several systems to classify the great diversity of vegetation characteristics in few fuel types are described, as well as methods for

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Wildland Fire Danger Estimation and Mapping

mapping them with special emphasis on those based on remote sensing images.

5.1 Relevant Properties of Fuels for Fire Danger Estimation and Fire Propagation Studies Wildfires (and in general any fire) are created and maintained due to the co-existence of three elements: fuels, oxygen, and heat (Folch, 1976). They constitute the so-called fire triangle. If the fuel is exhausted, removed or isolated, then fire is extinguished and the triangle is broken. The same occurs if the oxygen is limited or if there is not enough heat to perpetuate the burning process (Pyne et al., 1996). In this chapter we will first cover the relevant properties of fuels that affect fire danger and fire propagation, which are mainly related to amoung, composition, chemistry, physiology and geometry. We will then discuss the use of remote sensing methods to map fuel type characteristics. Vegetation, either as tree crowns or material on the surface, is the source of most wildland fuels. Crown fuels consist of both live and dead material in the canopies of trees, while surface fuels include shrubs, herbs, leaves, and woody particles in contact with the ground. Surface fuel properties can be grouped into three classes: physical, chemical, and physiological. We will discuss physical and chemical properties in greater details in this chapter. Physiological properties deal primarily with moisture content and have been extensively covered in chapters 3 and 4. Several fuel characteristics are critical for fire propagation studies: crown bulk density, crown base height, canopy height, canopy closure, surface area to volume ratio, vertical and horizontal continuity, dead and live fuel load, live woody loads, and size of particles refer to vegetation geometry, while moisture content is related to vegetation physiology. Since the latter has been extensively covered in chapters 3 and 4, we will focus now on the former. 5.1.1 Crown fuel properties

Crown fuels are those that burn when a fire leaps into the canopies of trees. Properties of crown fuels that determine the rate and intensity of


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crown fires include canopy closure, canopy height, crown base height, and crown bulk density. Canopy closure. Canopy closure, measured as percent cover, affects fire behavior by shading surface fuels and thereby increasing fuel moisture. In addition, the more complete the cover, the more winds above the canopy are reduced at the level that affects the surface fire (Albini and Baughman, 1979). Canopy cover also affects the amount of canopy fuels available for a crown fire, as we will see in the discussion below on crown bulk density. Canopy closure is one of the fuel properties most easily determined through remote sensing methodologies. Canopy height. Like canopy closure, canopy height affects the reduction of wind speeds at the fire level. The higher the canopy, the greater the wind reduction. Canopy height also affects the lofting of embers from a torching tree (Albini, 1979). An ember from a tall tree will travel further than one lofted from a shorter tree. Finally, canopy height contribute to the amount of crown fuels when coupled with crown bulk density. Tree heights have been measured routinely using aerial photographs and other remote sensing technologies (Lefsky et al., 1999b). Crown base height. Crown base height is the vertical distance from the ground surface to the base of the live crowns (Finney, 1998). It should also include ladder fuels such as dead branches, shrubs, and young trees that extend upward toward the crown bases, effectively reducing the crown base height (Van Wagner, 1993). This height determines the threshold for transition from a surface fire to a crown fire. Although tradition remote sensing technologies have difficulty determining crown base height, new sensors such as Light Detection and Ranging laser scanning (Lidar) show promise. Crown bulk density. Crown bulk density is the amount of fuel per unit of volume of the forest canopy. It determines the threshold for active crown fires, i.e., the spreading of fire from tree to tree. Different

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species of trees have different crown bulk densities depending on branching habit and foliage characteristics. The overall bulk density of the forest canopy depends on species present and the canopy closure. As tree canopies become closer and denser, fire is able to spread more easily from on to the other. Initial efforts to estimate crown bulk density using Lidar have proven successful (Riatio et al., 2003). 5.1.2 Surface fuel properties Surface fuels typically include dead material on the ground as well as the live and dead fuels in the herbaceous and shrub layers. Litter and small woody fuels are those fuels that lie on the ground and contribute to the passing front of the fire. Not included are large woody fuels and deep duff fuels that can burn and smolder for long periods after the fire has passed. These slower burning fuels are extremely important in determining fire effects, however. Physical properties of surface fuels include size class and surface area to volume ratio, specific gravity, load by size class, and fuel bed depth. Chemical properties are heat content and ash content. The horizontal and vertical arrangement of the fuel is also critical at determining fire behavior and effects. Because of the difficulty of sensing objects beneath canopies, surface fuels have been difficult to sense remotely. A combined analysis of field plots, imagery and modeling appears to be the most fruitful direction to take to determine surface fuel properties (Keane et al., 2001). Size class and surface area to volume ratio. The size class of fuel particles plays a critical role in fire behavior. The smaller the size of a fuel particle, the larger is the ratio between the surface and the volume. This ratio is an extremely important fuel characteristic because as more surface area is available for combustion, heating of the entire particle is quicker, and moisture is driven off more easily. Size classes have been traditionally specified as from 0 to 6cm, 6 to 2.5cm, 2.5 to 7.6cm, and >7.6cm (Albini, 1976). Fuel particle density. The weight per unit of volume of a fuel particle is called the particle density. The more dense the fuel particle, the higher


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the load and the greater the heat content. Albini (1976) specifies a standard value of 51.25 kg In-3 for fuel particle density for use in the BEHAVE system calculations. Load. The amount of fuel that is potentially available for combustion is called the fuel load. It can have a differential effect on fire spread and intensity. As a heat source, the more fuel available the higher the reaction intensity. Rate of spread may actually decrease as load increases, however; the extra fuel becomes a greater heat sink, and more heat is required to raise it to ignition temperature. Much of the response depends on the size class of the fuel, its packing ratio, and whether or not it is dead or live fuel. Fuel bed depth. Fuel bed depth is the distance from the ground to the top of the surface fuel layer. Depth directly affects fuel bed bulk density, the total amount of fuel that is potentially available. Fuel bed bulk density is defined as the oven dry weight of the fuel per unit of fuel bed volume and is calculated by dividing the oven dry fuel bed load by the fuel bed depth. An increase in density will tend to cause a decrease in spread rate. Depth also affects the actual packing ratio, which is found by dividing the fuel bed bulk density by the oven dry fuel particle density. Heat content. Heat content is the amount of energy contained within a fuel particle per unit of weight and provides the energy to drive combustion. Rate of spread varies directly with heat content; doubling the heat content results in a two-fold increase in rate of spread. There is some variation in heat content for fuels of different species. Conifers tend to have higher values than hardwoods because of the presence of resins and higher lignin content. Albini (1976) uses a constant heat content of 18.61 MJ Ash content. The total mineral ash content of a fuel particle is that portion of the particle that does not burn. Loads need to be reduced by the amount of ash in order to calculate the contribution that fuel has to flaming combustion. Although is considerable variation in ash content

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(e.g.,(Van Wagtendonk et al., 1998)), Albini (1976) uses a constant value of 5.5 percent. 5.2 Fuel Types and Fuel Models

Since the combination of fuel properties of vegetation species are almost infinite, the description of those properties relevant for fire danger estimation and fire propagation studies is based on classification schemes, which tend to summarize large groups of vegetation characteristics. These groups are usually called "fuel types" (Pyne et al., 1996). More specifically, a fuel type has been defined as "an identifiable association of fuel elements of distinctive species, form, size, arrangement, and continuity that will exhibit characteristic fire behaviour under defined burning conditions" (Merrill and Alexander, 1987). Vegetation species can provide a clue of the morphology, branch fall or litter fall properties (Keane et al., 2001), but are not necessarily determinant for fire management, since the same species may present completely different fire propagation rates if their fuel load, density, vertical continuity, compactness, or surface area to volume ratio characteristics, among others, change (Anderson, 1982; Andrews, 1986; Deeming et al., 1978). There are several strategies to classify fuel types, according to the final use of the fuel classification. Firstly, two general groups may be distinguished according to the type of fire that one intends to model. Modelling of surface fires is mainly concerned with fuels in contact with the soil, while analysis of crown fires require a description of the forest overstory. Surface fuels have received the most attention, since fire behaviour simulation programs dealing with surface fires are also the most widely known and tested. In North America, the National Fire Danger Rating System (NFDRS) uses 20 fuel types adapted to the vegetation conditions of the USA (Deeming et al., 1978). The description included both the understory and the canopy layer, but the main criterion to discriminate fuels was the ground vegetation. Examples of the descriptions are: Western grasslands; Brush dense; Open Pine, Palmetto-Galberry with pine, etc.


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The development of the Behave mathematical model carried along a new fuel type classification, known as the NFFL (Northern Forest Fire Laboratory or Behave) fuel types (Albini, 1976; Anderson, 1982). The authors distinguished 13 fuel types based on the primary nature of the surface fuel. The first three are herbaceous fuels, the 4 th to 6th shrub groups, the 8 th to 10th are dead leaves under forest canopy and the 1 to 13th are slash residues and basal accumulation material. This classification strategy has been widely used in fire propagation studies, since the Behave program has been used in many different ecosystems. Table 5.1 includes a more detailed description of the NFFL models. Additional classification schemes of fuel types are the Canadian Forest Fire Behaviour Prediction (FBP) System (Lawson et al., 1985), part of the Canadian Forest Fire Danger Rating System (CFFDRS). European researchers developed a new system, in the framework of the Prometheus project ( http://www.algo.com.gr/), which is better adapted to fuels found in Mediterranean ecosystems. This system simplifies and adapts the Behave classification to Mediterranean conditions. The Prometheus system is mainly based on the type and height of the propagation element and it identifies 7 fuel types (Figure 5.1). It includes the following categories: Ground fuels (cover >60%): Grass. Surface fuels (shrub cover >60%, tree cover 60%, tree cover 60%, tree cover 4 m) with a clean ground surface (shrub cover 4 m) with medium surface fuels (shrub cover >30%): In this fuel type the base of the canopies is well above the surface fuel

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layer (>0.5 m). The fuel consists essentially of small shrubs, grass, litter and duff. 7. Tree stands (>4 m) with heavy surface fuels (shrub cover >30%): Stands with a very dense surface fuel layer and with a very small vertical gap to the canopy base ( 60 % Grass

Fuel type 2

Average height (0.30-0.60 m) Fuel type 3

> 60 % Shrubs & < 50 % Trees (>4m)

Average height (0. 60-2.00 m)

000-103.

Average height (2.00-4.00 m)

Fuel type 4

Fuel type 6 •

= 50%

Average height difference between drubs and trees

!6'

> 0.50 m

Trees (> 4 in) > 30 %Shrubs Average height difference between shrubs and trees