Resurfacing of underground peat fire: smouldering transition to flaming wildfire on litter surface

Background Smouldering wildfires in peatlands are one of the largest and longest-lasting fire phenomena on Earth, but it is unclear whether such underground peat fires can resurface to the ground and ignite a flame on the litter layer.Methods This work conducted a laboratory experiment by putting a 5-cm thick litter layer (banyan tree leaves with a density of 27-53 kg/m3) onto a 10-cm thick peat sample (moisture content of 10-100%).Key results and conclusions Tests confirmed that a smouldering peat fire, ignited at the bottom, can propagate upwards and resurface to ignite a flaming wildfire on the surface litter layer. The propensity of litter to be flaming ignited decreased with increasing peat moisture content and litter layer density. We found the threshold of such surface flaming as a function of temperature and temperature increase rate at the interface between peat and litter. Finally, large field experiments successfully reproduced and validated the laboratory observations.Implications This work reveals an important wildfire ignition phenomenon that has received little attention but may cause new spot fires, accelerate fire progression and exacerbate its hazards. Smouldering peat fires can survive underground for months, and may re-emerge and start a flame above ground when the dry and hot season arrives. This work demonstrates that the resurfacing of underground peat fire can ignite a flame on the surface litter layer and increase wildfire hazards. Photograph by Yichao Zhang et al.