Using observed fire behavior to compare custom and standard fire behavior fuel models: A case study of grass-invaded shrublands at Hawai‘i Volcanoes National Park

Background/Question/Methods

Invasive grasses in Hawai‘i have long been understood to be a critical factor affecting fire size in dry to mesic systems. However, observations comparing actual fire behavior with in situ fuels measurements and fire behavior model results are sparse. We measured fuel loads in a molasses-grass (Melinis minutiflora) invaded kipuka at Hawai‘i Volcanoes National Park (HAVO) and calculated expected flame lengths and rates of spread for it and for several standard fuel models using BehavePlus5®. We compared our model results with fire behavior observations recorded during a prescribed fire in August 2011 using a stationary video camera. Flame heights and flame angles were measured and converted to flame lengths. Rates of spread were measured for six intervals, and a boot-strapping algorithm was used to construct estimates of the mean, standard deviation, and 95% confidence intervals of rate of spread and flame length.

Results/Conclusions

Observed wind speeds ranged from 8 to 30 kmph and averaged 24 kmph. Average flame lengths were 1.19 m, maximum flame lengths were 3.73 m, and rate of spread was 0.53 m/s. We compared these observed fire behaviors to 6 standard FBFM and 1 custom FBFM. The custom model reproduced observed rates of spread accurately (0.47 m/s). All of the standard FBFMs over-predicted rates of spread. All of the fuel models—custom and standard—over-predicted flame lengths with the custom fuel model performing the worst.  Our results highlight the difficulty of predicting fire behavior in Hawaiian fuels specifically, and tropical fuels more generally. Even with custom models based on field measurements of Hawaiian specific fuels, our fire behavior predictions only marginally agreed with observations. Fire behavior science in the mainland United States has invested considerable time and money into developing accurate FBFM for mainland fuel types, and similar investments could improve Hawai‘i specific FBFM.