FLOOD05499

• County: Statewide
• Community: State of Colorado
• Basin: Statewide
• Title: Urban Storm Runoff Inlet Hydrograph Study - Volume 5
• Date: 5/1/1975
• Prepared By: Cheng-lung Chen
• Floodplain - Doc Type: Flood Mitigation/Flood Warning/Watershed Restoration

Musgrave and Holtan (1964), Powell and Beasley
(1967), Gifford (1968), and Hermanson (1970)
among many other investigators. An extensive review
of previous investigations reveals that all the studies
reported have considered many common factors that
affect infiltration and hence runoff. Some of the
most common factors are soil properties (including
grain size and its distribution, porosity, compactness,
etc.), plant cover, slope, degree of aggregation of soH,
antecedent soil moisture, and rainfall intensity and
duration. In the following paragraphs, some of these
factors are reviewed in terms of previous investiga-
tors' findings and results.

Effect of antecedent moisture

The rate of infiltration into a soil is at a
maximum when a soil is fairly dry, for after water is
added, the pore space becomes full of water and the
infiltration rate of additional water declines to a low
but uniform level. Neal (1938), in a laboratory study
of the effects of degree of slope, .surface conditions,
and rainfall intensity on infiltration, found that initial
soil moisture content had a greater effect on the rate
of infiltration during the first 20 minutes of sim-
ulated rainfall than any other factors, Other workers
(Arend and Horton, 1943; Free et aI., 1940) obtained
similar results in field measurements with infiltro-
meters, Hansen (1955) in the study of surface
irrigation, has concluded that in spite of the fact that
the rate of entry in moist soils is less than in drier
soils, the wetting front advances more rapidly when
the soil is wet than when it is dry, Powell and Beasley
(1967) have also found that the antecedent soil
moisture has its greatest effect on infiltration when
the soil is bare, This is expected, since the beating
action of rain and sorting of particles to fill the pores
is greatest where the soil is bare. Meeuwig (1970)
concluded that the initial moisture content exerts a
significant effect on the amount of water retained.
Retained water decreases about 0.1 inch for each
0,05 inch in initial moisture content of the surface 2
inches of soil. These findings are, in general, in
agreement with previous studies conducted by
Brakensiek and Frevert (1961), Jamison and Thorn.
ton (1961), and Thames and Ursie (1960), Green
(1962) found a significant effect of antecedent
moisture content on infiltration under field condi-
tions, In addition, he found that the rate of advance
of the wetting front is proportional to the antecedent
moisture level, and concluded from field infiltration
rates measured on two different soils that in some
cases, antecedent moisture differences in a given soil
may influence infiltration rates as much as tillage,
surface sealing. or profile differences between soils.

Effect of soil texture and structure

Horton (1933) recognized maximum and min.
imum infiltration rates of a soil. The maximum
infiltration rate for a given rain occurs at the

beginning of the rain, He has indicated that the
infiltration rate decreases rapidly because of changes
in the structure of the surface soil and increases in
soil moisture, and then gradually approaches to a
somewhat stable minimum. Powell and Beasley
(1967) have reasoned that when the soil is dry, the
high initial infiltration rate is primarily the result of
the filling of the pore spaces larger than capillary size,
Once these pores are filled, the infiltration is due
mainly to the advance of water by capillary potential.

Musgrave and Holtan (1964) indicated that the
soil characteristics affecting infiltration most were
largely ones in relation with pore size and pore-size
distribution as well as their relative stability during
storms. In sands the pores are relatively stable,
while soils with appreciable amounts of clay are
subject during a storm to the disintegration of
the surface crumbs or aggregates which in their
dry state may provide relatively large pores but
swell appreciably upon wetting. During a storm,
sands may slowly rearrange themselves into a more
dense mix than formerly, whereas in silts and
clays a breakdown of soil crumbs and a melting of
aggregates take place upon the impact of raindrops,
causing the very small particles of silt and clay to
move across the surface and penetrate previously
existing pores, thus clogging them and greatly reduc-
ing infiltration, The degree of swelling that occurs in
soil also is affected by the content and kind of clay
minerals present.

For certain soils which develop large cracks or
sun checks when dry, it seems more convenient to
treat infiltration as the disappearance of water below
the general ground surface, This results in including,
as part of innItration capacity, the rate at which
surface water runs into open cracks. The effect of this
inclusion produces relatively high infiltration.capacity
rates at the beginning of the storm, decreasing sharply
when the cracks are filled or become closed (Horner,
1944),

Rauzi and Fly (1968) have found that the
ability to absorb rainfall may have been impaired,
even on highly permeable sand range sites, where
poor structure (i.e.. poor arrangement of the soil
particles with respect to functions as determined by
the capacity of the soil to hold water and the
movement of water through soils) has been developed
either by excessive trampling by livestock, by sealing
of surface pores through splash erosion, or by
deposition of wind or water carrying sediments.
Unfavorable surface soil conditions markedly reduce
water intake rates. In addition, they have found that
soils with compact or blocky clay subsoils and clay or
clayey soils affected by alkali, clay soils having good
structure and no alkali, clay soils of poor structure,
and alkali or saline-alkali soils all have low intake rate,
Clay soils with good structure take water at rates

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