n. The rate of canopy fuel consumption per unit time per unit area of a canopy profile.
Mass-flow rate is an important factor affecting the propagation of crown fires. It is a product of canopy bulk density (kg m-3) and the rate of spread of the crown fire (m sec-1). One way to conceptually picture mass-flow rate is to imagine the flaming front as stationary (rather than the forest as stationary and the flaming front moving though it) and to imagine the forest moving through this stationary front as on a conveyor belt. The mass consumed per unit time through any square meter of the flaming front depends on how fast the “conveyor belt” is moving (the crown fire rate of spread) and the density of the forest that is moving along the conveyor belt (the canopy bulk density).
Van Wagner (1977) empirically determined the minimum mass-flow rate required to sustain a crown fire as 0.05 kg m-2 sec-1. Using this relationship and a model to predict crown fire spread rate, the minimum wind speed needed to sustain a crown fire can be calculated. Alternatively, the canopy bulk density below which a crown fire is unlikely can be calculated. See Agee and Skinner 2005, Scott and Reinhardt 2001, and Van Wagner 1977 for further discussion.