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<br />LITERATURE REVIEW
<br />
<br />Rain infiltration is a process of major im-
<br />portance in the hydrologic cycle. The importance of
<br />rain infiltration has been recognized for several
<br />decades, as is evidenced by the detailed studies of
<br />Wollny in Germany as early as 1874 (Baver, 1938), In
<br />the United States, extensive research on infiltration
<br />was undertaken during the 1930's when soil and
<br />water conservation became a matter of national
<br />concern. Hydrologists, interested in the prediction of
<br />surface runoff from watersheds, sought quantitative
<br />estimates of water intake rates of soils over a wide
<br />range of cover and soil conditions.
<br />
<br />Ramser (1927) began his pioneering studies on
<br />small mixed-cover watersheds ranging in size from
<br />1.12 to 1.25 acres. His prime objective was to
<br />determine the values of C, the coefficient in the
<br />so.called rational formula (Mulvaney, 1851; Kuich-
<br />ling, 1889; L1oyd.Davis, 1906) for computing the
<br />maximum rate of runoff from an agricultural water-
<br />shed, Ramser's (1927) data were the first to show on
<br />the basis of direct field measurements that there were
<br />many interdependent factors influencing runoff from
<br />watersheds, such as the size and shape of the
<br />watershed, surface slope, nature and amount of
<br />vegetation, character of the soil regarding perme-
<br />ability, drainage channels, evaporation, storage and
<br />underground conditions, and the duration and inten-
<br />sity of rainfall, Hydrologists are still trying to
<br />determine the full significance of most of the factors
<br />influencing runofL Among them, infiltration that is
<br />the entry into soil of water through its soil-
<br />atmosphere interface (Rose, 1966), plays one of the
<br />most important roles in the rainfall-runoff process.
<br />Historical developments in infiltration studies includ-
<br />ing concepts, factors affecting infiltration, charac-
<br />teristics, and modeling are reviewed herein. For
<br />convenience, all the previous findings and results are
<br />presented without elaborating justification of their
<br />accuracies.
<br />
<br />Early Concepts on Inmtration
<br />and Mechanism
<br />
<br />Horton (1933, 1940) defined infiltration cap-
<br />acity as the maximum rate at which the soil, when in
<br />a given condition, can absorb falling rain. The term
<br />"capacity" in this connection has no relation to total
<br />volume absorbed but is a limiting infiltration rate.
<br />Opposition to using this confusing terminology has
<br />
<br />been seen in latcr publications with different names
<br />being used such as infiltration rate (Richards, 1952),
<br />potential infiltration rate (Smith, 1944), and in.
<br />filtrabiIity (Hillel, 1971; Swartzendruber and Hillel,
<br />1973), Horton (1933. 1940) also stated that the rate
<br />of infiltration is the actual rate at which the rainfall
<br />can be absorbed into a given soil under a given
<br />cnndition, Fletcher (1949) considered infiltration as
<br />the total amount of water entering the soil from the
<br />time of its addition to the end of the first hour.
<br />Richards (1952) also defined the infiltration rate of
<br />soil as the maximum rate at which a soil, in a given
<br />condition at a given time, can absorb water applied in
<br />excess to the soil surface. either as rainfall or shallow
<br />impounded water. Quantitatively, inf1ltration rate,
<br />defined as the volume of water passing into the soil
<br />per unit area per unit time, has the dimension of
<br />velocity.
<br />
<br />Turner (1963), Musgrave and Holtan (1964),
<br />and Hermanson (1970) among many other previous
<br />investigators considered innttration as a three-step
<br />sequence: (1) surface entry, (2) transmission through
<br />the soil, and (3) depletion of storage capacity in the
<br />soil. They have found that after saturation, the rate
<br />of infiltration is limited to the lowest transmission
<br />rate encountered in the saturated profile by the
<br />infiltrating water, and that if the surface entry rate is
<br />slower than transmission capacity of any horizon,
<br />infiltration is limited to the surface entry rate
<br />throughout an entire storm.
<br />
<br />Colman and Bodman (1944) distinguished five
<br />zones in the soil during infiltration; (I) saturated
<br />zone (a zone reaching a depth of about 1.5 em), (2)
<br />transition zone (a zone of about 5 em in which a
<br />rapid decrease in moisture occurs), (3) transmission
<br />zone (a zone in which moisture content is nearly
<br />constant), (4) wetting zone (a zone of fairly rapid
<br />change in moisture content), and (5) wetting front (a
<br />zone of very steep moisture gradient which shows a
<br />visible limit of moisture penetration into the soil).
<br />
<br />Factors Affecting Infiltration
<br />
<br />Factors affecting infiltration have been studied
<br />by Baver (1933), Lewis and Powers (1938), Horton
<br />(1940), Fletcher (1949), Diebold (1951), Musgrave
<br />(1955), Miller and Gardner (1962), Lull (1964),
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