Live Fuel Moisture Content: The ‘Pea Under the Mattress’ of Fire Spread Rate Modeling?

Carlos Rossa,Paulo Fernandes

doi:10.3390/fire1030043

Abstract

Currently, there is a dispute on whether live fuel moisture content (FMC) should be accounted for when predicting a real-world fire-spread rate (RoS). The laboratory and field data results are conflicting: laboratory trials show a significant effect of live FMC on RoS, which has not been convincingly detected in the field. It has been suggested that the lack of influence of live FMC on RoS might arise from differences in the ignition of dead and live fuels: flammability trials using live leaves subjected to high heat fluxes (80–140 kW m−2) show that ignition occurs before all of the moisture is vaporized. We analyze evidence from recent studies, and hypothesize that differences in the ignition mechanisms between dead and live fuels do not preclude the use of overall fine FMC for attaining acceptable RoS predictions. We refer to a simple theory that consists of two connected hypotheses to explain why the effect of live FMC on field fires RoS has remained elusive so far: H1, live tree foliage FMC remains fairly constant over the year; and H2, the seasonal variation of live shrubs’ FMC correlates with the average dead FMC. As a result, the effect of live FMC is not easily detected by statistical analysis.

Highlights

Fire behavior modeling has been around for nearly a century [1]
One would expect that a complete understanding of the basic mechanisms driving fire propagation would have been achieved
A lack in understanding the effect of live fuel moisture content (FMC) on RoS hinders the development of prediction systems that are applicable to generic fuel complexes, which often are composed of mixed live and dead vegetation

Summary

Introduction

Fire behavior modeling has been around for nearly a century [1]. Intuitively, one would expect that a complete understanding of the basic mechanisms driving fire propagation would have been achieved . A lack in understanding the effect of live FMC on RoS hinders the development of prediction systems that are applicable to generic fuel complexes, which often are composed of mixed live and dead vegetation. The process of accomplishing a physically correct detailed description of the mechanisms of fire spread would greatly benefit from fully confirmed guidance on the expected main factors influencing fire propagation and how they exert that influence. There are studies confirming that ignition mechanisms differ between live and dead fuels [8,9,10,11]. Fire 2018, 1, 43 to establish that those differences in the ignition process do not preclude RoS modeling from overall fine FMC (live and dead fuels) such that the resulting accuracy is acceptable for operational purposes

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