Canadian Forest Service Publications

Estimating the heat transfer to an organic soil surface during crown fire. 2014. Thompson, D.K.; Wotton, B.M.; Waddington, J.M. International Journal of Wildland Fire 24(1):120-129.

Year: 2014

Issued by: Northern Forestry Centre

Catalog ID: 35849

Language: English

Availability: PDF (request by e-mail)

Available from the Journal's Web site.
DOI: 10.1071/WF12121

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The Peatland Smouldering and Ignition (PSI) model was developed to quantify the heat transfer from a wildfire to an organic soil or moss surface in a Sphagnum–black spruce peatland. The Canadian Fire Behaviour Prediction system was used as a basis for the relationship between wind speed and rate of spread. Convection, conduction, and radiation processes were modelled before and during the arrival of the flaming front. The net heat flux to the soil from fire varied between 1.1 and 8.6 MJ m–2, with moderate-intensity fires transferring more energy to the surface compared with higher-intensity fires under higher winds. Radiative heat transfer to the soil surface both before the fire’s arrival and within the flaming front were the primary mechanisms of energy gain to the peatland surface. The role of convective and conductive cooling was no greater than 30% of gross energy gain. Peatland surface ignition in hummock and hollow microforms was modelled under normal and drought conditions. Hollow microforms dried out significantly during the course of a summer and increased their ignition vulnerability. Small-scale changes in slope and aspect of the peatland surface increased the amount of heat transferred by radiation by up to 30%, allowing some areas of higher soil moisture content to ignite. While no direct model validation is available, model outputs showing the preferential combustion of lichen and feathermoss, and the lack of ignition in Sphagnum in all but the most severe drought generally mimic observed ignitions patterns. The modelled peak of net energy input to the surface occurred at moderate wind speeds, suggesting that high-intensity fires do not necessarily lead to greater energy transfer and risk of smouldering combustion.

Plain Language Summary

About 1,500 square kilometres of forested peat bogs in western Canada are burned in wildfires each year, but current models of wildfires and studies of smouldering in peat bogs both fail to capture what happens when wildfire goes through peat bogs. In this paper, the researchers develop a model to predict wildfires in bogs of sphagnum and black spruce trees with a moss surface, a common type of forest cover in Canada’s boreal zone. The model will be tested in future studies. To develop the model, the researchers estimated the amount of heat transferred to the organic soil surface in this type of bog from a wildfire. Radiant heat was found to be the greatest contributor to the total heat input, with a lesser contribution by convection and conduction. The maximum heat input was found not in fire during the windiest conditions, but rather in fires during moderate wind conditions. This finding is important, as it differs from models and observations of the rate that fire spreads across the soil surface, in which the highest rates of spread are found at the highest wind speeds. This study contributes to knowledge of wildfires in a common forest type in Canada, which will help predict fires and their effects.