Mixed composition of top layer

Since thermal properties of humus and mineral soil differ markedly (as described in detail in the next section on thermal properties), special treatment is required for a thin humus layer when numerical requirements demand that the top compartment represents a layer thicker than the humus layer, i.e. eq (1.3) has to be modified. Three special cases for heat conduction at the soil surface, q_h(0), are given, depending on the depth of the insulating litter or humus layer.

For negligible depths, i.e., less than 5 mm, thermal conduction in humus is neglected:

                                                     (1.9)

where k_hm is the conductivity in a mineral soil, T_s is the surface temperature and T₁ is the temperature in the first soil compartment.

For a humus layer thicker than 5 mm but less than half the depth of the top soil layer a steady-state solution, analogous to the one for snow, gives the boundary temperature between humus and mineral soil:

                                                              (1.10)

where

                                               (1.11)

where k_ho is the conductivity of the organic soil, k_hm is the conductivity of the mineral soil and ∆z_humus is the thickness of the humus layer. The temperature, T_b, is used to calculate q_h(0) instead of T₁, in eq.(1.3).

For humus layers thicker than half the top soil layer, the calcualtion of q_h(0) degenerates into the standard solution, i.e.:

                                                   (1.12)

where k_ho is the conductivity in the organic soil, T_s is the surface temperature and T₁ is the temperature in the first soil compartment.