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

Infiltration Characteristics

In the early years of his infiltration studies,
Horton (1935) assumed that infiltration capacity
might be satisfactorily approximated as having a
constant uniform value during the first hour or two
of a precipitation period. Since then the importance
of the role the shape of the early segment of
infIltration capacity curves plays in the calculation of
runoff from a watershed, particularly a small area
under intense precipitation, has been widely recog-
nized by most investigators. Many methods have been
developed to derive infiltration capacity curves from
oa'tural or artificial rainfall (Musgrave and Holtan,
1964), There have been the detcntion.flow-
relationship method (Hydrology, SCS, 1969; Sharp
and Holtan, 1940 and 1942), the time-condensation
method (Holtan, 1945), and the block method
(Horner and Lloyd, 1940; Sherman, 1940; Sherman
and Mayer, 194]). Infiltration capacity curves so
determined all have characteristically similar shapes,
their values being relatively high in the beginning of
precipitation, decreasing rapidly as precipitation con-
tinues, and tending to reach rather definite minimum
values, for a particular precipitation period. The use
of such characteristic curves represented a long
advance over the earlier conception of uniform
infiltration capacity,

The results of extensive research on infiltration
capacity conducted by Horner and Jens (1942)
indicated that: (1) Infiltration capacity varied little
with surface slope; (2) it probably varied materially
with soil porosity and soil moisture, possibly with soil
moisture deficiency below field capacity; (3) it might
change rapidly with an alteration of soil surface
condition such as might occur under the puddling
action of rain impact, or under erosion and in-
working of fines where the soil is not protected by
good vegetal cover; (4) it might be quite different for
bare cultivated soils as compared with grass or other
good vegetal cover; and (5) for bare soils it might vary
with precipitation intensity, but under good vegetal
cover it was relatively independent of intensity. From
Horner and Jens' research results, basic infIltration
capacity curves might be selected so that they would
be satisfactorily representative of any particular
combination of soil and cover under specific seasonal
conditions.

Every soil and cover complex has a related
characteristic curve of decreasing infiltration capacity
during a precipitation period. As a rule, infIltration
capacity of a given soil passes through a cycle from
storm to storm, If the character of the soil and its
moisture history are known for a time preceding a
given rain, the infiltration capacity which it will have
at the time of rain can, in general, be closely
predicted (Horton, 1935), Suffice it to say that each
soH and cover complex has a unique infiltration-

capacity curve, and that the values of infiltration
capacity will follow a definite decay curve during a
period of precipitation where rainfall intensities are in
excess of infiltration capacity and adjust to a some-
what modified curve during the period of precipita-
tion where intensities are less than infiltration
capacity (Horner, 1944),

Although, for any soil, cover, and seasonal
condition, the curve representing the decay of in-
filtration capacity appears to have a quite definite
form under continuous excess rainfall, the appearance
of infiltration capacity during a particular precipita-
tion period may also vary with the initial
(antecedent) soil moisture and with intermittent or
varying precipitation. Adjustment of infiltration capa-
city to antecedent conditions and precipitation pat-
tern must be in order. However, with the present
knowledge in the mechanics of infiltration, there is
no definite rule that can be followed in adjusting
initial and continuing infiltration capacities for a
range of soils and covers in question.

For selection of an infiltration capacity curve
representative of any soil and cover condition under
an average antecedent condition, the use of a
standard infiltration-capacity curve is becoming quite
a common practice (Musgrave and Holtan, 1964).
Curves derived from analyses of a number of storms
on single-practice watersheds are used in arriving at
standard curves of infiltration capacity, Jens (1948)
derived infiltration capacity curves for wet and
normal antecedent conditions of turf areas. These
curves may be accepted as reasonably representative
of the infiltration capacity curve for a turfed cover
for a rather wide range of clay subsoils. In view of the
little artificial compaction that would occur from
trampling of the surface under intensive recreational
use or from walking over city lawns, representative
curves for city lawns would be slightly lower than
those derived by Jens (1948), For sandy loams or
sands, infiltration capacity would be materially
higher, Holtan and Kirkpatrick (1950) derived three
typical standard infiltration curves for hay, grain, and
bare soil, respectively, on certain soils of the Pied.
mont.

For all practical purposes, three factors (soils,
vegetation, and antecedent soil moisture) may be
used as bases for grouping infiltration capacity and
hence the rainfall.runoff relationship within each
soil-cover-moisture complex, The result should be a
family of curves representing infiltration capacity and
hence rainfall-runoff relationships for the various
complexes (Musgrave and Holtan, 1964).

Soil

Musgrave (1955) grouped soils in accordance
with their infiltration capacity, after a period of

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