Approximate solutions to the two-phase formulation of the problem of infiltration (i.e. accounting for both water and air movement) give more accurate predictions than exact solution to the unsaturated flow formulation (i.e., accounting only for water movement). The two-phase theory has the significant advantage of accounting for observed experimental results which cannot be modeled by the one-phase flow theory. The approximate solutions are not only more accurate, but also more economical. The costs of computer runs for prediction of infiltration rates by the analytic approximate method were on the order of one-tenth of the costs involved in the numerical finite-difference solution of the two-phase flow equations. (Author)