Soil heating and evaporation under extreme conditions: Forest fires and slash pile burns

Heating any soil during a sufficiently intense wild fire or prescribed burn can alter soil irreversibly, resulting in many significant and well known, long term biological, chemical, and hydrological effects. To better understand how fire impacts soil, especially considering the increasing probability of wildfires that is being driven by climate change and the increasing use of prescribe burns by land managers, it is important to better understand the dynamics of the coupled heat and moisture transport in soil during these extreme heating events. Here I describe the development of a new model designed to simulate soil heat and moisture transport during fires where the surface heating often ranges between 10,000 and 100,000 Wm-2 for several minutes to several hours. Model performance is tested against laboratory measurements of soil temperature and moisture changes at several depths during controlled heating events created with an extremely intense radiant heater. The laboratory tests employed well described soils with well known physical properties. The model, on the other hand, is somewhat unusual in that it employs formulations for temperature dependencies of the soil specific heat, thermal conductivity, and the water retention curve. In general qualitative terms, the model agrees with the laboratory observations, viz., it simulates an increase in soil moisture ahead of the drying front (due to the condensation of evaporated soil water at the front) and a hiatus in the soil temperature rise during the strongly evaporative stage of the soil drying. Nevertheless, further diagnostics of the model's performance show that it is basically incapable of producing a fully physically realistic solution. This is because it employs, as do most soil evaporation studies, the Kelvin equation to model the relationship between the soil relative humidity and the soil water potential, which is shown to autonomously produce a spurious vapor gradient in the dry zone of the soil profile that effectively forces the soil moisture deeper into the soil rather than allowing it to escape (evaporate) out of the soil. Nonetheless, this specific issue with the Kelvin equation is symptomatic of a more profound lack of knowledge of the dynamics of soil evaporation and the soil moisture potential under the combined conditions of high temperatures and very low soil moisture content.