Fig. 1. Model for measured solar radiation S_r(t) as the input signal. R_T(t) = time dependent transfer function for the surface and the top centimeter of soil; I₁(t) = heat flow at the surface and within the top centimeter of soil; I₂(t) = heat flow transmitted below the 1-cm depth; T₁(t) = soil temperature at depth of 1 cm; R-C = ladder of resistances and capacitances for modeling soil below the 1-cm depth.