Nonlinear impacts of fuel load on rate of spread and residence time in forest surface fires

Frequent and extreme forest fires not only damage the environment but also pose a serious threat to firefighting personnel. Understanding the rate of spread (ROS) is crucial for developing effective firefighting strategies. Surface fuel load serves as the foundation for forest combustion, yet its effect on the ROS of surface fires remains contentious. To investigate the impact of fuel load on surface fire ROS, this study uses pine needles as the fuel material, simulating the accumulation of surface combustibles in forests by increasing fuel load, while also examining fuel depth and packing ratio under each load condition. Using an experimental system capable of measuring the ROS at multiple points along the fire line, we evaluated the ROS of surface fires within a fuel load range of 0.4–3.0 kg/m². The effects of packing ratio and fuel depth on ROS under different load conditions were discussed, and the influence of fuel load on fire line residence time was also explored. The results indicate that increasing fuel load leads to higher packing ratios and greater fuel bed depth due to the compressibility and weight of the pine needles. Within the studied load range, ROS does not follow a simple linear trend; instead, it initially rises, then decreases, and finally shows a slow increase. This non-linear behavior arises from the combined, and at times opposing, effects of packing ratio and fuel depth on ROS. Additionally, as fuel load increases, the residence time of the fire line follows a pattern of slow increase, sharp rise, and gradual further increase. The growth in flame depth, driven by higher fuel loads, ultimately becomes the dominant factor in prolonging the residence time. These findings provide valuable experimental data to enhance the understanding of forest surface fire dynamics and fuel load effects.