Fig. 4. Illustration of the principles implemented in the air-to-soil transition model. S_h(z*) is the surface height distribution and v_h^soil(z*) the cumulated value corresponding to the volume fraction of the soil material; z = 0 is the average surface height of a soil area A of the order by ; z* = 0 and z* = h refer to the highest peak and the lowest depression within A. The TDR and temperature probes are located at depths of 2 and 7 cm. _t^soil(z*) is the dielectric constant of the bulk soil material (including soil water), and _h,t^app(z*) is the apparent dielectric constant of the two-phase (air, soil material) transition layer of thickness h.