Based on the analysis of the hydraulic properties of unsaturated soil, a scaling solution of Richard's equation is presented in this paper. It may be easily transformed to the particular solution for a certain site if the scaling factors are known. That is, scaling of the vertical infiltration equation of unsaturated soil in a heterogeneous media to an invariant form with respect to certain boundary conditions can offer a unique solution for vertical infiltration. Thirty infiltration tests have been carried out in grassland at Richmond, 60km west of Sydney, and the data from each test were used to determine the sorption S and the hydraulic conductivity A in Philip's equation I = St^1/2+At. Owing to large variability of the parameters S and A, the infiltration data have been scaling according to the concept of similar media, and one value of scaling factor has been assigned to the infiltration data of each site. How to determine the scaling factors is discussed and the frequency distribution of the scaling factors in the present tests is log-normal. It was found that the numerical simulation results agreed very closely with the field experimental data.

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