SCS
CURVE
NUMBE
R
METHO
D
R O W E L L S I N S AY
Introduction
SCS-CN method, developed by Soil Conservation
Services (SCS) of USA in 1969, is a simple,
predictable, and stable conceptual method for
estimation of direct runoff depth based on storm
rainfall depth. It relies on only one parameter,
CN. Currently, it is a well-established method,
having been widely accepted for use in USA and
many other countries.
SCS-CN
method
Estimating
R u n o f f Vo l u m e
The SCS-CN method is based on the water balance equation of the rainfall in a
known interval of time Δt, which can be expressed as
P = Ia + F + Q
(5.17)
Where:
P = total precipitation
Ia = initial abstraction
F = Cumulative infiltration ex-cluding Ia
Q = direct surface runoff (all in units of volume occurring in time Δt).
Two other concepts as below are also used with Eq.(5.17).
• The first concept is that the ratio of actual amount of direct
runoff (Q) to maximum potential runoff (= P-Ia) is equal to the
ratio of actual infiltration (F) to the potential maximum retention
(or infiltration), S. This proportionality concept can be
schematically shown as in Fig. 5.7
Thus
(5.18)
• The second concept is that the amount of initial abstraction (Ia) is some
fraction of the potential maximum retention (S).
Thus
(5.19)
Combining Eqs. (5.18) and (5.19), and using (5.17)
(5.20a)
Further
(5.20b)
For operation purposes a time interval Δt = 1 day is adopted. Thus P = daily
rainfall and Q = daily runoff from the catchment.
Curve Number (CN) The parameter S representing the potential maximum retention depends upon
the soil-vegetation-land use complex of the catchment and also upon the antecedent soil moisture
condition in the catchment just prior to the commencement of the rainfall event. For convenience in
practical application the Soil Conservation Services (SCS) of USA has expressed S (in mm) in terms of
a dimensionless parameter CN (the Curve number) as
(5.21)
The constant 254 is used to express S in mm.
The curve number CN is now related to S as
(5.22)
and has a range of 100 ≥ CN ≥ 0. A CN value of 100 represents a condition of zero potential
retention (i.e. impervious catchment) and CN = 0 represents an infinitely abstracting catchment
with S = ∞. This curve number CN depends upon
• Soil type
• Land use/cover
• Antecedent moisture condition
Soils In the determination of CN, the hydrological soil classification is adopted. Here,
soils are classified into four classes A, B, C and D based upon the infiltration and
other characteristics. The important soil characteristics that influence hydrological
classification of soils are effective depth of soil, average clay content, infiltration
characteristics and permeability. Following is a brief description of four hydrologic
soil groups:
 Group-A: (Low Runoff Potential): Soils having high infiltration rates even when
thoroughly wetted and consisting chiefly of deep, well to excessively drained
sands or gravels. These soils have high rate of water transmission. [Example:
Deep sand, Deep loess and Aggregated silt]
 Group-B: (Moderately Low runoff Potential): Soils having moderate infiltration
rates when thoroughly wetted and consisting chiefly of moderately deep to
deep, moderately well to well-drained soils with moderately fine to moderately
coarse textures. These soils have moderate rate of water transmission. [Example:
Shallow loess, Sandy loam, Red loamy soil, Red sandy loam and Red sandy soil]
 Group-C: (Moderately High Runoff Potential): Soils having low infiltration rates when
thoroughly wetted and consisting chiefly of moderately deep to deep, moderately well
to well drained soils with moderately fine to moderately coarse textures. These soils
have moderate rate of water transmission. [Example: Clayey loam, Shallow sandy
loam, Soils usually high in clay, Mixed red and black soils]
 Group-D: (High Runoff Potential): Soils having very low infiltration rates when
thoroughly wetted and consisting chiefly of clay soils with a high swelling potential,
soils with a permanent high-water table, soils with a clay pan, or clay layer at or near
the surface, and shallow soils over nearly impervious material.
[Example: Heavy plastic clays, certain saline soils and deep black soils].
Antecedent Moisture Condition (AMC) Antecedent Moisture Condition (AMC) refers to
the moisture content present in the soil at the beginning of the rainfall-runoff event
under consideration. It is well known that initial abstraction and infiltration are
governed by AMC. For purposes of practical application three levels of AMC are
recognized by SCS as follows:
AMC-I:
place.
AMC-II:
Soils are dry but not to wilting point. Satisfactory cultivation has taken
Average conditions
AMC-III:
Sufficient rainfall has occurred within the immediate past 5 days.
Saturated soil conditions prevail.
The limits of these three AMC classes, based on total rainfall magnitude in the
previous 5 days, are given in Table 5.5. It is to be noted that the limits also depend upon
the seasons: two seasons, viz. growing season and dormant season are considered.
Table 5.5 Antecedent Moisture Conditions (AMC) for Determining the Value of CN
Land Use The variation of CN under AMC-II, called CNII, for various land use conditions
commonly found in practice are shown in Table 5.6(a, b and c).
Table 5.6(a) Runoff Curve Numbers [CNII] for Hydrologic Soil Cover Complexes [Under AMC-II
Conditions]
Note: Sugarcane has a separate
supplementary Table of CNII values
(Table 5.6(b)). The conversion of CNII to
other two AMC conditions can be made
through the use of following correlation
equations.10
For AMC-I:
(5.23)
Table 5.6(b) CNII Values for Sugarcane
Table 5.6(c) CNII Values for Suburban and Urban Land Uses (Ref. 3)
For AMC-III:
(5.24)
The equations (5.23) and (5.24) are applicable in the CNII, range of 55 to 95 which covers
most of the practical range.
Value of λ On the basis of extensive measurements in small size catchments SCS (1985) adopted λ= 0.2
as a standard value. With this Eq. (5.20-a) becomes
(5.25)
Where:
Q = daily runoff
P = daily rainfall
S = retention parameter
all in units of mm. Equation 5.25, which is well established, is called as the
Standard SCS-CN equation.
SCS-CN Equation for Indian Conditions Values of λ varying in the range 0.1 ≤ λ ≤ 0.4 have
been documented in a number of studies from various geographical locations, which
include USA and many other countries. For use in Indian conditions λ = 0.1 and 0.3 subject
to certain constraints of soil type and AMC type has been recommended (Ref. 7) as below:
Example:
Solution:
Example:
Solution:
Example:
Solution:
Example:
Solution:
Example:
Solution:
THANK
YOU
2/1/20XX
PRESENTATION TITLE
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