FLOOD04389

• County: Statewide
• Basin: Statewide
• Title: Chapter 4 Graphical Peak Discharge Method
• Date: 6/1/1986
• Floodplain - Doc Type: Educational/Technical/Reference Information

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Chapter 2: Estimating runoff

SCS plmoff Curve Number method

The SCS Runoff Curve Number (CN) method is
described in detail in NEH-4 (SCS 1985). The SCS
runoff f>quation i:;

Q

(P - I.JI
(P - I.) + S

IEq.2-I)

where

Q = runoff (in),
P = rainfall (in),
S = potential maximum retention after runoff
be~.;ns (in), ar.d
I. = initial abstraction (in).

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Initial abstraction (I.) is all losses before runoff
begiw. It includes water retained in sur'~.ce
d~pres..~ion~', water intercepted by veKt'tation,
e\'aporation, and infiltration. I. is hi!(hl:; variable but
Kenerally is correlated with soil and cover
parameters. Throu!(h studies of many .maIl
aj(Iicul'.ur:d watersheds, I. was found to be
appro'dmated b)' tbe 1'ollo,,;ng empirical equation:

I. = 0.28.

IEq.2.2)

By temoving I. as an ir,dependent parameter, this
approximation allows u.. of a combination o( Sand P
to prod'lce a unique runoff amolint. Substitutin!(
equation 2.2 into equation 2.1 gives

Q = (P - 0.28)2
(P + O.SS)

[Eq. 2-3J

S is related to the ""il and cover conditions o( the
watershed throu!(h the CN CN has a ranJ(e o( 0 to
lOll, and S is related to CN by

s = 1000 - 10. IE 2 ')
''1. ...
CN
Fil(\lre 2.1 and table 2.1 solve equations 2.3 and 24
for a ran!(e of CN'. and rainfall.

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t

Factors considered in detennining
runoff curve numbers

The mlljor factors that detennir.e CN are the
hydrologic soil group (HSG), cover type, treatment,
hydrologic condition, and antecedent runoff condition
(ARC). Another factor e>>n..idercJ ;,. whether
impervious areas outlet directly to the drainage
systelT. (connected) or whether the flow spreads over
pervious areas before entering the drainage sYPte'h
(uncor.nected). Figure 2.2 is provided to aid in
selecting the appropria~,' figure or table (or
detenninin!( cur'e numbers.

I
f
!
I
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CN's in t.al>le 2.2 (n '0 Ii) represent average
antecedent runoff condition for urban, cultivated
agricultural, other agricultl1J'al, and aricl and semiarid
rangeland uses. Table 2.;1 assumes impervious areas
are directly collnected. T~e following sections explain
how to ,Ietennine eN'. and how to modify them (or
urban conditions.

Hydrologic lOil J{rolIp.

Infiltration rate. of ""il. vary widely and Ill.. afi'ected
by .ubsurface pennt:ability as well as 8urfact' ;ntake
rates. Soil. are classified into four HSG's (A, 3, C,
and D) according to their minimum infiltration rate.
which is obtained for bare soil after prolol1glOd
wetting. Appendix A defines tbe (our group' and
provide. a list of mo.~ o( the soil. in the United
States and their group clasaification. The soils in the
area o( interest may be identified (rum a soil survey
report, which can be obtained (rom local SCS ofr1coa
:;r ""il and water conservation district ofr1cea.

t

;

f

Most urban areas are only partially covered by
impervious .urfaceR: the .oil remain. an important
factor in runoff e.timates. Urbanization has a greater.
effect on runoff in watersheds with soils having high
infiltration rates (p'm"" and j(l'avel.) tban in
watersheds predominantly of silts and clay., which
ger.erally have low infiltration rates.

Any disturbance of a soil profile can .ignificantly
change illl infilt....tion characteristics. With
urbanization, native soil profiles may be mixed or
removed or fill material from other areas may be
introduced. Therefore, a method based on lIOil

(21()' VI. TR.55. Second Ed., June 1986)

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