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<br />prolonged wetting, The array of these soils arranged
<br />in the order of the minimum infiltration rates
<br />was derived by Musgrave mostly from analyses
<br />of runoff hydrographs, It is tentative, subject to
<br />revision or verification by further testing. Also noted
<br />is the fact that Musgrave's grouping of soils (A, B, C,
<br />and D) is somewhat different from the SCS hydro-
<br />logic soil groups (Hydrology, 1969; Ogorsky and
<br />Mockus, 1964) to which no ranges of quantitative
<br />final infiltration values are given. A thorough review
<br />of soil classifications (see Appendix A) reveals that
<br />the SCS hydrologic soil group classification, if supple-
<br />mented by the catena concept (Chiang, 1971), would
<br />probably give the most practical soil array for use as a
<br />basis in the derivations of standard infiltration capa-
<br />city curves.
<br />
<br />The SCS hydrologic soil group classification (A,
<br />B, C, and D) was based on the premise that similar
<br />soils (Le., similar in depth, organic matter content,
<br />structure, and degree of swelling when saturated)
<br />would respond in an essentially similar manner during
<br />a rainstorm having excessive intensities. In applica-
<br />tion, it is cautioned that some of the soils in the table
<br />were classified, for example, under the D group
<br />because of a high water table that creates a drainage
<br />problem, Once these soils are effectively drained,
<br />they can be placed in an alphabetically higher group.
<br />In order to supplement and refine the SCS classifica-
<br />tion, Chiang (1971) suggested a rating table using the
<br />catena concept. The Chiang rating table allows for an
<br />intermediate class between each of the four groups
<br />classified by SCS, The rating was given according to
<br />internal drainage, depth, and texture of the soil, as
<br />well as subsurface soil conditions.
<br />
<br />Cover
<br />
<br />Detailed information about the vegetative cov-
<br />er, such as plant density and height, root density
<br />and depth, extent of plant cover, and extent and
<br />amount of litter, is seldom available, Therefore,
<br />data on the effect of vegetative cover on the
<br />inf1ltration capacities of various soils may rely on the
<br />land use as an index of cover conditions, The SCS
<br />(Hydrology, 1969) listed various land-use practices in
<br />the estimated order of their influence upon the
<br />inf1ltration capacities of various soils. The order is
<br />that indicated by analyses of hydrographs from plots
<br />and single-practice watersheds and by infiltrometer
<br />tests,
<br />
<br />Antecedent soil moisture
<br />
<br />The four points of soil-water equilibrium,
<br />i.e., saturation, field capacity, wilting point, and
<br />hygroscopic moisture were suggested for use in
<br />moisture classification to determine their effects
<br />on innItration capacity (Musgrave and Holtan, 1964),
<br />Many schemes have been devised for estimating
<br />
<br />the antecedent moisture status relative to these
<br />points of equilibrium. For practical purposes, how~
<br />ever, the index of watershed wetness used in con-
<br />nection with the SCS runoff estimation method
<br />(Hydrology, 1969) may be more convenient than
<br />those previously developed, The following three levels
<br />of antecedent moisture condition (AMC) were used in
<br />the SCS method:
<br />
<br />AMC-L Lowest runoff potentiaL The watershed
<br />soils are dry enough for satisfactory plowing or
<br />cultivation to take place.
<br />
<br />AMC-II, Average runoff potentiaL
<br />
<br />AMC-IIL Highest runoff potentiaL The water-
<br />shed is practically saturated from antecedent rains.
<br />
<br />Inclusion of this index in the estimation of infiltra-
<br />tion capacities for various soils-cover complexes will
<br />be investigated later.
<br />
<br />Infiltration Modeling
<br />
<br />Infiltration-capacity decay curve (or more re-
<br />cently called infiltrability-time curve by
<br />Swartzendruber and Hillel (1973)) of a given soil.
<br />cover-moisture complex, beginning with a very high
<br />infiltration rate, and eventually approaching a con-
<br />stant non-zero value asymptotically with time, has
<br />been hypothetically portrayed as a solution to a
<br />boundary.value problem of rain infiltration (Philip,
<br />1957a and 1969b; Hanks and Bowers, 1962; Wang
<br />and Lakshminarayana, 1968; Rubin and Steinhardt,
<br />1963; Rubin, 1966b; Braester, 1973; Bruce and
<br />Whisler, 1972; Whisler and Klute, 1965 and 1969),
<br />using a nonlinear form of the Fokker-Planck equation
<br />(Philip, I 969b) as the flow equation for water moving
<br />through a rigid, unsaturated soil, subject to various
<br />initial and boundary conditions of interest. Many of
<br />the concepts leading to the nonlinear Fokker-P\anck
<br />equation were implicit in Buckingham's (1907) mon-
<br />ograph, but Richards (1931) formerly presented the
<br />equation in 1931 (philip, 1969b), It is now well
<br />known as, and for convenience henceforth referred to
<br />as, the Richards equation. Many natural soils contain
<br />swelling elay, which can cause movement of the soil
<br />particles as well as of the water, and produce air
<br />bubbles upon ponding on the soil surface, These
<br />phenomena have made the Richards equation more
<br />difficult to be accepted as the basic flow equation,
<br />For infiltration with counter flow of air, Peck (1965),
<br />Adrian and Franzini (1966), Morel.Seytoux and
<br />Noblanc (1973), and Morel-Seytoux and Khanji
<br />(1974) developed a method of moving strained
<br />coordinates that greatly facilitates the study of
<br />two-fluid systems, For infiltration into deforming
<br />porous media, Smiles and Rosenthal (1968) and
<br />Philip (1969a) did some work in an attempt to derive
<br />the flow equations representing the more realistic
<br />
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