Spot ignition of a wildland fire and its transition to propagation

Background: The prediction of the propagation of wildland fires is an important socio-Technical problem. Wildland fires are often initiated by small spot ignition sources and then spread to larger burning areas. Methods: Experiments are conducted for the spotting ignition of a forest surface fuel (pine needles) in a relatively large (up to 1 m2), horizontal laboratory bed, and the subsequent fire spread without wind. The spotting ignition sources are a cluster of steel particles, an ember and a small pilot flame. Key results and conclusions: Wildfire spread has an initial acceleration phase, with the growth of the burned area in the fuel bed following a power law dependence in time, almost independent of the ignition source. Comparison with previous larger-scale experiments and FARSITE modelling of the fire spread over similar fuel beds shows that the power function with time describes well the combined results of the initial wildfire growth and the transition to larger fire propagation for relatively long times. Implications: The Rothermel equation under different environmental conditions may be extended to describe the initial accelerative growth of a spot fire. This work supports the modelling of fire propagation that currently is geared to a later time in the development of a wildfire.