In the soil, heat transfer occurs primarily by conduction, although convective heat transfer can be important in some cases. Heat conduction is governed by Fourier’s Law, which was first documented in 1807 and published in 1822 in France and may have influenced the later development of Darcy’s Law (1856) . Just as Darcy’s Law says that the water flux is proportional to the hydraulic gradient, Fourier’s Law states that the heat flux by conduction is proportional to the temperature gradient:
where qh is the heat flux (W m-2), l is the thermal conductivity of the substance through which heat is being conducted (W m-1 °C-1), and is the temperature gradient. Fourier’s work had tremendous influence, and the basic form of Fourier’s Law was followed subsequently in key breakthroughs such as Ohm’s Law for electrical conduction and Fick’s Law for chemical diffusion, along with Poiseuille’s Law and Darcy’s Law.