Estimating the heat transfer to an organic soil surface during crown fire

Abstract

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.