SURFACE RUNOFF ESTIMATION USING SCS- CN METHOD AND … · Chow V T, Maidment D K, Mays L W, 2002. Applied Hydrology, Publisher McGraw Hills, New York, USA. 4. Dhvani Tailor, Narendra

SURFACE RUNOFF ESTIMATION USING SCS- CN METHOD AND GIS TOOL

FOR THE PALAR WATERSHED, KARNATAKA

Amritha Thankachan1, Mohammed Badiuddin Parvez2, Chalapathi K3, M. Inayathulla4,

Chithaiah.E5

1,2,3 Research Scholar, Department of Civil Engineering, UVCE, Bangalore University

4 Professor, Department of Civil Engineering, UVCE, Bangalore University

5Guest Faculty, Department of Civil Engineering, UVCE, Bangalore University

Abstract:

Adequate knowledge about the hydrology is very much required for the proper planning

and management of water resources in an area. Rainfall and runoff are the important

constituents in determining the hydrology of an area to determine the water management

strategies. SCS- CN method is a widely used method for the calculation of surface runoff

considering the land use pattern, soil type and antecedent moisture condition. In the present

study runoff of the Palar watershed, Karnataka state, South India has been calculated using the

SCS-CN method. The watershed consists of a total area of 2872.357 km2. The maximum

rainfall of 1231.67 mm in the year 2005 and a minimum of 418.7 mm in the year 2003. The

average annual runoff is calculated as 218.26 mm and 626.91MCM. The rainfall- runoff

correlation value is 0.8253. The study results can be effectively coordinated for the watershed

management activities.

Key Words: Antecedent Moisture Condition, Hydrological Soil Group, Runoff, SCS- CN

method

Introduction:

A watershed is defined as an area of land that feds all the water running under it and

draining off into a waterbody. Rainfall and runoff are important processes involved in

hydrologic cycle. Rainfall is the common form of the precipitation in which liquid water is

received from the condensed atmospheric water vapour. Where ever rainfall is involved in that

region’s hydrologic cycle, it serves major role not only to serve the needs of the humanity, but

also for the existence of the life.

Surface runoff is the term used to mention the flow of excess water through the surface

of earth. Runoff depends on many factors like meteorological factors, physical characteristic

of the basin etc. in which rainfall and its characteristics have major influence. Also it is very

much important to study how the runoff varies with rainfall under the prevailing conditions as

rainfall- runoff studies are very essential for the construction of hydraulic structures, watershed

management, flood forecasting, water harvesting and also for any hydrologic studies.

Soil Conservation Services Curve Number method abbreviated as SCS-CN method is

a widely accepted and used method for the calculation of runoff. This method was developed

Compliance Engineering Journal

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by United Stated Department of Agriculture and Natural Resources Conservation Services. It

is also called as CN method. It is mainly based on the concept of water balance.

The major hypothesis behind SCS-CN method are the ratio of ‘Q’ and (P-Ia) is equal to

the ratio of ‘F’ and ‘S’; where ‘Q’ is the actual runoff, (P-Ia) is the potential runoff, ‘P’ is

rainfall, ‘Ia’ is the initial abstractions, ‘F’ is the cumulative infiltration excluding Ia and ‘S’ is

the potential infiltration and the amount of initial abstraction is some fraction of potential

infiltration.

Description of the study area:

Palar is a river which originates in Nandi Hills in Chikkaballapura , Karnataka. It pours

out to Bay of Bangal at Vayalur, Tamil Nadu after flowing a distance of 348 km through

Karnataka, Andra Pradesh and Tamil Nadu. It has seven tributaries. The study area situates

between latitude 12051’39’’ and 13028’08’’ and longitude 77052’47’’ and 78034’53’’. It is having

a total area of 2872.357 km2.

Fig. 1. Location of study area

Methodology:

The methodology adopted for the study is expressed by the flow diagram. The data

required for the study is collected from different centres eg. LU and LC map and soil map from



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KSRSAC, rainfall data from the year 1986 to 2015 from IMD, SCS-CN method is adopted for

the calculation of runoff.

In SCS-CN method, boundary of the watershed, soil map and land cover and land use

map and rainfall data serve as major inputs. Curve numbers are assigned to the different land

use classes based on the soil groups. Soil is grouped into hydrological soil groups A, B, C and

D based on their infiltration capacity. Rainfall data from the six rain gauge stations are used

for the study. CN I and CN III are calculated from the obtained CN II value based on the

antecedent moisture conditions using the equations. Potential maximum retention ‘S’ is

calculated from the equation:

Runoff ‘Q’ is calculated from rainfall ‘P’ and ‘S’ value from the equation.

Fig. 2 . Methodology Adopted for the study



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Fig. 3. Curve Number Map

Fig. 4. Theissen polygon Map

Table. 1. Hydrological Soil group classification



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Table. 2. Antecedent Moisture Condition

Results and Discussions:

The Palar river basin had an average rainfall of 776.31 mm. The rainfall ranges from

418.7 mm to 1231.67 mm in 2003 and 2005 respectively and the runoff varies from 96.32 mm

in 2003 and 398.37 mm in 2005. The annual rainfall and runoff is listed in the table. The

average annual runoff is found to be 218.26 mm and 626.91 MCM. The rainfall- runoff

correlation is shown in the figure and correlation value is observed as 0.8253.

Table. 4. Runoff estimation

Year

Rainfall

(mm)

Runoff

(mm)

Rainfall

(MCM)

Runoff

(MCM)

1986 772.30 207.29 2218.32 595.42

1987 676.47 159.97 1943.07 459.49

1988 869.93 282.53 2498.75 811.54

1989 585.58 197.97 1682.00 568.64

1990 669.34 173.23 1922.57 497.58

1991 1088.95 391.45 3127.84 1124.38

1992 738.06 206.45 2119.98 593.00

1993 1031.03 329.00 2961.49 945.01

1994 645.33 175.76 1853.61 504.86

1995 563.95 126.38 1619.87 363.00

1996 968.98 257.57 2783.25 739.83

1997 774.15 194.87 2223.63 559.75



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1998 921.67 353.47 2647.38 1015.28

1999 728.51 221.13 2092.54 635.17

2000 764.97 198.07 2197.27 568.94

2001 1025.30 376.95 2945.04 1082.74

2002 545.90 129.55 1568.02 372.10

2003 418.71 96.32 1202.68 276.68

2004 509.54 138.51 1463.60 397.85

2005 1231.67 398.37 3537.79 1144.27

2006 668.19 156.28 1919.28 448.90

2007 803.52 178.54 2307.99 512.84

2008 1031.78 293.44 2963.64 842.85

2009 805.18 215.48 2312.77 618.94

2010 858.92 188.99 2467.14 542.84

2011 713.80 146.33 2050.28 420.32

2012 700.40 219.05 2011.79 629.18

2013 653.73 139.11 1877.76 399.58

2014 585.92 160.17 1682.98 460.07

2015 937.61 235.47 2693.16 676.36

Fig. 5. Rainfall- runoff in mm.



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Fig. 6. Rainfall- runoff in MCM

Fig. 7. Rainfall- runoff correlation

Conclusion:

The SCS-CN method works basically on the principle of water balance and this method

is well suited not only for watersheds with similar characteristics but also for ungauged

catchments. A good correlation is found between the rainfall and runoff for the study area. The



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runoff is found to be 28% of the rainfall obtained in the watershed. Even though the method is

designed for a single storm event, it can be scaled to find average annual runoff values. This

study can be effectively useful for the watershed management of Palar watershed.

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Documents

SURFACE RUNOFF ESTIMATION USING SCS- CN METHOD AND … · Chow V T, Maidment D K, Mays L W, 2002. Applied Hydrology, Publisher McGraw Hills, New York, USA. 4. Dhvani Tailor, Narendra