Moisture Sorption Models for Fuel Beds of Standing Dead Grass in Alaska

Eric Miller

doi:10.3390/fire2010002

Abstract

Sorption models were developed to predict the moisture content in fuelbeds of standing dead grass from ambient weather measurements. Intuition suggests that the response time of standing dead grass to diurnal changes in weather is negligible and that moisture content tracks the equilibrium moisture content under most field conditions. This assumption suggests that moisture content could be modelled by empirically fitting coefficients to equations of equilibrium moisture content using field measurements. Here, six equations commonly used in wildland fire management and other industries were fit using 293 measurements of weather and moisture content in standing dead grass from Alaska, U.S.A. Predictors were air temperature and either relative humidity or dewpoint depression. Mean absolute errors of the best three models were approximately 1.16% of moisture content. The models predicted well the moisture content of an independently collected dataset from Canada but less so a set from Australia. The models may be used in wildland fire danger rating and fire behavior prediction systems.

Highlights

The moisture content of the fuel bed is an important driver of wildland fire behavior and an integral input to fire behavior prediction and fire danger rating systems
For some nominal one-hour fuels the effective timelag may be zero if the ratio of surface area to volume is high enough to allow the rate of internal moisture diffusion to equal or exceed the diurnal rate of change in moisture content (Me) determined by air temperature and relative humidity [9]
The primary objective of this paper is to provide predictive models of moisture content in standing dead grass for use in fire danger rating systems, fire behavior prediction, and fireline monitoring

Summary

Introduction

The moisture content of the fuel bed is an important driver of wildland fire behavior and an integral input to fire behavior prediction and fire danger rating systems. The moisture content (M) of thermally and sorptively thin fuels with a high ratio of surface area to volume drives attributes of flaming combustion such as ignition probability, flame length, fireline intensity, and rate of spread. These fine dead fuels carry fire across the landscape [1]. For some nominal one-hour fuels the effective timelag may be zero if the ratio of surface area to volume is high enough to allow the rate of internal moisture diffusion to equal or exceed the diurnal rate of change in Me determined by air temperature and relative humidity [9].

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Discussion

Conclusion