Comparing the Fire Danger Rating Systems of Canada and the US: FWI versus NFDRS

Inhaltsverzeichnis

Abstract

1. Introduction

2. Description of the Canadian Forest Fire Weather Index (FWI)

3. Structure of the National Fire Danger Rating System (NFDRS)

4. Comparison and Discussion

5. Conclusion

6. References

Abstract

This paper describes and compares conceptually the Fire Weather Index (FWI) system of Canada and the National Fire Danger Rating System (NFDRS) of the USA. The relatively simple FWI system rates fire danger for all Canada and is based on empirical field- data derived from a single fuel type. The laboratory- based NFDRS, in contrast, allows more specification for distinct fire danger areas and models fuel moisture more abstract and in different classifications. Relative strengths and weaknesses with regard to the vegetative conditions in the particular country are discussed. The use of empirical data and the good and simple representation of soil moisture are the major strengths of the FWI system. The NFDRS wins through its possibility to model specifically a distinct fire danger area and through the consideration of live fuel moisture content. The conclusion of this paper is that both systems can benefit from each other. A combination of the simplicity of the FWI and specialization on a distinct area through the site descriptors similar to the ones of the NFDRS is proposed.

1. Introduction

Deeming et al. (1972) defined Fire danger as the “resultant descriptor of the combination of constant and variable factors which affect the initiation, spread and difficulty of control of wildfires on an area” . This descriptor helps fire managers to make decisions and pre- allocate resources so that fires can be fought quickly and efficiently (Schlobohm & Brian 2002).

In North America there are two systems that rate fire danger: the Canadian Fire Weather Index (FWI) and the American National Fire Danger Rating System (NFDRS). Van Wagner (1975) already compared both system structures almost 40years ago. Since then especially the NFDRS changed significantly. In this paper, I will describe and compare the current models regarding model basis, structure and outputs. In the end strengths and weaknesses of each model will be discussed.

2. Description of the Canadian Forest Fire Weather Index (FWI)

The FWI system rates the potential of fire in a single standard fuel type (jack- /lodgepole pine) with six numerical values (Wotton et al. 2009). It is basically a set of indices that represent fire danger at the midafternoon peak at 1600 hours LST (van Wagner 1987). The FWI estimates fire behavior through three moisture content classes and the effects of wind (van Wagner 1987). The system is mostly based on data that is derived from field experiments and observation (van Wagner 1987). Laboratory experiment data was only used to complement the field data (van Wagner 1987). Key assumptions of the model are that fuels and their moisture are homogenous and that wind is only blowing from one single direction without any interaction with topography (van Wagner 1987). Also, every rainfall is considered as overnight rain that dries out during the day (van Wagner 1987). The system components are determined by daily weather inputs like air- temperature, relative humidity, open wind speed at ten meters above ground and precipitation measured at noon LST (van Wagner 1987). The structure of the FWI system is relatively simple and easy to understand as Figure 1 shows.

[...]