A Non-Equilibrium Model for Soil Heating and Evaporation Under Extreme Conditions

The increasing probability of wildfires being driven by climate change and the increasing use of prescribe burns by land managers requires better understanding and modeling of the coupled heat, moisture, and vapor transport in soil during extreme heating events. Here I describe a model designed to investigate non-equilibrium evaporation and the transport of moisture (liquid) and water vapor during fires, which can have surface heating ranging between 10,000 and 100,000 Wm-2 for several minutes to several hours. Although the present model is a more general model than Massman's (2012) equilibrium evaporation model (Massman, Water Resources Research 48: doi10.1029/2011WR011710), it does employ some of the same parameterizations. On the other hand, the present model accommodates different formulations of hydraulic conductivity, the water retention curve, water activity, and the non-equilibrium evaporative source term, none of which the 2012 model did. Since the basis of the new model is three coupled partial differential equations, both the Crank-Nicolson (CN) and Newton-Raphson (NR) methods are used for numerical solutions. All of these model improvements were made in the hope of shedding some light on the original 2012 model's inability to produce a physically meaningful solution. Similar to the 2012 model, the present model's 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. Results to date suggest that the CN and NR solutions can be different, but that neither method yields a fully satisfactory solution or one that necessarily approximates the laboratory observations.