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

sod. an average of 8.9 inches of water entered the
ground in 2 hours as compared with 7.5 inches under
mesquite and 3.5 inches under bare soil.

There are other studies dealing with the effect
of plant cover on infiltration. Some results were not
very clear-cut and conclusive because other factors
were also taken into consideration in their studies,
whereas others analyzed data statistically without
giving a due account of their results. It is not
intended to elaborate a review of such studies herein,
but those who are interested in the subject may refer
to, for example, Bertoni, Larson, and Shrader (1958),
Smith and Leopold (1942), Woodward (1943),
Osborn (1952), Hanks aod Anderson (1957),
Meeuwig (1970), and Fletcher (1960).

In the preceding review, the kind of cover was
mainly stressed, but none of the studies compared
different cover densities on the same soil as it is
affecting infiltration,

Effect of rainfall

Linsley, KoWer, and Paulhus (1949) have reo
ported that rainfall intensity has little effect on the
rate of infiltration when it exceeds the capacity rate.
This agrees with the findings of Schreiber and Kincaid
(1967), but disagrees with those of Fletcher (1960),
Willis (1965) has found that the infiltration rate of a
bare soil was reduced by an increase in kinetic energy
of rainfall which is a function of the velocity of
impact of raindrops and of the rainfall intensity,
However, Duley and Kelley (1939) observed no
significant difference in either total intake or infIltra.
tion rate, although there is a difference in the rate of
application of water which materially exceeded the
rate of intake. Local experimentation on the variation
of infiltration capacity with rainfall intensity showed
predominant variation for bare soil, as noted by
Horner and Jens (1942), and a lesser amount of
variation for sodded areas.

Duley and Kelley (1939) also found that when
the rate of application of water was sufficient to give
runoff, a fairly definite amount of water entered the
soil and any amount of application in excess of this
intake appeared in the runoff. During the progress of
their tests on cultivated plots of four soil types with
different slopes and different rates of application, it
became increasingly clear that rainfall and its related
factors were exerting only a minor influence on the
intake rate.

Duley (1939) observed that the rapid reduction
in the rate of intake by cultivated soils, as rain
continuously fell on the soil surface, was
accompanied by the formation of a thin, compact
layer at the soil surface, and that the water was able

to pass through this layer very slowly, He postulated
that this thin, compact surface layer was apparently
the result of severe structural disturbance due in part
to the beating effects of the raindrops, and in part, to
an assorting action, as water flowed on the soil
surface, fitting fine particles around the larger ones to
form a relatively impervious seal. His data showed
that this thin, compact layer had a greater effect on
intake of water than the soil type, slope, moisture
content, or soil profile characteristics. Later, Duley
and Kelley (1939) successfully prevented the forma.
tion of this semi-impervious layer, often a few
millimeters thick, by breaking down the soil structure
by the impact of raindrops on the soil surface, using a
cover of straw or a growing crop.

Ellison (1950) in his study of soil erosion by
rainstorms has reported that raindrops working
through the splash (impact plus spatter) process break
down clods and crumbs of soil and' compact these
broken materials. The inflow of surface water made
muddy by splash further seals surface cracks and
pores, and tends to waterproof the soil surface. Tests
on open ranges showed that with good grass cover
only about a ton of soil per acre was splashed and the
.water intake was 2.66 inches during a 15 minute
period. On other areas where there was less forage,
the splash tended to increase and water intake tended
to decrease with reduction in vegetal cover. Finally,
on bare areas where there was no cover at all, 70 tons
per acre of soil were splashed and water intake was
reduced to 0,10 inches in 15 minutes.

Green (1962) has also concluded that surface
sealing diminishes the effect of antecedent moisture
on infiltration because the hydraulic conductivity of
the immediate soil surface controls water flow into
the soil and surface sealing does not allow suction
gradients to control the rate of infiltration,

Duley and Domingo (1949) found that on an
area affected by overflow deposits and trampling of
animals, intake rate on bluegrass land was reduced to
a very low point. Le., from a normal 2.02 inches per
hour down to 0.14 inches per hour under dry
conditions and from a normal of 0.85 inches per hour
to 0.13 inches per hour under wet conditions,

Effect of soil-surface slope

Duley and Kelley (1939) on four soils tested
four different slopes, 2 percent, 4 percent, 6 percent,
and 10 percent. They noted that there was a
tendency for the amount of water intake to decrease
slightly with increase in slope, The greatest intake was
found on the gentlest slope, particularly 2 percent
slope or less. Their observations that the degree of
slope has only a slight effect on infiltration were
reported to be in line with earlier investigations.

8