ABSTRACT
The modeling of water flow and heat transport processes in the subsurface has to be based on a mathematical formulation of the various processes occurring in a considered spatial domain of the subsurface. Such a mathematical formulation provides a compact description of the relevant processes and includes initial and boundary conditions thus representing a model of the complex reality. Furthermore, it lists all assumptions and simplifications, which are postulated. The considered domain extends horizontally and vertically, the size depending on expected length scales of the processes, as well as on possibilities to formulate proper boundary conditions for the related variables like temperature or infiltration rate. For the modeling of heat transfer in shallow subsurface systems, the considered domain typically includes the soil surface where it is feasible to formulate thermal boundary conditions. This means, for the case of shallow unconfined groundwater systems, that, in general, the unsaturated zone (also referred to as capillary zone) has to be taken into account. Physical processes essentially comprise hydraulic and thermal processes in porous media. Heat generation by chemical or biochemical reactions or by radioactive decay is not assumed to be of importance in the case of heat transport in the subsurface. Thus, the relevant processes considered here comprise the flow of water and the heat transport in both the unsaturated and the saturated zones of the subsurface.
