spatial distribution of rainfall in salem and namakkal … distribution of rainfall in salem and...
TRANSCRIPT
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
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
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 978
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.
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 979
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
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 980
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
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 981
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.
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 982
Figure 4: Seasonal rainfall distributions
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 983
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
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 984
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
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 985
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.
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 986
Figure 6: Seasonal rainfall variability
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 987
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
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 988
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
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 989
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.
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 990
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
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 991
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
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 992
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.
5. References
1. A Gazetteer of Southern India with the Tenasserim provinces and Singapore, Printed
and Published by Pharoah and Co, Madras, pp 489-499.
2. Afzal Sharieff et al., (2010)¸ Agricultural Meteorology, Sarap Book Publishers private,
Ltd, New Delhi.
3. Alaguraja. P Manivel.M, Nagarathinam, S.R Sakthivel. R and Yuvaraj D., (2010),
Rainfall Distribution Study in Coimbatore District Using GIS, Recent Trends in
Water Research Remote Sensing and General Perspectives, I.K International
Publishing House Pvt. Ltd. New Delhi, pp 92-115
4. Ananthakrishnan R., (1979), Some feature of the southwest monsoon rainfall along
the west coast of India, Proceedings in Indian academy of Science, 88 A, Part II, pp
177-199.
5. Bishnoi, O.P., (2010), Applied Agroclimatology, Oxford Book Company, Jaipur,
India.
6. Gadgil A., (1986), Annual and weekly analysis of rainfall and temperature for Pune:
A multiple time series approach, Institute of Indian Geographers, 8(1), pp 14-20.
7. Gangai, P., Aruchamy, S., and Selvam, K., (2008), Rainfall Characteristics and
Drought Prone Area of Pudukkottai District, Tamilnadu, Indian National Geographer,
23(1&2), pp 59-70.
8. Hema malaini, B., (1986), Climate and landuse of Ananthapur Dt, Andhra Pradesh,
Transactions, Institute of Indian Geographers, 8(1), pp 45-49.
9. Ishappa Muniyappa Rathod, Aruchamy S., (2010), Spatial Analysis of Rainfall
Variation in Coimbatore District Tamilnadu using GIS International Journal of
Geomatics and Geosciences, 1(2), pp 106-118.
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 993
10. Ishappa muniyappa rathod. Aruchamy.S., (2010), Rainfall Trends and Pattern of
Kongu Upland, Tamilnadu, India using GIS Techniques, International Journal of
Environmental Sciences, 1(2), pp 109-122
11. Jagannadha Sarma V.V (2005), Rainfall pattern in the coastal zone of Krishna-
Godavary basin Andhra Pradesh, India, Journal of applied hydrology, 28(1&2), pp 1-
11.
12. Kusre B.C, Singh Kh.S., (2012), Study of spatial and temporal distribution of rainfall
in Nagaland (India), International Journal of Geomatics and Geosciences, 2(3), pp 712
- 722
13. Lalitha Devi., (1992), Climatic Characteristics, Water Balance of Utter Pradesh,
Journal of Scientific Research, pp 25-56
14. Nagarathinam, S.R., (1990), Agroclimatological Aspects of Coimbatore District,
Unpublished Ph.D., Thesis, Madras University, Chennai.
15. Raghavendra Ramanan, S., (2006), Water Balance Modeling of Rainfed Crop: A
Study of Sorghum and Groundnut in Salam and Namakkal District, Unpublished
Ph.D., Thesis, Madras University, Chennai.
16. Ramos., (2001), Rainfall distribution pattern and their over time in a Mediterranean
area. Theoretical and Applied Climatology, 69, pp 163-170
17. Sahu,D.D., (2003), Agrometreology and Remote Sensing Principles and Practices,
Argobios (India), Jodhpur, pp 126 - 133
18. Saravanan, E., (1994), Agroclimatological Studies of Kerala State for Application in
Land Use Planning, Unpublished Ph.D., Thesis, Cochin University, Science and
Technology, Cochin.
19. Sastri, A.S.R.A.S., Divakar Naidu. and Somnath Choudhury. (2009). Agro-topo
Climatological Studies for Crop Planning – A Case Study for the Northern Hills
Agroclimatic Zones of Chattisgarh State, Journal of Agrometeorology, 11(1),pp 33-36.
20. Subramaniam.A. R., (1992), Climatic variability in India. Annals of the National
Association of Geographers, India, 20(3), pp 50-59.
21. Subrahamanyan, V.P. (1983), General Climatology, Heitage Publishers, New Delhi,
India.
22. Syiemlieh,H.J.(2004), Orographic effects on the distribution of rainfall in
north-east India, Transactions of Institute of Indain Geographers, 26(1), pp 12-25.
23. Thornthwaite, C.W. (1948), An Approach toward a Rational Classification of Climate,
Geographical Review, 38(1), pp 55-94.
24. Tripathi, S. K (2009). Rainfall analysis for crop planning: A lesson from Uttarakhand,
Journal of applied hydrology, 22(1), pp 111.
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 994
25. Umamathi, S., Aruchamy, S.,(2011) Rainfall Rhythm of Suruli AR Watershed, Theni
District, Tamilnadu – A GIS Approach, International Journal of Geomatics And
Geosciences, 20(1), pp 219-230.
26. Venkatraman, S(1992), Crops and Weather, Indian Council of Agriculture, New
Delhi.