Spatial distribution of rainfall in Salem and Namakkal … distribution of rainfall in Salem and Namakkal districts Jegankumar.R, Nagarathinam S.R, Kannadasan.K International Journal

INTERNATIONAL JOURNAL OF GEOMATICS AND GEOSCIENCES

Volume 2, No 4, 2012

© Copyright 2010 All rights reserved Integrated Publishing services

Research article ISSN 0976 – 4380

Submitted on April 2012 published on May 2012 976

Spatial distribution of rainfall in Salem and Namakkal districts Jegankumar.R

1, Nagarathinam S.R

2, Kannadasan.K

3

1- Assistant Professor, Department of Geography, School of Geosciences, Bharathidasan

University, Tiruchirappalli

2- Associate Professor & Head, Department of Geography, Govt., Arts College

(Autonomous), Coimbatore

3- Guest Lecturer, School of Geosciences, Bharathidasan University

[email protected]

ABSTRACT

The spatial distribution of rainfall shows a unique pattern in Salem and Namakkal Districts.

By analyzing the long term rainfall data, the results shows that Summer contributes 18.73%,

Southwest monsoon produces 40% and North east 40%. The average mean rainfall of the

study area is 844.49mm. The variability indicates higher variability in winter to compare

other seasons. Similarly precipitation ratio is carried out to bring out the anomalies in the

distribution and the rainfall frequency is calculated to understand the occurrences. GIS is

effectively used in this attempt to compute and produce maps.

Keywords: Rainfall, rainfall variation, Salem district rainfall, Namakkal district rainfall.

1. Introduction

Rainfall is unique variable, which reflects multiple factors locally and globally. The

distribution of rainfall depends upon various factors existing at regional scale. Rainfall is

being a single most important factor for success of crops in the farming areas. South west

monsoon (June – September) is the most important season as it brings more amounts of

rainfall to major parts of India and the northeast monsoon is the returning monsoon from

October to December, it pours supplementary rainfall and apart from conventional rainfall

during summer. Study of rainfall over a long period is called rainfall climatology. It brings

out the general pattern and characteristics of rainfall of a particular region (Sahu, 2003).

Usually rainfall is studied as weekly, monthly, seasonally and annually. The best

understanding of rainfall is useful for various agronomic activities (Mavi, 1994). In this

study GIS is effectively used to interpolate the rainfall for various analysis and preparation of

maps.

2. Study area

The study area lies between 11°00' and 12

°00' North latitude and 77

°40' and 78

°50' of East

longitude. It occupies an area of 8, 658 sq.km. The present study area is land locked by Erode

in the west, Karur and Tiruchirappalli in the south, Perambalur and Cuddalore in the east,

Villupuram in the northeast and Dharmapuri in the northern direction. It has many hills and

valleys. Many structurally controlled hills of the Eastern ghats are present in the study area.

They are Shevorys, Kolli hills, Cheteri hills, Kalarayan hills and Pachamalai . In the North of

study area Shevorys (Yercaud) is present with the maximum elevation of 1,652m. East of

shevorys part of Cheteri Hills and Kalrayan Hills are present. Kolli hill is located in the south

eastern direction. The maximum elevation of the Kolli hill is about 1,300m and a part

Spatial distribution of rainfall in Salem and Namakkal districts

Jegankumar.R, Nagarathinam S.R, Kannadasan.K

International Journal of Geomatics and Geosciences

Volume 2 Issue 4, 2012 977

Pachamalai is present in eastward of Kolli hills. The well marked Attur Gap is located

between Kalrayan hill and Pachamalai. In the Northwestern part of study area Stanley

Reservoir is located and it is surrounded by hills elevation with 768m. Apart from the major

hills few hillocks also seen in the plain region. Kongu upland is located in the west, Karur

plain and Musri plain in south and Ponnaiyar plain and Vellar plain are located in the east.

The present study area comprises of two districts such as Salem and Namakkal Districts of

Tamilnadu. At present Salem District contains 9 Taluks, 20 Blocks, 33 town panchayats, four

revenue divisions, 631 revenue villages, four municipalities and one corporation. The District

occupies an area of 5,226.06 sq.kms. The Namakkal District consists of four taluks, fifteen

blocks, two revenue divisions, nineteen town panchayats, 391 revenue villages, 331

panchayat villages and five municipalities, the Namakkal district is spread about 3,431.98

km2. The total area of the study area is 8,658.00 sq.kms. The population of Salem district is

34,80,008 according to 2011 Census. It ranked fifth among the District in population of the

Tamilnadu. Similarly Namakkal district consists of 17,38,376 people and it is in 21th

position

among the districts. The location of the study area is given in the figure1. The figure 2

illustrates the location of rain gauge stations in and around the study area.

Figure 1: Study area map





Figure 2: Location of rain gauge stations

2.1 Data and analysis

There are thirty seven rainfall stations selected for the present study. The daily rainfall data

for the period of 1980 – 2010 have been collected for major stations and available rainfall

stations which are installed in last five years. The daily rainfall data has been tabulated as

monthly for the respective rain gauge stations. The tabulated data are analyzed to calculate

mean rainfall, coefficient of variation, precipitation ratio and frequency. The results are

mapped in GIS environment by applying spline interpolation.

2.2 Mean annual rainfall

The location of the study area is an important factor for the distribution of rainfall. In the

study area, three seasons have significant amount of rainfall out of the four seasons. The well

marked hills in the north and south directions play a key role in the spatial distribution of

rainfall.





Table 1: Annual mean and seasonal rainfall

Sl.

No Station Winter Summer

Southwest

Monsoon

Northeast

Monsoon

Mean

Annual

1 Salem Junction 7.50 160.20 461.60 286.70 916.00

2 Salem 5.90 173.00 502.30 312.10 993.30

3 Attur 7.91 108.36 314.03 333.25 763.55

4 Omalur 8.70 176.00 465.70 309.80 960.20

5 Namakkal 8.95 171.78 371.70 316.00 868.43

6 Paramathi 4.80 80.40 161.40 229.60 476.20

7 Mettur 9.20 196.50 375.70 307.50 888.90

8 Yercaud 22.40 268.10 870.20 540.90 1701.60

9 Rasipuram 8.60 148.80 403.70 272.20 833.30

10 Puduchatram 1.40 167.60 246.30 282.44 697.74

11 Edappadi 4.60 245.70 315.20 374.10 939.60

12 Pottaneri 0.60 163.70 403.90 316.00 884.20

13 Kolathur 6.25 192.28 242.47 558.58 999.58

14 Sankari 8.30 153.00 326.50 283.20 771.00

15 Tiruchengode 10.80 177.10 307.40 314.40 809.70

16 Erode 5.00 217.00 286.90 420.60 929.50

17 Ammapet 10.46 181.98 271.92 344.98 809.34

18 Kumarapalayam 11.34 137.77 274.65 289.50 713.26

19 Mohanur 0.40 122.40 119.30 319.80 561.90

20 Senthamagalam 6.40 133.10 343.80 258.70 742.00

21 Thampampatti 6.30 111.20 274.77 398.20 790.47

22 Dainshpet 1.70 153.60 389.90 319.80 865.00

23 Karur 9.20 112.80 211.40 292.60 626.00

24 Musri 14.90 93.30 247.20 359.20 714.60

25 Thathaiangarpet 6.10 89.82 182.90 271.30 550.12

26 Anaimadu 2.33 155.00 386.33 453.50 997.16

27 Penagram 9.40 192.40 383.30 267.70 852.80

28 Dharmapuri 9.30 186.00 456.60 301.30 953.20

29 Harur 13.50 140.00 384.90 350.20 888.60

30 Modakurichi 2.80 127.34 205.41 315.44 650.99

31 Paramathi 8.02 124.54 179.12 316.90 628.58

32 Karia Kovil 6.80 125.40 367.20 431.16 930.56

33 Kullampatti 15.10 196.40 345.80 299.46 856.76

34 Pillukurichi 15.00 178.80 308.10 437.80 939.70

35 Vazaphadi 3.70 147.90 342.75 347.60 841.95

36 Mangalapuram 8.40 143.70 377.70 354.06 883.86

37 Gangavalli 2.33 198.95 395.93 419.10 1016.31

Average 7.69 158.16 337.95 340.69 844.49

There are thirty seven rainfall stations found in and around the study area. The general pattern

of the rainfall over this area shows that the southwest monsoon and northeast monsoon shares

80% totally, but spatial pattern is different, summer showers due to conventional rainfall

contribute 18.73 % and winter rain is about 1% annually. The annual average rainfall is about

844.49 mm, 340.69mm during northeast monsoon, 337.95mm occur in southwest, 158.16mm

in summer and 7.69mm occur in winter





2.3 Mean annual rainfall

The mean annual rainfall is the sum of twelve month long term monthly average is computed.

The annual average rain fall of the study area is 844.49mm, the annual mean varies from

476.20mm to 1701.60mm. Southwest and northeast monsoon shares 80 percent of rainfall

and summer contributes by 18 percent and winter shares 1%. The stations Yercaud and

Gangavalli record more than 1000mm rainfall, the following stations receive between 900mm

and 1000mm. They are Kolathur, Anaimadu, Salem, Omalur, Pillukurichi, Edappadi,

Kariakovil, Erode and Salem Jn. Less than 500mm of rainfall occurs in Mohanur and

Paramathi. The mean annual rainfall of the study area is classified into five categories less

than 700mm of rainfall occurs in north of Kadayampatti, Paramathi, Kabilarmalai,

Senthamagalam and Veerapandi blocks of the study area. Rainfall between 700 and 800mm

is found on west of Senthamagalam, eastern part of Tiruchengode, Vennandur,

Panamarathuppatti, Vazaphadi, East of Attur and in Mettur in the western part of the study

area. The mean annual rainfall for the all stations shown in table 1 and figure 3 illustrates the

mean annual rainfall of the study area.

Figure 3: Mean annual rainfall

2.4 Winter

The winter season contributes small amount of rainfall to the annual share. In this season

rainfall varies from 2mm to 22mm. In the winter season the maximum rainfall occur in the

hilly areas of Yercaud. Apart from the Yercaud block all parts of the study area receive less

than 10mm of rain fall during winter season. The seasonal pattern of rainfall of the study area

is shown in figure 4.

2.5 Summer

The average rainfall of the summer season is about 158.16 mm (20%) of total annual rain of

the study area, (844.49 mm). During the summer season showers were common in this study

area (Ramanan, 2006) in the summer season rainfall varies from 80.4mm at Paramathi and

268.1 mm at Yercaud. Less than 100mm of rainfall was found in north of Kadayampatti,

Kabilarmalai and northern part of Erumaipatti. From 100 to 125mm of rainfall occurs in





Gangavalli, Attur, Senthamagalam, Namagiripettai, Erumaipatti, northern part of

Kabilarmalai, Paramathi and few parts of Kadayampatti. Between 125 and 150mm amount of

rainfall during the summer season occurs in Vazaphadi, northern parts of

Pethanaickanpalayam, Vennandur, Veerapandi, west of Namakkal, and few parts of northern

Omalur block. More than 150mm of rain fall is present during the summer in the following

blocks namely Thalaivasal, Tiruchengode, Pallipalayam, Edappadi, Kolathur, and “Yercaud.

2.6 Southwest monsoon

The winds from Arabian Sea bring more rainfall to the western part of the country. A few

places in Tamilnadu receives more rainfall during the southwest monsoon season. A Branch

of wind belongs to southwest monsoon is able to reach over Salem and Namakkal through

Palaghat Gap, it was obstructed by hills present in the study area. As result of orographic

effect, the windward side around Namakkal, Sankari, Omalur, Salem and Yercaud gets more

rain. Areas between Kolli hills and around Attur become rain shadow during this period.

During the southwest monsoon the rainfall varies between 119 mm at Mohanur and 828mm

at Yercaud. The average rainfall of this season is about 337.95mm (40%) of the total mean

annual rainfall. Less than 150mm of rainfall is found only in the northern part of

Kadayampatti, western part of Kolathur, and southern part of Kabilarmalai. Between 150 and

250mm of rainfall occurs in few parts of Kolathur, Mecheri, southern part of Tiruchengode,

and Erumaipatti. This category (250 350mm) of rainfall occur in vast area, which include,

Sankari, northern part of Tiruchengode, Senthamagalam, Kollimalai, western part of

Gangavalli, Attur, southern Vazaphadi & Ayothiyappattinam, and Tharamangalam in west.

From 350 to 450mm of rainfall found in the following areas of Omalur, Veerapandi,

Namakkal, Panamarathuppatti, northern part of Pethanaickanpalayam and western part of

Thalaivasal and eastern part of Gangavalli. More than 450mm of rainfall prevails over

Yercaud, Salem and Thalaivasal.

2.7 Northeast monsoon

The retreating monsoon is called northeast monsoon, it is prevail in the period from October

to December. From this monsoon, east cost of Peninsular India and interior parts of

Tamilnadu receive rain; through the depressions originate from Bay of Bengal. The study

area receives 40.34% of total mean annual rain fall from northeast season and the table 2

indicates the northeast monsoon season rainfall of the study area. The rainfall varies during

the season from 229mm at Paramathi to 558mm at Kolathur. Attur Gap Plays significant role,

it provides a path to the rain clouds from Bay of Bengal to the inner parts of the study area.

The Yercaud and Kolli Hills create a rain shadow effect on the west of each hills, it is clearly

seen in the spatial distribution. During this period Kolathur and Mettur located straight to the

Attur Gap is benefited in the season by receiving more than 475mm rainfall. The windward

side of Kolli hills, Yercaud and Southern side of Kalarayan receive more than 475mm of

rainfall. Rainfall between 150 and 250mm occur only in the few places of southern part of

Kadayampatti, Veerapandi, Senthamagalam, and Kabilarmalai. Rainfall from 250 to 300mm

is found in Kolathur, Omalur, Salem, Attur, Sankari, Tiruchengode, Veerapandi, Rasipuram,

Paramathi, Erumaipatti, Vennandur, Puduchatram and middle parts of Magudanchavadi.

Rainfall of 350 and 450mm is the second major category found in this region, which covers

south of Thalaivasal, eastern part of Gangavalli, Yercaud, northern parts of

Pethanaickanpalayam, eastern part Kolathur and central Edappadi.





Figure 4: Seasonal rainfall distributions





3. Rainfall variability

The coefficient of variation (CV) defined as the standard deviation divided by the mean value

of rainfall. It shows the variability of rainfall in percentage. The higher the variability

percentage, the lower is the dependability and vice versa. For the monthly case less than

100% of CV is dependable, while more than 100% not dependable rainfall.

CV = (SD/Mean) X 100 where SD = Standard deviation. - 1

3.1 Mean annual variability

The mean annual variability of the study area is calculated for the thirty eight rainfall gauge

stations located in and around the study area. Coefficient of variability is calculated from the

long term mean annual rainfall and standard deviation of the each rainfall station. The annual

variability is 30.41% and it varies from 20% to 50% in the study area. The minimum

variability is found at Kariakovil at north (14%) and the Maximum variability is found at

Paramathi in the south (53%). The low variability (less than 20%) is found in the east,

northeast and northwestern part of the study area, which includes around Gangavalli and

Thalaivasal in the east, the area around Kariakovil in the northeast, Pottaneri and Pillukurichi

on the northwestern part of the study area. The Salem, Omalur, Kolathur, Pillukurichi,

Mangalapuram, Paramathi, Kumarapalayam, Mettur dam, Tiruchengode, Yercaud, Sankari,

Edappadi, Puduchatram, Senthamagalam and Namakkal are covered with 20- 30 percentage

of variability. The variability range of 30-40 percentage is found in the following places

Vazaphadi, Kullampatti, Attur, Rasipuram, Thampampatti and Dainshpet. Areas around north

of Dainshpet in the north and areas around Mohanur in south were under 40-50 percentage of

variability. More than 50 percentage of variability is found in the west of Yercaud and down

south of Paramathi. The mean annual variability shown in figure 5 and in table 3 annual and

seasonal variability is shown.

Figure 5: Mean annual variability





3.2 Winter rainfall variability

The mean rainfall variability of winter season is 222.87 % and varies between 66.20% and

318%, maximum variability is found in Gangavalli and minimum is present over Kariakovil.

The low variability of less than 150% is present around Kariakovil in the northeast, Attur and

Vazaphadi in the east and the portions of east face of Yercaud are in less than 150% of

variability. Similar conditions prevail over Puduchatram in the central part of Namakkal

District. Variability of 150 – 175 % is seen over in the following places, Middle portions of

Yercaud, Kullampatti, Anaimadu and Edappadi. The variability range of 175 – 225

percentage is seen over in the following places Ammapet, Attur, Tiruchengode, Mettur,

Pottaneri, Rasipuram, and Kolathur. The range variability between 175 and 225 concentrates

over northeast and southwest central portions of the study area. High variability range

between 225 – 300% is seen more in the study area than other variability categories, it

spreads around central, southeast and a small area in the north. The places records high

variability include Pillukurichi, Salem, Senthamagalam, Mangalapuram, Thampampatti,

Paramathi, Omalur, Namakkal, Kumarapalayam and Sankari. The very high variability of

more than 300% is seen towards east of Gangavalli, Mohanur south of Omalur and north of

Dainshpet. The high variability in winter season of among all season states that the

occurrence of less rainfall during winter.

Table 3: Annual and seasonal rainfall variability

Sl.No Stations Winter Summer Southwest

monsoon

Northeast

Monsoon

Annual

Mean

1 Salem Junction 285.30 43.80 31.30 43.50 21.25

2 Salem 228.80 46.20 27.40 42.90 21.12

3 Attur 184.60 60.50 36.90 49.90 33.90

4 Omalur 255.20 46.50 26.30 77.40 21.32

5 Namakkal 259.30 50.60 46.90 90.90 30.49

6 Paramathi 243.80 75.50 71.70 63.50 53.30

7 Mettur 196.70 47.90 37.50 49.40 25.10

8 Yercaud 153.60 42.20 47.70 43.20 26.05

9 Rasipuram 210.50 53.20 46.40 53.90 34.66

10 Puduchatram 142.90 56.00 38.50 47.60 28.22

11 Edappadi 160.90 36.60 21.70 49.80 28.15

12 Pottaneri 200.00 60.50 29.80 42.40 17.15

13 Kolathur 210.90 61.40 61.90 35.10 22.30

14 Sankari 269.90 61.80 35.40 47.10 27.50

15 Tiruchengode 195.40 52.20 43.90 45.90 25.10

16 Erode 174.00 77.50 46.10 53.90 44.90

17 Ammapet 182.30 54.70 44.30 55.10 36.51

18 Kumarapalayam 266.20 65.10 37.90 45.40 24.72

19 Mohanur 300.00 65.00 63.70 54.30 47.16

20 Senthamagalam 229.70 52.50 41.80 44.10 29.10

21 Thampampatti 240.30 81.40 58.80 54.10 40.10

22 Dainshpet 376.50 62.90 47.60 73.00 41.30

23 Karur 217.40 56.00 42.80 54.30 29.17

24 Musri 212.80 64.00 46.50 46.50 28.71

25 Thathaiangarpet 249.20 104.00 67.90 65.20 43.42





26 Anaimadu 157.50 58.30 58.00 50.40 16.65

27 Penagram 280.90 54.60 52.90 53.20 32.18

28 Dharmapuri 257.00 49.60 40.00 58.10 33.88

29 Harur 263.70 54.60 39.20 53.60 58.42

30 Modakurichi 282.10 91.50 41.60 42.10 30.90

31 K. Paramathi 217.00 50.50 46.10 42.10 24.17

32 Kariakovil 66.20 24.60 34.70 31.80 14.00

33 Kullampatti 156.30 56.90 47.30 55.50 33.63

34 Pillukurichi 226.00 58.80 35.00 54.40 23.70

35 Vazaphadi 143.20 73.40 22.00 41.70 32.96

36 Mangalapuram 232.10 57.90 28.60 44.50 24.00

37 Gangavalli 318.00 51.70 37.30 47.60 19.82

Average 222.87 58.39 42.79 51.44 30.41

3.3 Summer rainfall variability

The summer variability of the study area is 58.39 percentage, it varies between 24.6 and 104

percentage of variability. Less than 50 percentage of variability is found in the east of Attur

and around Thalaivasal, similar variability prevails also in Kariakovil, Edappadi, Yercaud,

Salem Jn, Salem, Omalur, Mettur, and Namakkal, the central part of Namakkal, around

Thampampatti in the east and north of Mettur in the 50-60 percentage variability range. The

variability between 60 and 70 percentage occur in the following areas Kolathur, Sankari,

Dainshpet, Mohanur, and Kumarapalayam. The high variability of more than 70% is found in

the southeast of Kollimalai, west of Paramathi north of Dainshpet. The drastic down

variability during summer season indicates that the rainfall during this season is higher than

winter. The summer showers contribute 158.16mm (18.73%) of rainfall to the total annual

rain. In general the central part and the eastern sector receive more rain which was indicated

by the less variability.

3.4 Southwest monsoon rainfall variability

The rainfall variability of the southwest monsoon is about 42.79%. The season contributes

40 % of total annual rainfall as equal to northeast monsoon. The variability of southwest

monsoon season varies between 21% at Edappadi and 71 % at Paramathi. The regions

influenced by southwest monsoon have less variability than other places. Less than 30

percentage of variability is found along the west around Omalur, Edappadi, Sankari and

Kolathur. The block around Thalaivasal and places around Attur exhibit less than 30% of

variability. The zone of 30 – 40 percentage of variability is found periphery of the zone of

less than 30 percentage of variability, the places include Salem Jn, Kariakovil, Pillukurichi,

Sankari, Attur, Gangavalli, Mettur, Kumarapalayam and Puduchatram. The moderate amount

of variability between 40 and 50 percentage of variability covers the major parts of

Rasipuram, Namakkal, Tiruchengode and other parts of Namakkal and the northern boundary

of the study area. The major portions of southeast and few places around north were covered

by 50 to 60 percentage of variability. High variability of more than 30 percentage of

variability exists in the north of Yercaud, the west of Kolathur and the south of Mohanur.





Figure 6: Seasonal rainfall variability





3.5 Northeast monsoon rainfall variability

The variability of the northeast monsoon had shown the strength of the northeast monsoon.

The spatial pattern clearly indicates that the prominent hills located in the study area create

rain shadow effect in the northeast monsoon season. The regional variability of the northeast

monsoon season is 91% and it varies from 52.31 % at Kariakoil and 31.8 % at Namakkal.

The eastern part of the study area, north of Kolli hills and west of Mettur are found in less

variability of < 45%. Variability of 45 – 55 percentage is found in the western part of the

study area, similar variability occur in the area between Attur, Salem and around

Thampampatti and west of Gangavalli. A linear area in the central and the west of Kolathur

is covered under 55 – 65 percentage of variability. The maximum variability more than 75

percentage is found in the west of Kollimalai and west of Yercaud hills. This region may

have a shadow during the northeast monsoon. Even though this season shares equal rain

along the southwest monsoon and the spatial distribution pattern is different.

3.6 Precipitation ratio

The abnormalities of rainfall at a specific region can be drawn through a simple ratio of

precipitation. The ratio may give the stability of rainfall with spatial pattern. Higher the ratio

is the higher the abnormality in rainfall and the lower in ratio indicates the less anomaly. The

formula used to calculate precipitation ratio is as follow.

Precipitation Ratio = (Px – Pn)/Pm x 100

Where: Px = Maximum of Rainfall,

Pn = Minimum of Rainfall and Pm is the mean rainfall.

3.7 Mean annual precipitation ratio

The mean annual precipitation ratio of the study area is 19.75%, the ratio calculated from the

monthly mean of the respective season., It varies between 14% at Penagram and Maximum at

28% at Mohanur. The majority of the area under less than 20 percentage area Yercaud,

Namakkal, Edappadi, Mangalapuram, Senthamagalam, Dainshpet, Salem, Kullampatti,

Sankari, Salem Jn, Omalur, Rasipuram, ,Vazaphadi, Mettur Dam, Attur, Puduchatram,

Tiruchengode and Erode. The mean annual precipitation ratio between 20 and 25 percentage

is found in the following areas Kariakovil, Pillukurichi, Anaimadu, Kolathur, Paramathi,

Thampampatti, Gangavalli and more than 28 % found in Mohanur. (Table 4)

3.8 Winter precipitation ratio

The seasonal precipitation ratio is shown in the figure 6. It is depicts the seasonal and annual

precipitation ratio. During the winter season the average precipitation ratio of this region is

811.48%, it varies at maximum of 1381 % at Salem Jn and minimum of 85.80 % at

Anaimadu. Zones with less than 500% cover the northeastern portion and areas around

Namakkal. The following regions such as such as Kullampatti, Yercaud, Attur, Tiruchengode,

Musri, Pillukurichi, Rasipuram, Gangavalli, Paramathi and Mohanur were under 500 – 700

percentage of precipitation ratio. The central part of the study area intercepted by

precipitation ratio zone of 700 – 1000, the following areas were partly covered in this

category, Senthamagalam, Mettur, Paramathi, Dainshpet, Namakkal. More than 1000





percentage of precipitation ratio is found in southeast of Namakkal, west of Salem and

Kolathur.

3.9 Summer precipitation ratio

The summer precipitation ratio of the study area is 203.72 % and it varies between 13% and

443%. The south eastern sector, west of Mettur and towards north of Yercaud express high

precipitation ratio of more than 250 % and following areas show high abnormality during this

season they are K. Paramathi, Salem, Senthamagalam, Sankari, Attur, Dainshpet,

Thampampatti, Pillukurichi, Mangalapuram and Gangavalli. Less than 150 percentage of

precipitation ratio is found in and around of the following places Paramathi, Kariakovil,

Anaimadu, Edappadi, Puduchatram and Vazaphadi, these areas under 150 % have fewer

anomalies to compare the other areas.

3.10 Southwest precipitation ratio

Southwest monsoon season is an important monsoon season for the study area, it contributes

40% rain to annual total. The average precipitation ratio of the region in southwest season is

152.26% , which is lesser than other three seasons. The precipitation ratio varies between

51 % and 287 %. The lower category of less than 100 percentage is found in northeast

portion of Attur Thalaivasal, Anaimadu, Kariakovil and west of Sankari, Yercaud along

Omalur. The category 100 – 150 percentage prevails most part of the Namakkal Districts of

the study area, places around Yercaud and west of Kolathur. 200 – 250 percentage of

precipitation ration prevails around Rasipuram west of Thampampatti and Mettur. The

maximum amount of precipitation ratio is found over Yercaud hills and a small area around

Thampampatti and the west of Kolathur.

3.11 Northeast precipitation ratio

Northeast monsoon equally shares 40% rainfall as southwest monsoon to the total annual

rainfall. The seasonal average precipitation is 188% prevails in this region during this period.

The northeast precipitation ratio varies between Kariakovil at minimum of 83 % and

maximum of 371% over Namakkal. The categories 100- 150 and 150 – 200 percentage

occupies western part of the study area. The eastern part of the study area has minimum

abnormality, which is indicated by the lower precipitation ratio. The precipitation ration

between 200 and 250 percentage prevails over the western middle part of the study area, it

covers the areas include Paramathi, Rasipuram, Omalur, Salem and north of Dainshpet. The

maximum precipitation ratio is found in Namakkal, east of Paramathi and north of Rasipuram.

Table 4: Precipitation ratio

Sl. No. Stations Winter Summer Southwest

Monsoon

Northeast

Monsoon

1 Ammapet 629.10 170.70 141.80 159.00

2 Anaimadu 85.80 81.90 80.20 142.40

3 Attur 616.90 246.30 139.60 211.80

4 Dainshpet 958.80 247.50 152.20 226.30

5 Dharmapuri 1223.70 256.10 165.10 263.90

6 Edappadi 323.90 104.40 51.90 146.30





7 Erode 540.00 262.70 137.30 200.90

8 Gangavalli 733.90 311.90 122.50 186.60

9 Harur 1133.30 278.40 193.30 199.40

10 Paramathi 859.10 207.50 181.50 172.90

11 Kariakovil 176.50 59.00 86.30 83.30

12 Karur 758.70 184.50 140.30 255.00

13 Kolathur 1187.20 182.30 271.40 102.70

14 Kullampatti 551.00 198.70 171.70 230.60

15 Kumarapalayam 1349.20 271.80 62.10 103.90

16 Mangalapuram 1071.40 278.40 134.70 191.20

17 Mettur 857.60 190.60 153.10 205.70

18 Modakurichi 1035.70 294.00 102.30 117.60

19 Mohanur 775.00 178.60 137.40 150.90

20 Musri 720.10 241.20 178.10 195.10

21 Namakkal 983.20 165.00 195.00 371.70

22 Omalur 1158.60 182.30 135.90 249.70

23 Paramathi 770.80 53.50 215.20 269.70

24 Penagram 1319.10 186.10 218.20 179.60

25 Pillukurichi 729.30 277.90 136.80 203.80

26 Pottaneri 500.00 13.10 93.30 132.10

27 Puduchatram 142.90 136.60 106.60 127.80

28 Rasipuram 731.40 167.20 231.60 234.60

29 Salem 1052.50 208.30 97.30 190.30

30 Salem Junction 1381.30 201.20 113.60 211.20

31 Sankari 1294.00 235.20 148.40 201.20

32 Senthamagalam 843.80 211.10 123.00 167.00

33 Thampampatti 1060.30 277.40 277.70 170.40

34 Thathaiangarpet 824.60 443.80 222.30 214.40

35 Tiruchengode 694.40 190.90 173.00 201.90

36 Vazaphadi 375.70 144.70 55.00 136.80

37 Yercaud 575.90 197.00 287.80 164.90

Average 811.48 203.72 152.26 188.45

3.12 Rainfall frequency

Rainfall frequency is the number of time rainfall occurs at a specific class interval. There are

28 rainfall stations which have more than 15 consecutive years of rainfall data are having

been taken for the frequency analysis. The mean annual rainfall data is classified into

following categories less than 600, 600 – 700, 700- 800, 800 – 900 and more than 900. There

are 730 times rainfall occurred in the study area for the selected period. More than 900mm of

rainfall occurred 270 times in the study area, which accounts maximum contribution. Less

than 600mm of rainfall occurred 133 times. Rainfall amount of 700-800mm and 800-900mm

rainfall events occurred 115 and 112 times respectively. The range of 600mm to 700mm of

rainfall occurred 100 times. The station wise frequency of occurrence is given below in the

table 5.





Figure 7: Seasonal precipitation ratio

Table 5: Rainfall frequency (Annual)

Sl.

No.

Rain Gauge

Stations < 600

600

-

700

700

-

800

800

-

900

>

900

1 Attur 7 6 6 3 9

2 Dainshpet 6 3 2 2 11

3 Kolathur 0 2 3 1 13

4 Kullampatti 2 2 9 8 9

5 Kumarapalayam 7 10 5 6 3





6 Mettur 2 3 4 8 13

7 Namakkal 6 4 3 4 14

8 Nangavalli 0 1 3 4 10

9 Omalur 1 2 3 5 20

10 Paramathi 20 3 2 2 1

11 Pillukurichi 4 2 9 6 10

12 Rasipuram 7 6 2 4 12

13 Salem 0 2 4 6 19

14 Salem Jn 0 2 8 6 15

15 Sankari 6 8 4 5 8

16 Senthamagalam 6 4 8 5 8

17 Thampampatti 5 5 3 3 11

18 Musri 9 7 5 6 4

19 Thathaiangarpet 14 2 6 2 4

20 Karur 13 9 4 4 1

21 Penagram 5 3 3 5 15

22 Erode 3 1 1 2 8

23 Modakurichi 4 5 4 1 1

24 Tiruchengode 5 4 7 6 9

25 Yercaud 0 0 1 0 30

26 Mangalapuram 1 4 6 8 12

Total number of

occurrences 133 100 115 112 270

Figure 8: Frequency curve for selected station from the above table





4. Conclusion

The study of thirty seven rainfall station with long term rainfall data shows the annual mean

rain is 844.49 mm, south west and northeast monsoon season contributes 337.95 and

340.69mm respectively. Both the monsoon seasons give 40% of rainfall each to the annual

rain. The spatial distribution pattern is different because of the hills with different elevation

spread across the study area. Significantly the summer season contributes 18.73%. The

winter season receives minimum rainfall among the other season. The variability indicates

more 100 % of variability observed in the winter season and the other three season the

variability was below 100%, which indicates the dependable rainfall is available during these

period. By observing the precipitation ratio of the east and south eastern side, more

abnormality is found than the western side. In the south west and northeast season the area

having more abnormality is very less. The rainfall frequency indicates more than 900 mm

rainfall has higher frequency followed by 133 for less than 600. The overall observation

shows except winter season, all seasons have rainfall without much variability.

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