Lower boundary condition

Different options exist for the lower boundary (see switch Lower Boundary). The lower boundary condition for heat conduction can be given as a temperature or as a constant flow equal to a constant geothermal contribution parameter, q_h,low. In the former case the temperature, T_lowB is calculated from the assumed value of annual mean air temperature, T_amean, and amplitude, T_aamp, from an analytical solution of the conduction equation:

                      (1.13)

where t is the time, t_ph is the phase shift, ω is the frequency of the cycle and d_a is the damping depth. The frequency is defined as:

                                                                  (1.14)

where y_cycle is the length of the temperature cycle (diurnal or annual) and the damping depth, d_a, is given as:

                                                                (1.15)

where D is the thermal diffusivity which is given as the ratio between the thermal conductivity, k_h, and the heat capacity, C, of the soil at a moisture content that equals the selected initial conditions.

Heat convection at the lower boundary condition depends on the presence of a ground water table in the profile. For an unsaturated profile convection follows percolation from the lowest soil layer. When a horizontal net ground water flow is present, convection follows this flow and is neglected for all layers below ground water level.