It has been well established that under certain circumstances wildfire is capable of producing water repellent or hydrophobic soils. Hydrophobic soils can dramatically alter runoff and erosion processes and as such have been the subject of considerable research activity. Wildfires in chaparral vegetation are recognized as being particularly susceptible to hydrophobic soil development. A comparison of chaparral fire soil heat profiles from DeBano (1989) and Weirich (unpublished) indicates that under higher fire intensity situations in chaparral a different soil heating mechanism other than just conduction heating may be at work. In contrast to the slow moving low temperature increases expected in conduction heating a much faster heat pulse resulting in more rapid temperature rises and higher temperatures at depth can also occur in chaparral wildland fires. This suggests that a better understanding of the heat transfer processes that occur at extreme fire intensities is both important and is needed. The specific aim of this study was to observe heat flow under a variety of particle sizes using a laboratory based wildfire simulator operating at intensities and durations similar to those experienced in chaparral wildfires.
The wildfire simulator system consisted of a propane burner array, an array of thermocouples to measure temperatures at varying locations and depths, and a data logging system to record the results of the heating experiments. Using the simulator homogenous sand, silt, clay, and heterogeneous clay loam were subjected to 600ºC, 900ºC, and 1200ºC peak intensities with two different heating durations or treatments (H1 and H2). The heating levels and durations used were based on data from field based chaparral fire experimental temperature data previously collected by Weirich (unpublished). The system design allowed the user to control the intensity and duration of the heat treatments and the thermocouple sensor arrays measured temperatures at the flame to a soil depth of 15cm. The apparatus and experimental treatments allowed for the investigation of peak heat intensity, heat duration, slope, and most importantly particle size on heat transfer processes.
The higher soil temperatures at depth, shorter times to peak temperatures at depth, and observed temperature spiking seen during some of the simulator experimental runs (specifically with respect to larger particle sizes such as sand) call into question the view that slow moving conduction may not be the only soil heat transfer process at work in high fire intensity situations such as those seen in chaparral wildfires and in particular chaparral wildfire underlain by larger particle sizes fractions such as sand.