The present theoretical base for mass transport in porous media rests mainly on empirical observations and resultant mathematical descriptions made at the macroscopic [pore size or above] level. Recently, several attempts have been made to extend the theoretical base to the molecular domain. Laroussi and De Backer [1975] derived the diffusion equation for soil moisture flow by considering the displacement of a fluid particle through an unsaturated porous medium as a Markov's stochastic process. However, Bhattacharya, Gupta and Sposito [l976a] point out that the Laroussi‐De Backer interpretation of the diffusivity is invalid because their analysis does not allow for Markovian coefficients that are space and time dependent.