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<br />system of water and soil in which both the water and <br />the soil particles are moving, but the full implications <br />for such complicated infiltration models are as yet <br />not clear. A general treatment of transport in an <br />unsaturated soil consisting of a mixture of a solid <br />phase, an aqueous phase, and a gaseous phase in <br />relation to deformable soils was given by Raats and <br />Klute (1968a, 1968b), <br /> <br />All of the solutions to the boundary-value <br />problems of infiltration process will yield infiltration- <br />capacity decay curves, but they are not generally <br /> <br />expressible in closed form. In application, however, <br />several algebraic (empirical) infiltration equations <br />have been developed. Because most algebraic equa- <br />tions are expressed as a function either of time or of <br />the total quantity of water infiltrated into the soil, <br />they are judged to be in the most convenient form for <br />use in the runoff study, The algebraic infiltration <br />equations in their historical order of development <br />inciude the Green.Ampt equation (1911), the <br />Kostiakov equation (1932), the Horton equation <br />(1940), the Philip equation (1957a), and the Holtan <br />equation (1961). All of these parametric equations <br />will be discussed at some length later in this report. <br /> <br />11 <br />