Estimation of Runoff Using NRCS-CN Method: A Study from South India

doi:10.21203/rs.3.rs-1543304/v1

The water resource on earth surface is circulated and attains different forms; of which runoff is a major component. The runoff in any area depends on the amount of precipitation, terrain, soil type and characteristic, slope, land use activity in that region and many other location specific factors. There are several methodologies adopted world wide to estimate the runoff; one important and widely used techniques is the NRCS-CN method. This method uses precipitation, soil group and land use as the major input factors. The present study is carried out in a watershed, named the Vannathangarai watershed, a part of Noyyal river basin, which is a tributary of River Cauvery, a major river draining through southern India. The study reveals that runoff is positively correlated with precipitation and is very high in areas with slope

NRCS-CN

Hydrological Soil Groups

Antecedent Moisture Conditions.

The rain that falls on the earth surface does not fully reach the underground sources, a part of it runs off the surface. The amount of water that percolates inside and that is lost as runoff depends on the local terrain, geologic and geomorphic characteristics, soil condition etc. The portion of precipitation which appears in the surface streams of either perennial or intermittent nature is called the runoff. Runoff is expressed as volume per unit time. It is very essential to estimate the runoff of a region in order to understand the amount that is recharged into the earth’s sub surface layers. Several empirical methods are available for the runoff estimation and a widely used method is the SCS CN method, which was renamed as the NRCS CN method. The present study is based on the Natural Resources Conservation Service (NRCS) Curve Number (CN) method which was devised by the USDA (1986).

The present study concentrates on the Vannathangarai watershed, of River Noyyal, which spreads over the parts of Coimbatore and Tiruppur districts of Tamil Nadu. This watershed includes the head waters of River Noyyal, which originates near the Vellingiri hills. River Noyyal is a right bank tributary of river Cauvery (Kaveri), The shape of the watershed is roughly oval, extending in a South West- North east direction. The study area is underlain by both porous and fissures formations. The major aquifers of the watershed are constituted by unconsolidated formations and weathered and fractured crystalline rocks. The hard consolidated crystalline rocks in this area are represented by weathered and fractured granite gneisses, granite, charnockites etc. In the fractured zones, the occurrence of groundwater is found in the weathered mantle and under semi confined conditions in fractured zones in phreatic conditions (CGWB, 2008). The major drainage feature of this study area is River Noyyal. The study area is endowed with a network of systematic tanks that meets the drinking and irrigation water needs of the area. The whole Noyyal river system has 21 anicuts and 31 tanks for facilitating direct irrigation. Apart from Tanks, there are several drains that flow through the study area.

The present study is carried out using the NRCS- CN methodology. This model uses, antecedent moisture condition, land use and soil type as the major input. For the calculation of runoff, daily rainfall data for 15 rain gauge stations were collected from the Institute for Water Studies, Public Works Department, Government of Tamil Nadu, Chennai, Tamil Nadu. The soil data is derived from the National Bureau of Soil Survey and Land Use Planning. The Land use and Land cover is analysed by remote sensing data. Accordingly, the Runoff is given by

Which is expressed in unit depth spread over the watershed and where Q = runoff in mms, P is Depth of precipitation over the entire watershed, S = maximum potential retention, and I_a is the initial abstraction. The I_a is the lose of water before the runoff begins, and includes the water collected in the depressions and absorbed by vegetation, and lost through evaporation and infiltration. I_a varies with the type of terrain and soil characteristics. USDA (1956) has given out the relationship between I_a and S as

I_a= 0.2S

Rao et al., (2010) has revised this relationship to suit the Indian conditions and has expressed it as

I_a= 0.3S

Substituting this value in the equation, it can be revised as

Where S is related to CN, and is a function of HSG (Hydrological Soil Group) and AMC (Antecedent Moisture Condition). The Curve Number is a dimensionless feature and its value varies from 0 to 100. Patila et al., 2008 derived the value of S as

a. Hydrological Soil Group (HSG)

To understand the properties of infiltration, USDA (1986) has developed a classification system, where the soils are categorized into 4 groups namely A, B, C, and D. Soils were assigned different grouping based on measured rainfall, runoff and infiltrometer data (Musgrave,1955). The classes are based on intake and transmission of water under the conditions of maximum yearly wetness, soil not frozen, bare soil surface and maximum swelling of expensive clays. The following table explains the different soil groups.

Table I: Hydrological Soil Groups and its characteristics

Soil Groups	Soil Characteristics
A	Low runoff potential when thoroughly wet, water transmitted freely, < 10% clay &>90% sand or gravel. Texture: - sand, gravel, loamy sand, sandy loam, loam or silt loam. Saturated hydraulic conductivity exceeds 40.0 micrometers/second, depth to water permeable layer- >50cm, depth to water table > 60cm.
B	Moderately low runoff potential when thoroughly wet. Water transmission unimpeded. Clay between 10% & 20%. Sand between 50% & 90%. Texture: - loamy sand, sandy loam, sandy clay loam. Saturated hydraulic conductivity- between 10.0–40.0 micrometer/second.
C	Moderately high runoff potential when thoroughly wet. Clay between 20% & 40%. Sand < 50%. Texture: - loam, silt loam, sandy clay loam, clay loam and silt clay loam. Saturated hydraulic conductivity- between 1.0–10.0 micrometer/second.
D	High runoff potential when thoroughly wet, water movement restricted or very restricted. Clay > 40% and Sand- < 50%. Saturated hydraulic conductivity less than or equal to 1.0 micrometer / second.
Source : USDA, Hydrology National Engineering Handbook, 2007.

b. Antecedent Moisture Conditions (AMC)

AMC refers to the moisture content present in the soil at a given period of time. It is determined by the amount of total rainfall in 5-day period preceding a rainfall event (McCuen, 1982). Three AMC conditions (AMC I, II & III) are identified by USDA (1986) based on different soil conditions and rainfall limits for dormant and growing seasons. Low, medium and high runoff potentials are respectively indicated by AMC I, II and III conditions.

Table II: Antecedent Moisture Conditions and its corresponding Soil Groups

AMC	Soil Characteristics	5-day total Rainfall (mm)
AMC	Soil Characteristics	Dormant Season	Growing Season
I	Soils dry, but not to wilting point.	< 13	< 36
II	Average moisture condition.	13–28	36–53
III	Heavy or light rainfall and low temperature occurred within last 5 days, soil is saturated.	> 28	> 53
Source: Source: USDA, Hydrology National Engineering Handbook, 2007.

Based on all these the CN value can be estimated for different AMC classes. As a standard rule, these values are first applied to AMC II and then it is extended to AMC I and III.

Accordingly, Rao et al.(2010) revised the form CN formula as follows:

Where, CNi is the curve number for LU-HSG class,

A is the area of each class and

N is the number of LU-HSG polygons.

Table III: Curve Number for corresponding LULC& HSG class of the watershed

LU/LC CLASS	HSG NUMBER
LU/LC CLASS	A	B	C	D
Settlement	48	66	78	83
River and stream	90	94	98	100
Barren/wasteland	64	75	83	85
Crop land	67	78	85	89
Degraded forest	-	72	80	87
Dense forest	-	67	77	83
Fallow land	76	86	90	93
Land with scrub	48	67	77	83
Open forest		79	86	89
Plantation	65	73	79	81
Land without scrub	-	66	77	-
Source: USDA, 1986 & Rao et al., 2010

Land use/ land cover and its changes were analysed for the year 2016. The image used for the LU/LC analysis was obtained from LANDSAT 8 (Operational Land Imager & Thermal Infrared Sensor). The date of the acquired image is 02.03.2016. The errors related to geometry and the brightness of the pixels values were rectified using image enhancement technique. Redistributing the pixel value is done by histogram equalisation, which is a non linear stretching method. This helps in flattening the histogram by increasing the contrast at the peaks and reducing at the tails (Rao et al., 1989). The next step is the classification of the image, by labelling a pixel based on its grey value and grouping them on the basis of their spectral signature. This present study is based on supervised classification done in ERDAS Imagine 2014. The LU/LC classes were derived based on NRSC level 2 classification, with slight modifications to suit the local environment. For the present study, 11 LULC classes have been identified in the watershed, based on the modified NRSC level two classification system. The major land use types found in Vannathangarai watershed are Built up, Dense forest, Degraded Forest and Open Forest, coming under Forest Class, water bodies, land with and without scrub and barren wasteland coming under Waste land class, Plantation, cropland and fallow land under agriculture.

For the estimation of runoff, daily rainfall events for 1 year (2016) of the 15 rain gauge stations were considered. All rainfall events of 5mm and above were considered for this purpose. The weighted curve number for AMC I is 89, AMC II is 78 and AMC III is 60. The total observed rainfall events of all the stations are given below.

Table V: Runoff estimation for various Rain gauge stations using NRCS-CN method- 2016.

Rainfall Date	Daily Rainfall in mms	5 day antecedent rainfall (mm)	AMC	CN	Q (mm)
Station: Agriculture College
09-05-2016	11.20	0.50	I	89	0.10
12-05-2016	32.00	14.30	II	78	1.34
15-05-2016	10.50	34.60	III	60	12.59
09-06-2016	6.40	0.00	I	89	0.32
18-06-2016	6.20	0.00	I	89	0.37
29-06-2016	6.20	2.20	I	89	0.37
10-07-2016	5.40	3.10	I	89	0.59
29-07-2016	6.50	4.50	I	89	0.30
14-10-2016	19.50	0.00	I	89	2.45
31-10-2016	50.20	0.00	I	89	23.04
03-11-2016	16.00	51.70	III	60	9.00
02-12-2016	7.40	0.00	I	89	0.14
03-12-2016	9.60	7.40	I	89	0.00
04-12-2016	18.00	17.00	II	78	0.18
14-12-2016	5.20	0.40	I	89	0.65
Station :Annur
14-04-2016	7.00	0.00	I	89	0.20
07-05-2016	5.00	0.00	I	89	0.72
09-05-2016	12.00	5.00	I	89	0.20
12-05-2016	7.00	17.00	II	78	3.68
13-05-2016	9.00	19.00	II	78	2.64
15-05-2016	5.00	28.00	III	60	16.98
01-06-2016	7.00	0.00	I	89	0.20
02-06-2016	16.00	7.00	I	89	1.14
20-06-2016	5.00	0.00	I	89	0.72
26-07-2016	15.00	0.00	I	89	0.84
21-09-2016	7.00	0.00	I	89	0.20
14-10-2016	12.00	0.00	I	89	0.20
03-12-2016	7.00	1.00	I	89	0.20
Station :Chithiraichavadi Anicut
09-04-2016	7.80	0.00	I	89	0.09
07-05-2016	33.60	0.00	I	89	10.52
09-05-2016	14.20	33.60	III	60	10.09
12-05-2016	18.20	51.60	III	60	7.77
09-06-2016	15.00	0.00	I	89	0.84
23-06-2016	12.40	7.00	I	89	0.26
25-06-2016	7.80	19.00	III	60	14.64
29-06-2016	9.40	9.00	I	89	0.00
07-07-2016	5.40	7.60	I	89	0.59
23-07-2016	5.40	0.60	I	89	0.59
02-12-2016	5.80	0.00	I	89	0.47
03-12-2016	8.80	5.80	I	89	0.01
Station: Coimbatore airport
09-04-2016	41.10	0.00	I	89	15.91
05-05-2016	17.60	2.90	I	89	1.69
15-05-2016	16.80	4.30	I	89	1.41
17-05-2016	7.70	3.78	I	89	0.10
14-10-2016	15.20	0.00	I	89	0.90
31-10-2016	68.60	0.00	I	89	38.67
02-12-2016	8.60	0.00	I	89	0.02
03-12-2016	12.00	8.60	I	89	0.20
04-12-2016	11.90	20.60	II	78	1.48
14-12-2016	7.80	0.30	I	89	0.09
15-12-2016	10.50	8.10	I	89	0.04
Station :Mettupalayam
01-01-2016	0.00	0.00	I	89	4.04
22-01-2016	220.00	0.00	I	89	183.26
23-01-2016	8.20	220.00	III	60	14.32
09-04-2016	11.00	0.00	I	89	0.08
11-05-2016	47.00	6.20	I	89	20.48
12-05-2016	19.40	53.20	III	60	7.15
18-05-2016	10.30	6.40	I	89	0.02
29-05-2016	8.10	0.00	I	89	0.06
18-06-2016	6.10	0.00	I	89	0.39
21-06-2016	5.00	8.30	I	89	0.72
29-06-2016	8.00	4.10	I	89	0.07
17-07-2016	9.00	0.00	I	89	0.01
26-07-2016	25.30	0.00	I	89	5.34
27-07-2016	18.40	25.30	II	78	0.14
29-07-2016	8.20	43.70	III	60	14.32
31-07-2016	9.20	51.90	III	60	13.55
14-10-2016	52.30	1.10	I	89	24.76
31-10-2016	11.00	0.00	I	89	0.08
02-11-2016	14.00	11.00	I	89	0.58
03-12-2016	13.40	3.20	I	89	0.45
04-12-2016	19.00	16.60	II	78	0.09
14-12-2016	10.20	4.40	I	89	0.02
15-12-2016	5.40	14.60	II	78	4.66
Station :Periyanaickenpalayam
07-05-2016	18.00	1.00	I	89	1.84
11-05-2016	6.00	21.00	II	78	4.27
12-05-2016	9.00	27.00	II	78	2.64
15-05-2016	40.00	17.00	II	78	3.80
16-05-2016	5.00	57.00	III	60	16.98
03-06-2016	7.00	0.00	I	89	0.20
18-07-2016	8.00	0.00	I	89	0.07
26-07-2016	30.00	2.00	I	89	8.15
23-08-2016	6.00	0.00	I	89	0.42
01-09-2016	24.00	0.00	I	89	4.62
15-09-2016	9.00	0.00	I	89	0.01
14-10-2016	42.00	0.00	I	89	16.59
31-10-2016	52.00	0.00	I	89	24.51
03-11-2016	24.00	52.00	III	60	5.04
02-12-2016	5.00	0.00	I	89	0.72
03-12-2016	18.00	5.00	I	89	1.84
04-12-2016	20.00	23.00	II	78	0.03
05-12-2016	5.00	43.00	III	60	16.98
14-12-2016	15.00	4.00	I	89	0.84
15-12-2016	15.00	19.00	II	78	0.65
Station :Podanur
13-03-2016	27.00	0.00	I	89	6.31
05-05-2016	9.30	0.00	I	89	0.00
09-05-2016	6.20	10.60	I	89	0.37
15-05-2016	15.00	2.00	I	89	0.84
17-05-2016	6.00	17.00	II	78	4.27
07-07-2016	28.00	0.00	I	89	6.91
13-10-2016	40.00	0.00	I	89	15.09
12-12-2016	11.00	0.00	I	89	0.08
13-12-2016	10.00	11.00	I	89	0.01
14-12-2016	6.00	21.00	II	78	4.27
15-12-2016	17.00	3.00	I	89	1.47
Station :Sultanpet
08-05-2016	7.00	4.00	I	89	0.20
09-05-2016	7.00	11.00	I	89	0.20
12-05-2016	12.00	18.00	II	78	1.45
17-05-2016	14.00	16.00	II	78	0.88
18-05-2016	20.00	18.00	II	78	0.03
28-06-2016	6.00	0.00	I	89	0.42
03-08-2016	6.00	0.00	I	89	0.42
27-08-2016	5.00	0.00	I	89	0.72
14-10-2016	55.00	0.00	I	89	26.99
31-10-2016	16.00	0.00	I	89	1.14
01-11-2016	7.00	16.00	II	78	3.68
03-11-2016	33.00	23.00	II	78	1.59
02-12-2016	8.00	0.00	I	89	0.07
03-12-2016	10.00	8.00	I	89	0.01
13-12-2016	8.00	0.00	I	89	0.07
Station :Thiruppur
09-05-2016	5.10	0.00	I	89	0.69
17-05-2016	25.60	3.00	I	89	5.50
18-05-2016	17.00	25.60	II	78	0.30
15-06-2016	5.10	0.00	I	89	0.69
27-07-2016	14.00	0.00	I	89	0.58
14-09-2016	15.00	0.00	I	89	0.84
20-09-2016	20.00	0.00	I	89	2.67
14-10-2016	68.00	0.00	I	89	38.14
21-10-2016	38.00	0.00	I	89	13.62
31-10-2016	64.00	0.00	I	89	34.65
02-11-2016	23.00	64.00	III	60	5.46
02-12-2016	22.00	0.00	I	89	3.60
Station :Thondamuthur
06-05-2016	18.00	0.00	I	89	1.84
08-05-2016	24.00	18.00	II	78	0.08
11-05-2016	37.00	42.00	III	60	1.22
31-08-2016	12.00	0.00	I	89	0.20
14-10-2016	24.00	0.00	I	89	4.62
31-10-2016	72.00	0.00	I	89	41.68
03-11-2016	21.00	72.00	III	60	6.36
02-12-2016	5.00	0.00	I	89	0.72
03-12-2016	18.00	5.00	I	89	1.84
04-12-2016	20.00	23.00	II	78	0.03
05-12-2016	5.00	43.00	III	60	16.98
12-12-2016	15.00	4.00	I	89	0.84
15-12-2016	15.00	19.00	II	78	0.65
Station :Sulur
08-04-2016	19.20	0.00	I	89	2.32
09-05-2016	17.20	0.00	I	89	1.55
12-05-2016	33.00	20.40	II	78	1.59
14-05-2016	18.20	53.40	III	60	7.77
17-05-2016	5.20	55.30	III	60	16.81
14-10-2016	10.20	0.00	I	89	0.02
31-10-2016	43.00	0.00	I	89	17.36
03-11-2016	14.20	43.00	III	60	10.09
02-12-2016	7.20	7.20	I	89	0.17
03-12-2016	13.20	7.20	I	89	0.41
04-12-2016	14.50	20.40	II	78	0.76
15-12-2016	6.20	3.20	I	89	0.37
Station : Avinashi
09-05-2016	26.00	0.00	I	89	5.73
15-05-2016	54.00	0.00	I	89	26.16
17-05-2016	10.00	54.00	III	60	12.95
18-05-2016	33.00	64.00	III	60	2.09
29-05-2016	12.40	0.00	I	89	0.26
14-06-2016	8.60	0.00	I	89	0.02
18-06-2016	20.00	8.60	I	89	2.67
26-07-2016	11.00	3.50	I	89	0.08
28-07-2016	13.00	11.00	I	89	0.37
14-08-2016	17.00	0.00	I	89	1.47
15-08-2016	16.00	17.00	III	60	9.00
28-08-2016	15.00	0.00	I	89	0.84
29-08-2016	12.00	15.00	III	60	11.53
30-08-2016	5.00	27.00	III	60	16.98
31-08-2016	22.00	32.00	III	60	5.90
02-09-2016	23.00	54.00	III	60	5.46
07-09-2016	7.00	27.00	II	78	3.68
16-09-2016	11.00	0.00	I	89	0.08
20-09-2016	18.00	11.00	I	89	1.84
14-10-2016	27.50	0.00	I	89	6.61
21-10-2016	9.00	0.00	I	89	0.01
31-10-2016	59.90	0.00	I	89	31.13
04-11-2016	18.00	64.40	III	60	7.88
02-12-2016	14.00	0.00	I	89	0.58
Station :Kallipalayam
07-05-2016	22.00	0.00	I	89	3.60
09-05-2016	21.00	22.00	III	60	6.36
17-05-2016	25.00	2.00	I	89	5.17
18-05-2016	37.00	25.00	III	60	1.22
28-05-2016	17.00	0.00	I	89	1.47
14-10-2016	9.00	0.00	I	89	0.01
31-10-2016	56.00	0.00	I	89	27.83
01-11-2016	18.40	56.00	III	60	7.67
03-11-2016	10.00	74.40	III	60	12.95
02-12-2016	9.00	0.00	I	89	0.01
03-12-2016	9.50	9.50	I	89	0.00
15-12-2016	16.00	2.00	I	89	1.14
Station :Palladam
12-05-2016	13.00	1.00	I	89	0.37
16-05-2016	19.00	14.00	II	78	0.09
18-05-2016	27.00	19.00	II	78	0.39
21-06-2016	17.00	0.00	I	89	1.47
13-10-2016	6.00	0.00	I	89	0.42
21-10-2016	16.00	3.00	I	89	1.14
30-10-2016	52.00	0.00	I	89	24.51
31-10-2016	16.00	52.00	III	60	9.00
02-11-2016	22.00	68.00	III	60	5.90
01-12-2016	7.00	0.00	I	89	0.20
02-12-2016	11.00	7.00	I	89	0.08
03-12-2016	6.00	18.00	II	78	4.27
14-12-2016	25.00	2.00	I	89	5.17
Station :Pongalur
17-05-2016	24.00	2.00	I	89	4.62
18-05-2016	37.00	24.00	II	78	2.76
14-10-2016	18.00	0.00	I	89	1.84
22-10-2016	27.00	4.00	I	89	6.31
31-10-2016	78.00	0.00	I	89	47.05
01-11-2016	10.00	78.00	III	60	12.95
02-12-2016	6.00	0.00	I	89	0.42
03-12-2016	7.00	6.00	I	89	0.20
15-12-2016	5.00	0.00	I	89	0.72
Source: compiled by author from various sources

Based on the analysis, the watershed is divided into 5 zones based on the runoff, viz. regions of very low (less than 58mms), low (between 58 and 77mms), moderate (between 77 and 96mms), high (between 96 and 114cms) and very high (more than 114mms) runoff. It can be noticed that maximum runoff occurs along the north western and north eastern side of the watershed. The slope of these region is very steep. Lower occurrences of runoff is observed near the southern extremity of the watershed. The average runoff estimated in the watershed is 5.58mm in the year 2016. The highest runoff is noticed in Mettupalayam, while Annur experiences the lowest runoff with 2.15mm. The highest run off of 23.04mm in Agriculture college was noticed in the month of October. Annur has recorded the highest runoff during May, which is about 16.98mm. In Chithiraichavidi Anicut, 14.64mm runoff occurred during June. Maximum run off in Coimbatore airport of 38mm occurred during October. Among all the stations, the highest runoff and one of the maximum rainfall event (23) was observed in Mettupalayam. During January, the highest run off of 118.mm occurred here. The next highest runoff event happened in October where 24.76mm was lost. 20 storm events had occurred in Periyanaickenpalayam, with the maximum in December. Highest run off happened in October, with 24.51mm. In Podanur, the maximum run off was during October. It was around 15.09mm. In Sultanpet also, highest occurred during October, with 26.99mm of runoff. Two equal intensities of runoff event happened in Thiruppur, 38.14mm and 34.65mm, both during October. Thondamuthur recorded the highest run off event during October (41.68mm). Sulur, registered 10.9mm run off in November. Avinashi has recorded the maximum rainfall event, that is 24 days registered rainfall over 5mm. Maximum run off occurred during October, 31.13mms. Kallipalayam and Palladam also registered the highest run off during October, with 27.83mms and 24.51mms respectively. In Pongalur, even though, the number of rainfall events are less, an intense run off event of 47.05mm occurred in October.

Runoff estimation for Vannathangarai watershed was done for the year 2016. The Natural Resources Conservation Service (NRCS) Curve Number (CN) method of runoff estimation was used for the present study. The National The daily rainfall data, Land Use- Land Cover classes derived from satellite imagery, soil data derived from National Bureau of Soil Survey and Land Use Planning were the used as the main datasets. The study proves to be helpful in analyzing the runoff in a very efficient way. The runoff estimation is very integral part of landscape analysis. This helps in understanding the amount of water percolating inside, which in turn becomes the groundwater. The runoff estimation is significant for flood analysis, as lower the rate of runoff, higher is the chance for flooding. The study points out that highest runoff is associated with maximum precipitation event and is closely related with the slope of the terrain. The maximum rainfall in the watershed happens during the north west monsoon season, during the months of November and December; incidentally runoff is also higher during these months.

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No competing interests reported.

Estimation of Runoff Using NRCS-CN Method: A Study from South India

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Abstract

Figures

1. Introduction

2. Study Area

3. Methodology

4. Result & Discussion

5. Conclusion

References

Additional Declarations

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Version 1