CLIMATE RESEARCH AND SERVICES

INDIA METEOROLOGICAL DEPARTMENT

MINISTRY OF EARTH SCIENCES
PUNE

Observed Rainfall Variability and Changes over

Tamil Nadu State

Met Monograph No.: ESSO/IMD/HS/Rainfall Variability/24(2020)/48

Pulak Guhathakurta, Usha Krishnan, Preetha Menon,

Ashwini Kumar Prasad, S T Sable and S C Advani
 GOVERNMENT OF INDIA
MINISTRY OF EARTH SCIENCES
INDIA METEOROLOGICAL DEPARTMENT

Met Monograph No.: ESSO/IMD/HS/Rainfall Variability/24(2020)/48

Observed Rainfall Variability and Changes Over

Tamil Nadu State

Pulak Guhathakurta, Usha Krishnan, Preetha Menon, Ashwini Kumar Prasad,

S.T. Sable and S C Advani

INDIA METEOROLOGICAL DEPARTMENT

PUNE - 411005

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 DOCUMENT AND DATA CONTROL SHEET

1 Document Title Observed Rainfall Variability and Changes Over Tamil

Nadu State
2 Issue No. ESSO/IMD/HS/Rainfall Variability/24(2020)/48
3 Issue Date January 2020
4 Security Unclassified
Classification
5 Control Status Uncontrolled
6 Document Type Scientific Publication
7 No. of Pages 28
8 No. of Figures 42
9 No. of References 3
10 Distribution Unrestricted
11 Language English
12 Authors Pulak Guhathakurta, Usha Krishnan, Preetha Menon,
Ashwini Kumar Prasad, S.T. Sable and S C Advani
13 Originating Climate Research Division/ Climate Application & User
Division/ Group Interface Group/ Hydrometeorology
14 Reviewing and Director General of Meteorology, India Meteorological
Approving Department, New Delhi
Authority
15 End users Central and State Ministries of Water resources,
agriculture and civic bodies, Science and Technology,
Disaster Management Agencies
16 Abstract India is in the tropical monsoon zone and receives plenty
of rainfall as most of the annual rainfall during the
monsoon season every year. However, the rainfall is
having high temporal and spatial variability and due to the
impact of climate changes there are significant changes in
the mean rainfall pattern and their variability as well as in
the intensity and frequencies of extreme rainfall events.
The report brings the result of the analysis based on the
rent 30 years of data (1989-2018) on the mean spatial
rainfall pattern as well as mean spatial pattern of different
rainfall events, trends and variability as well as extreme
rainfall events during the monsoon months and annual for
the state.

17 Key Words Rainfall trend, variability, extreme events, dry days

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 1. Introduction

Tamil Nadu lies roughly to the south of latitude 13°30'N. It is one of the southern- most states of
the country and is bounded by the Bay of Bengal on its east, Western Ghats on its west, Indian Ocean on
its south, and Nellore and Chittoor districts of Andhra Pradesh, on north and Kolar, Bangalore and Mysore
districts of Karnataka on its northwest.

The Western Ghats run along the whole length of the west coast at distances from the sea varying
between 80 and 160 km. The height of the Ghats average 1200 m and rise to even more than 2500 m at
some places. There is only one break, known as Palghat gap, 26 km wide in Kerala. Along the eastern coast
runs the chain of the Eastern Ghats with average height of 600 m. Its distance from the coast increases as
one goes southwards. They encompass a stretch of low country from about 160 to 240 km wide between
their easternmost spurs and the east coast. On their way southwards, these two ranges meet and at the point
of junction they rise into a striking upheaval known as the Nilgiri hills, on which stands the hill station of
Ooty (Uthagamandalam) at 2100 a.m.s.l., whose loftiest peaks are Dodabetta (2600 m.a.s.l) and Makurti
(2400 a.m.s.l.).Immediately south of this plateau, the range which now runs between the districts Palghat
(in Kerala) and Coimbatore is interrupted by the remarkable Palghat gap. South of this gap, the ghats rise
abruptly and at this point they are known as Anaimalai and a continuation of the same hills in Dindigul
Anna district is called Palani, Thereafter, the ghats run down to cape Comorin through the states of Kerala
and Tamil Nadu. There are several isolated peaks of hills which are not directly connected with the ghats.
Some of these are the Shevaroy in Salem, the Pachamalai and kollimalai in the same district and
Tiruchirappalli district and Javadi hills in the North Arcot-Ambedkar and South Arcot districts.

Many studies are available on the observed trends and variability of rainfall and also extreme
rainfall events, but all the studies are based on past 100 years or more data and also the recent years are not
included (Guhathakurta et al, 2015; Guhathakurta et al, 2011; Guhathakurta & Rajeevan, 2008 etc). Also
there are limited studies available on rainfall analysis and trend of the State Tamil Nadu. In the present
report all the analysis of observed rainfall patterns, trends and variability have been done based on recent
past 30 years (1989-2018) that will help to have idea of the recent changes for climate change adaptation
and management by the state authorities.

2. Data and Methodology

Daily Rainfall data from 1989 to 2018 is considered for analysis of trend variability and mean rainfall
patterns. From the daily rainfall data monthly rainfall series of each station is computed and then monthly

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district rainfall series has been constructed by considering arithmetic average of all the station rainfall
values within the district. The monthly rainfall series of the state has been computed by using area weighted
rainfall values of all the districts within the state. The objective of the analysis is to:

1. Identify the spatial pattern of the mean rainfall

2. Understand district wise observed rainfall trend and variability in annual and SW monsoon
season (June, July, august and September).

Daily station rainfall data is utilized for identification of the mean spatial patterns and rainfall
intensity trends. From mean and standard deviation (SD), the coefficient of variation (CV) is calculated as
follows:

Standard Deviation
Coeffcient of variation (CV) = × 100
Mean

Fig.1 shows the location of the districts of the state.

Fig. 1 Location of the districts of Tamil Nadu

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 3. State rainfall mean and variability and trend

Table 1 shows the mean rainfall (mm) and coefficient of variation of the state for the monsoon
months, southwest monsoon season and annual during the period 1989-2018. It can be seen that the state
gets highest rainfall (34%) of south west monsoon rainfall in September month while the August month
gets 28% of the south west monsoon rainfall. June and July receive 17% and 21% of south west monsoon
rainfall respectively. 35% of annual rainfall is received during the southwest monsoon season. The
variability of monsoon and annual rainfall is 26% and 25% respectively.

June July August September JJAS Annual

Mean 53.6 64.7 88.0 105.4 311.7 898.1
CV 46.7 41.9 40.9 37.8 26.3 25.5

Table 1 Mean rainfall (mm) and coefficient of variation of the state for the monsoon months,
southwest monsoon season and annual

Fig. 2 and 3 show the time series of rainfall in mm for the months of June, July, August, September
and southwest monsoon season, annual respectively. The trend lines are also displayed for each of the
series. Neither monthly rainfall nor seasonal or annual rainfall shows any significant
increasing/decreasing trend. In the monthly rainfall August shows increasing trend while June, July and
September show decreasing trend. Seasonal rainfall shows no trend whereas annual rainfall shows slight
decreasing trend. During the last 30 years highest rainfall of June (127.6 mm) and July (155.1 mm)
received in the year 1991 and 1989 respectively while highest rainfall of 159.5 mm in August received in
the year 2015 and of 207.2 mm in September received in the year 2004. Highest annual rainfall of 1324
mm received in the year 2005 and highest southwest monsoon rainfall of 436.2 mm received in the year
1996.

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Fig. 2 Time series of rainfall in mm for the months of June, July, August, September and trends

Fig. 3 Time series of rainfall in mm for the southwest monsoon season and annual trends

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 4. District rainfall mean, variability and trend
4.1 Mean and coefficient of variation
Table 2 gives the rainfall statistics for the districts of Tamil Nadu for the four monsoon months,
southwest monsoon season and annual while Fig.4-5 shows the spatial pattern of these statistics. It can be
seen that the district in the western part of the state viz. Nilgiris receives highest rainfall over other districts
during the monsoon months, monsoon season and annual period. Rainfall received over this district is
around 174-216 mm in June, 219-272 mm in July and 164-200 mm in August. In September, in addition to
Nilgiris highest rainfall is also received in the northern districts of the state viz Vellore, Thiruvannamalai,
Kanchipuram and Dharmapuri ranging from 138-165 mm. During SW monsoon highest rainfall is over
Nilgiris and annual highest is over Nilgiris and Kanyakumari ranging from 696-1567 mm. Lowest rainfall
received during the SW monsoon season is over Thoothukudi district (61.8 mm) while Tirupur district
received lowest annual rainfall (600.3 mm).

According to Table 2 variance in rainfall is maximum for Ariyalur district in the months of June,
August, September and monsoon months. Perambalur district shows maximum variation in July, August,
monsoon months and annual scale.

Fig.4 shows the overall picture of mean rainfall over districts of state. Except for a couple of
districts in the western part of the state not much rainfall is received in the months of June and July. In the
month of August, the northern part of the state gets more rainfall (90-164 mm) as compared to the previous
months and Nilgiris and Coimbatore districts in the western part of the state gets highest rainfall. In
September not only northern parts but the central parts too get comparatively more rainfall. The month of
September has better rainfall over the entire state in the monsoon season. If we see monsoon seasonal
rainfall only the northern part of the state and a couple of districts in the west show rainfall activity.
Similarly for annual rainfall, only a few north and central coastal districts and a couple of districts in the
west show better rainfall activity as compared to the rest of the state.

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 JUNE JULY AUGUST SEPTEMBER MONSOON ANNUAL
DISTRICT
MEAN CV MEAN CV MEAN CV MEAN CV MEAN CV MEAN CV
CHENNAI 85.8 152 94.1 61 125.4 61 133.3 58 438.6 47 1352.7 53
COIMBATORE 164.8 44 222.8 51 164.3 53 121.8 59 673.6 39 1206.1 32
CUDDALORE 48.0 83 55.7 58 106.4 54 107.2 56 317.4 32 1156.4 34
DHARMAPURI 51.6 64 60.3 91 103.2 69 140.8 50 355.9 30 852.1 26
DINDIGUL 36.7 62 34.8 84 67.6 75 113.0 67 252.0 43 884.2 29
ERODE 28.7 81 33.8 112 56.6 60 93.0 62 212.1 49 653.4 31
KANCHEEPURAM 69.4 98 79.2 48 129.3 46 139.0 41 416.9 33 1215.5 33
KANNIYAKUMARI 185.4 70 120.0 62 88.3 95 121.5 77 515.3 40 1408.1 27
KARUR 17.2 88 23.0 127 49.7 70 83.8 60 173.7 38 609.9 54
MADURAI 41.0 66 53.8 67 82.8 51 106.6 49 284.3 35 842.9 28
NAGAPATTINAM 36.0 111 37.1 58 78.5 58 80.7 62 232.3 44 1293.2 44
NAMAKKAL 38.3 500 46.6 68 88.0 54 97.7 212 270.7 91 689.9 75
NILGIRIS 216.4 38 272.3 51 200.4 36 165.0 43 854.2 30 1567.1 25
PERAMBALUR 30.9 618 31.5 605 81.6 253 107.0 198 251.0 135 762.4 104
PUDUKOTTAI 40.5 55 48.8 52 85.4 63 96.4 56 271.1 34 798.3 31
RAMANATHAPURAM 14.1 107 19.0 89 29.7 81 50.3 81 113.0 56 805.3 33
SALEM 54.3 55 70.2 59 112.7 53 127.9 44 365.1 25 850.8 24
SIVAGANGA 46.1 68 59.5 55 91.7 52 103.5 57 300.7 33 851.5 27
TANJAVUR 35.2 70 44.9 58 92.7 61 98.2 53 271.0 37 979.7 31
THENI 61.3 60 74.4 61 71.1 62 90.9 57 297.8 43 869.5 62
TIRUCHIRAPALLI 26.3 98 39.2 84 71.3 66 105.3 59 242.2 35 715.8 31
TIRUNELVELI 39.6 83 31.3 88 26.9 115 37.8 83 135.6 56 841.8 33
THIRUVALLUR 75.4 108 90.9 47 116.6 54 132.7 37 415.5 33 1103.9 33
TIRUVANNAMALAI 63.1 72 80.8 55 127.8 58 155.8 44 427.5 30 994.5 45
THIRUVARUR 33.7 88 50.4 71 86.8 61 100.8 58 271.6 35 1154.7 51
THOOTHUKUDI 5.7 133 7.9 141 16.0 81 32.2 82 61.8 56 593.1 33
VELLORE 75.4 66 85.2 62 115.5 59 146.4 43 422.5 33 884.8 32
VILLUPURAM 44.2 64 60.6 54 108.1 50 118.7 46 331.6 32 918.7 62
VIRUDHUNAGAR 18.5 90 29.6 114 49.1 67 72.7 54 169.9 39 742.1 29
KRISHNAGIRI 46.8 75 59.7 86 95.2 63 136.5 59 338.1 32 782.8 49
ARIYALUR 36.3 529 43.1 449 95.5 295 103.6 277 278.5 121 860.1 63
TIRUPPUR 21.4 80 20.5 94 37.4 80 67.5 75 146.9 53 600.3 57

Table 2. Rainfall statistics for the districts of Tamil Nadu for the four monsoon months, southwest monsoon
season and annual

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Fig.4 Mean rainfall pattern over districts of Tamil Nadu

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Fig.5 Coefficient of Variation (%) over districts of Tamil Nadu

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4.2 Trend in district rainfall
Fig.6 shows the trend in district rainfall for (a) June, (b) July (c) August (d) September,
(e) JJAS and (f) annual. It can be seen that June rainfall shows significant decreasing trend in Madurai
district and most of the state shows non significant decreasing trend except for Theni, Perambalur and
Villupuram districts. For the July month significant decreasing trend has been noticed in Tirupur,
Madurai and Nagapattinam districts. In August, significant increasing trend is seen in Theni, Sivaganga,
Chennai and Thanjavur districts. Rest of the state too shows increasing trend in rainfall except for
Namakkal, Nilgiris and Coimbatore districts. September rainfall of Dharmapuri district shows
significant decreasing trend. During the whole southwest monsoon season Madurai and Dharmapuri
districts show significant decreasing trend. For the annual rainfall only one district viz. Madurai shows
significant decreasing trend.

a b

c d

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 e f
Fig.7 Trends in district rainfall for (a) June, (b) July (c) August (d) September (e) JJAS and (f) annual

5. Analysis of Average frequencies for rainfall events of different intensities

5.1 Average frequency of Rainy days

The average frequency of rainy days is calculated for Tamil Nadu for June, July, August,
September, June to September and Annual. Figure 7 shows that in the month of June the maximum number
of rainy days lies in the range of 8 to 9days in southern part of Coimbatore and western part of Nilgiris
districts. While minimum number of rainy days lies in the range of 2 to 3 days in Nagapattinam, Thanjavur,
Thiruvarur, Cuddalore, Ariyalur, Perambalur, Thiruchirapalli, Pudukottai, Karur, Dindigul and parts of
Tirupur, Namakkal, Viluppuram and Madurai districts. Whereas in the remaining districts, the number of
rainy days lies in the range of 3 to 8 days.

Figure 8 shows that in the month of July the maximum number of rainy days lies in the range of 9
to 11 days in some parts of Coimbatore and Nilgiris districts. While minimum number of rainy days lies in
the range of 2 to 4 days in Nagapattinam, Thiruvarur, Thanjavur, Ariyalur, Pudukottai, Thiruchirapalli,
Karur, Dindigul, parts of Tirupur and Cuddalore, northeastern parts of Theni, southern part of Namakkal
and Perambalur, and northern part of Madurai districts. Whereas in the remaining districts, the number of
rainy days lies in the range of 4 to 9 days.

Figure 9 shows that in the month of August the maximum number of rainy days lies in the range of
8 to 10 days in most parts of Nilgiris, southern part of Coimbatore, Kanyakumari, some parts of Tirunelveli,
Thoothukudi, Kanchipuram and Salem. While minimum number of rainy days lies in the range of 3 to 5

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days in some parts of most parts of Tirupur and Karur, some parts of Coimbatore, Dindigul, Theni,
Tiruchirapalli, Thanjavur and Nagapattinamdistricts. Whereas in the remaining districts, the number of
rainy days lies in the range of 5 to 8 days.

Figure 10 shows that in the month of September the maximum number of rainy days lies in the
range of 7 to 8 days inNilgiris, Kanyakumari, southern parts of Coimbatore and Tirunelveli and some parts
of the districts located in the northern part of the state. While minimum number of rainy days lies in the
range of 3 to 5 days in most parts of Tirupur and some parts of Coimbatore, Dindigul, Karur and
Nagapattinamdistricts. Whereas in the remaining districts, the number of rainy days lies in the range of 5
to 7 days.

Figure 11 shows that during June to September the maximum number of rainy days lies in the range of 29
to 34 days in Kanyakumari, southern parts of Coimbatore and Tirunelveli and some parts of Nilgiris and
Tiruvallur districts. While minimum number of rainy days lies in the range of 12 to 16 days in most parts
of Tirupur, Karur, Tiruchirapalli, Tiruvarur and Thanjavur and some parts of Dindigul, Nagapattinam,
Pudukottai, Theni, Madurai and Thoothukudi districts. Whereas in the remaining districts, the number of
rainy days lies in the range of 16 to 29 days.

Figure 12 shows that during the entire year the maximum number of rainy days lies in the range of
57 to 63 days in most parts of Nilgiris, southern part of Coimbatore and some parts of Salem districts. While
minimum number of rainy days lies in the range of 34 to 40 days in most parts of Karur, Tirupur and
Tiruchirapalli and some parts of Coimbatore, Dindigul, Pudukottai, Viluppuram and Thoothukudidistricts.
Whereas in the remaining districts, the number of rainy days lies in the range of 40to 57 days.

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 Fig. 7 Average frequency of rainy days: June Fig. 8 Average frequency of rainy days: July

Fig. 9 Average frequency of rainy days: August Fig. 10 Average frequency of rainy days: September

Fig. 11 Average frequency of rainy days: JJAS Fig. 12 Average frequency of rainy days: Annual

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5.2 Average frequency of Heavy rainfall days

The average frequency of Heavy rainfall days is calculated for Tamil Nadu for June, July, August,
September, June to September and Annual. Figure 13 shows that in the month of June the maximum number
of heavy rainfall days lies in the range of 0.5 to 1 day in Kanyakumari, western parts of Nilgiris and Theni,
southern parts of Coimbatore, southwestern parts of Erode and Tirunelveli districts. While minimum
number of Heavy rainfall days lies in the range of 0.1 to 0.14 days in the central part of the state comprising
most of Nagapattinam, Thanjavur, Tiruvarur, Ariyalur, Cuddalore, Perambalur, Tiruchirapalli, Karur and
Pudukottai and some parts of Dindigul, Madurai, Theni, Tirupur, Namakkal, Salem, Erode, Viluppuram
and Tiruvannamalai districts. Whereas in the remaining districts, the number of Heavy rainfall days lies in
the range of 0.1 to 0.5 days.

Figure 14 shows that in the month of July the maximum number of heavy rainfall days lies in the
range of 0.7 to 1 day in some of the districts in western parts of the state comprising of most parts of Nilgiris
and Kanyakumari, southern part of Coimbatore and Tirunelveli, southwestern part of Theni and western
parts of Erode district. While minimum number of Heavy rainfall days lies in the range of 0.1 to 0.2 days
in most parts of Tiruvallur, Chennai, Kanchipuram, Cuddalore, Nagapattinam, Tiruvarur, Tanjavur,
Pudukottai, Tiruchirapalli and Karur and some parts of Dindigul, Madurai, Tirupur, Namakkal, Salem,
Vellore and Thoothukudidistricts. Whereas in the remaining districts, the number of Heavy rainfall days
lies in the range of 0.2 to 0.7 days.

Figure 15 shows that in the month of August the maximum number of heavy rainfall days lies in
the range of 0.4 to 1 day in most parts of Kanyakumari, Tirunelveli, southern parts of Thoothukudi and
Coimbatore, southwestern parts of Theni and Erode and western parts of Nilgiris districts. While minimum
number of Heavy rainfall days lies in the range of 0.1 to 0.2 days in some parts of Nilgiris, Coimbatore,
Tirupur, Karur, Madurai, Pudukottai, Tiruvarur and Erode districts. Whereas in the remaining districts, the
number of Heavy rainfall days lies in the range of 0.2 to 0.4 days.

Figure 16 shows that during September the maximum number of heavy rainfall days lies in the
range of 0.4 to 1 day in western parts of Erode,some parts of Nilgiris, Coimbatore, Krishnagiri, Dharmapuri,
Salem, Vellore, Tiruvannamalai, Kanchipuram, Tiruvallur, Viluppuram, Cuddalore and Theni, districts.
While minimum number of Heavy rainfall days lies in the range of 0.1 to 0.2 daymainly in central part of
the state comprising of Coimbatore, Tirupur, Dindigul, Madurai, Erode, Pudukottai, Tiruchirapalli,
Tiruvarur and Nagapattinamdistricts. Whereas in the remaining districts, the number of Heavy rainfall days

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lies in the range of 0.2 to 0.4 days.

Figure 17 shows that during June to September the maximum number of heavy rainfall days lies
in the range of 1.8 to 3 days in most parts of Kanyakumari and Tirunelveli, some parts of Nilgiris,
Coimbatore and western parts of Erode, Theni and Thoothukudidistricts. While minimum number of Heavy
rainfall days lies in the range of 0.4 to 0.8 day mainly in central part of the state comprising of Nagapattinam,
Tiruvarur, Tanjavur, Pudukottai, Tiruchirapalli, Karur, Dindigul, Tirupur, Erode, Namakkal, Ariyalur and
Cuddaloredistricts. Whereas in the remaining districts, the number of Heavy rainfall days lies in the range
of 0.1 to 1.2 days.

Figure 18 shows that during the entire year the maximum number of heavy rainfall days lies in the
range of 3 to 4 days in Chennai, western parts of Nilgiris and Erode, southern part of Coimbatore, eastern
part of Tiruvallur, Cuddalore and Nagapattinam and southern part of Kanchipuram districts. While
minimum number of Heavy rainfall days lies in the range of 1 to 2 days mainly in central parts of the state..
Whereas in the remaining districts, the number of Heavy rainfall days lies in the range of 2 to 4 days.

Fig. 13 Average frequency of heavy rainfall days: Fig. 14 Average frequency of heavy rainfall days:
June July

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 Fig. 15 Average frequency of heavy rainfall days: Fig. 16 Average frequency of heavy rainfall days:
August September

Fig. 17 Average frequency of heavy rainfall days: Fig. 18 Average frequency of heavy rainfall days:
JJAS Annual

5.3 Average frequency of Dry days

The average frequency of dry days is calculated for Tamil Nadu for June, July, August, September,
June to September and Annual. Figure 19 shows that in the month of June the maximum number of dry
days lies in the range of 25 to 27 days in most parts of Cuddalore, Nagapattinam, Thiruvarur, Tanjavur,
Pudukottai, Karur, Tiruchirapalli, Perambalur, Tirupur and Dindigul and some parts of Madurai, Sivaganga,
Erode and Viluppuramdistricts. While minimum number of dry days lies in the range of 18 to 20 days in
most of the parts of Nilgiris and southern part of Coimbatore districts. Whereas in the remaining districts,
the number of dry days lies in the range of 20 to 25 days.

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 Figure 20 shows that in the month of July the maximum number of dry days lies in the range of 25
to 28 days mainly in central part of the state comprising of most parts of Nagapattinam, Ariyalur,
Thiruvarur, Tanjavur, Perambalur, Tiruchirapalli, Karur, Pudukottai, Dindigul, Tirupur and Cuddalore and
some parts of Sivaganga, Madurai, Namakkal ,Viluppuram, Erode and Thoothukudidistricts. While
minimum number of dry days lies in the range of 16 to 19 days in most part of Nilgiris and southern part
of Coimbatore districts. Whereas in the remaining districts, the number of dry days lies in the range of 19
to 25 days.

Figure 21 shows that in the month of August the maximum number of dry days lies in the range
of 25 to 27 days in most parts of Tirupur and some parts of Dindigul, Tiruchirapalli, Pudukottai, Tanjavur,
Tirunelveli and Nagapattinamdistricts. While minimum number of dry days lies in the range of 18 to 20
days in most parts of Nilgiris, southern part of Coimbatore and some parts of Chennai, Dindigul and Salem
districts. Whereas in the remaining districts, the number of dry days lies in the range of 20 to 25 days.

Figure 22 shows that in the month of September the maximum number of dry days lies in the range
of 24 to 25 days in most parts of Tirupur and some parts of Coimbatore, Dindigul, Pudukottai and
Tanjavurdistricts. While minimum number of dry days lies in the range of 19 to 20 days in most parts of
Nilgiris, southern part of Coimbatore and some parts of Dindigul, Salem and Chennai districts. Whereas
in the remaining districts, the number of dry days lies in the range of 20 to 24 days.

Figure 23 shows that during June to September the maximum number of dry days lies in the range
of 91 to 98 days in most parts of Nagapattinam, Cuddalore, Thiruvarur, Tanjavur, Ariyalur, Tiruchirapalli,
Karur, Tirupur and Pudukottai and some parts of Perambalur, Dindigul, Madurai and Namakkaldistricts.
While minimum number of dry days lies in the range of 67 to 74 days in most of the districts of Nilgiris
and southern parts of Coimbatore districts. Whereas in the remaining districts, the number of dry days lies
in the range of 74 to 92 days.

Figure 24 shows that in the month of during the entire year the maximum number of dry days lies
in the range of 284 to 293 days in most of the parts of Tirupur and Tiruchirapalli and some parts of Erode,
Namakkal, Ariyalur, Thanjavur, Pudukottai and Madurai districts. While minimum number of dry days lies
in the range of 248 to 257 days in most of the parts of Nilgiris and some parts of Coimbatore, Dindigul and
Tiruvallur districts. Whereas in the remaining districts, the number of dry days lies in the range of 257 to
284 days.

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 Fig. 19 Average frequency of dry days: June Fig. 20 Average frequency of dry days: July

Fig. 21 Average frequency of dry days: August Fig. 22 Average frequency of dry days: September

Fig. 23 Average frequency of dry days: JJAS Fig. 24 Average frequency of dry days: Annual

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6 Trends in the frequencies of different rainfall events

6.1 Trend in frequency of Rainy days

The Trend in frequency of rainy days is calculated for the rain gauge stations of Tamil Nadu for
June, July, August, September, June to September and Annual. Figure 25 shows that in the month of June
there is a significant increase in Rainy days in stations in Coimbatore, western part of Viluppuram,
Tiruvallur, northern part of Pudukottai and central part of Theni districts. Whereas rest of the districts show
a significant decrease in Rainy days in stations except Dharmapuri, Kanchipuram, Thiruvarur, Perambalur,
Dindigul and Virudhunagar which do not show any trend.

Figure 26 shows that in the month of July there is a significant increase in Rainy days in stations
in western part of Nilgiri, southern part of Theni, northern part of Pudukottai, EasatTiruchirapalli,
Cuddalore, Viluppuram, Tiruvannamalai, Vellore, Chennai and Tiruvallur districts. Whereas rest of the
districts show a significant decrease in Rainy days in stations except Krishnagiri, Dharmapuri, Namakkal,
Perambalur, Ariyalur, Nagapattinam, Virudhunagar, Ramanathapuram and Sivaganga which do not show
any trend.

Figure 27 shows that in the month of August there is a significant decrease in Rainy
daysinstationsinKanyakumari, Tirunelveli and east Erode districts. Whereas rest of the districts show a
significant increase in Rainy days in stations except Krishnangiri, Tirupur, Dindigul, Perambalur, Ariyalur,
Cuddalore and Ramanathapuram which do not show any trend.
Figure 28 shows that in the month of September there is a significant decrease in Rainy daysin
stationsinNilgiris, Coimbatore, western part of Theni, Tiruvallur, western part of Viluppuram, eastern part
of Tiruchirapalli, north Pudukottai, western part of Tirunelveli. Whereas rest of the districts show a
significant increase in Rainy days in stations except Namakkal, Ariyalur and Cuddalore which do not show
any trend.

Figure 29 shows that in the month of June to September there is a significant increase in Rainy
daysinstationsin Tiruvallur, Kanchipuram, Viluppuram, Tiruvarur, Tanjavur, western Sivaganga and Karur
districts. Whereas rest of the districts show a significant decrease in Rainy days in stations except
Dharmapuri, Tiruvannamalai, Cuddalore, Nagapattinam, Ramanathapuram, Virudhunagar and Namakkal
which do not show any trend.

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 Figure 30 shows that in the entire year there is a significant increase in Rainy days in Karur,
western part of Theni, east Nagapattinam, east Cuddalore, Chennai, Thiruvannamalai, Viluppuram, central
part of Salem districts. Whereas rest of the districts show a significant decrease in Rainy days in stations
except Dharmapuri, Perambalur, Ariyalur, Ramanathapuram and Virudhunagar which do not show any
trend.

Fig. 25 Trend in frequency of rainy days: June Fig. 26 Trend in frequency of rainy days: July

Fig. 27Trend in frequency of rainy days: August Fig. 28 Trend in frequency of rainy days:
September

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 Fig. 29 Trend in frequency of rainy days: JJAS Fig. 30 Trend in frequency of rainy days: Annual

6.2 Trend in frequency of Heavy rainfall days

The Trend in frequency of Heavy days is calculated for Tamil Nadu for June, July, August,
September, June to September and Annual. Figure 31 shows that in the month of June there is a significant
increase in Heavy rainfall days in Nilgiris districts. Whereas there is a significant decrease in Heavy rainfall
days in Tiruvallur, Vellore, Tiruvannamalai, Tiruchirapalli, Dindigul, Sivaganga, Madurai, Virudhunagar,
Tirunelveli and Kanyakumari districts. While remaining districts did not show any significant change.

Figure 32 shows that in the month of July there is a significant increase in Heavy rainfall days in
Vellore, Dharmapuri, Viluppuram, Perambalur, Pudukottai, Theni districts. Whereas there is a significant
decrease in Heavy rainfall days in Nilgiris, Coimbatore, Tirupur, Karur, Madurai, Tirunelveli, Cuddalore,
Kanchipuram, and Tiruvallur districts.

Figure 33 shows that in the month of August there is a significant increase in Heavy rainfall days
in Krishnagiri, Western part of Vellore, Thiruvallur, Kanchipuram, Tiruvannamalai, Viluppuram,
Dharmapuri, Salem, Erode, Tirupur, Theni, Kanyakumari, Tirunelveli, Northern part of Sivaganga,
Tiruchirapalli, Tanjavur, northern part of Nagapattinam districts. Whereas there is a significant decrease in
Heavy rainfall days in northern part of Vellore, Cuddalore, Virudhunagar, south Sivagangadistricts. While
remaining districts did not show any significant change.

Figure 34 shows that in the month of September there is a significant increase in Heavy rainfall

22
days in Vellore, Tiruvannamalai, Viluppuram, eastern part of Madurai, Karur, southern part of Coimbatore
districts. Whereas there is a significant decrease in Heavy rainfall days in Nilgiris, Erode, Krishnagiri,
Dharmapuri, Cuddalore, Salem, Tirupur, Dindigul, southern part of Tiruchirapalli, Tanjavur, Tiruvarur,
Sivaganga, Virudhunagar, Tirunelveli, Theni and western part of Madurai districts. While remaining
districts did not show any significant change.

Figure 35 shows that during June to September there is a significant increase in Heavy rainfall
days in Tirupur, Dharmapuri, Vellore, Kanchipuram, Thiruvannamalai, Viluppuram, Pudukottai and Theni
districts. Whereas there is a significant decrease in Heavy rainfall days in Nilgiris, Coimbatore, Tiruvallur,
Cuddalore, Tiruvarur, Sivaganga, Madurai, Kanyakumari and Tirunelveli districts. While remaining
districts did not show any significant change.

Figure 36 shows that in the entire year there is a significant increase in Heavy rainfall days in
Krishnagiri, Tiruvannamalai, Viluppuram, Pudukottai. Sivaganga and Thoothukudi districts. Whereas there
is a significant decrease in Heavy rainfall days in Nilgiris, Coimbatore, Tirupur, Vellore, Tiruvallur,
Cuddalore, Tiruvarur, Kanyakumari, Tirunelveli, Madurai, Dindigul, Karur and Coimbatore districts.
While remaining districts did not show any significant change.

Fig.31 Trend in frequency of heavy rainfall days: Fig. 32 Trend in frequency of heavy rainfall days:
June June

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 Fig. 33 Trend in frequency of heavy rainfall days: Fig. 34 Trend in frequency of heavy rainfall days:
August September

Fig. 35 Trend in frequency of heavy rainfall days: Fig. 36 Trend in frequency of heavy rainfall days:
JJAS Annual

6.3 Trend in frequency of Dry days

The Trend in frequency of dry days is calculated for Tamil Nadu for June, July, August, September,
June to September and Annual. Figure 37 shows that in the month of June there is a significant decrease in
dry days in northern part of Pudukottai, eastern part of Virudhunagar, western part of Coimbatoredistricts.
Whereas there is a significant increase in dry days in remaining districts except Ariyalur, Tiruvarur and
Nagapattinam which do not show any significant change.

Figure 38 shows that in the month of July there is a significant decrease in dry days in Krishnagiri,
Tiruvallur and Tiruvannamalai districts. Whereas there is a significant increase in dry days in remaining
districts except Vilippuram, Cuddalore, Perambalur and Ariyalur districts which do not show any
significant change.

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 Figure 39 shows that in the month of August there is a significant increase in dry days in Nilgiris,
Coimbatore, Tirupur, Kanyakumari, south Tirunelveli and Tirupur districts. Whereas there is a significant
decrease in dry days in remaining districts except Krishnagiri, Tiruvallur, Dharmapuri, Nagapattinam,
Dindigul, Ramanathapuram and Virudhunagar which do not show any significant change.

Figure 40 shows that in the month of September there is a significant decrease in dry days in
Nilgiris, Coimbatore Tiruvallur, eastern part of Virudhunagar, western part of Tirunelveli and western part
of Theni districts. Whereas there is a significant increase in dry days in remaining districts except Cuddalore
and Ariyalur districts which do not show any significant change.

Figure 41 shows that in the month of June to September there is a significant decrease in dry days
in Nilgiris, western part of Coimbatore, Theni, Tiruvallur, Tiruvannamalai and Cuddalore districts. Whereas
there is a significant increase in dry days in remaining districts except Perambalur, Ariyalur, Tiruvarur,
Sivaganga and Erode districts which do not show any significant change.

Figure 42 shows that during the entire year there is a significant decrease in dry days in western
part of Pudukottai and Tiruvarur districts. Whereas there is a significant increase in dry days in remaining
districts except Perambalur, Ariyalur and Tiruvallur districts which do not show any significant change.

Fig. 37 Trend in frequency of dry days: June Fig. 38Trend in frequency of dry days: July

25
 Fig. 39 Trend in frequency of dry days: August Fig. 40 Trend in frequency of dry days:
September

Fig. 41 Trend in frequency of dry days: JJAS Fig. 42 Trend in frequency of dry days: Annual

7. Conclusions

In the present study we have investigated the rainfall pattern and it its variability and also changes based
on recent 30 years data. In the analysis we have considered monsoon months, the monsoon season and annual
scale. The spatial scale has been considered from state to district for study of rainfall total and stations are being
considered for seeing intensities of rainfall. The analysis brought many significant features of rainfall pattern and
can be used for water agricultural managements. Some of the important results can be summarized as:
Tamil Nadu gets maximum rainfall in September (34% of SW monsoon rainfall) followed by
August (28 % of SW monsoon rainfall).

26
35% of annual rainfall received during southwest monsoon rainfall (June –September).
Nilgiris district receives 55% of annual rainfall in SW monsoon season while Thoothukudi district
receives 10% of annual rainfall in SW monsoon season.
June rainfall shows significant decreasing trend in Madurai district
For the July month only significant decreasing trend has been noticed in Nagapattinam Tirupur
and Madurai districts.
Theni, Sivaganga and Thanjavur districts show significant increasing trend in August rainfall.
September rainfall of Dharmapuri district shows significant decreasing trend.
During the whole southwest monsoon season Dharmapuri and Madurai districts show significant
decreasing trend.
For the annual rainfall only one district viz. Madurai shows significant decreasing trend.
Maximum rainfall received during the SW monsoon season over Nilgiris district (696 mm - 854
mm) while Erode, Tirupur, Tiruchirapalli, Karur, Tirunelveli, Virudhunagar, Thoothukudi and
Ramanathapuram districts received lowest rainfall (62 mm-220 mm)
Maximum rainfall received during the year over Nilgiris and Kanyakumari districts (1372 mm-
1567 mm) while Krishnagiri, Erode, Tirupur, Namakkal, Karur, Tiruchirapalli, Perambalur,
Virudhunagar and Thoothukudi received lowest annual rainfall (593 mm-788 mm).
Maximum number of rainy days lies in the range of 29 to 34 days in Kanyakumari, southern parts
of Coimbatore and Tirunelveli and some parts of Nilgiris and Tiruvallur districts (daily rainfall
>=2.5mm) out of 122 days of SW monsoon season While minimum number of rainy days lies in
the range of 12 to 16 days in most parts of Tirupur, Karur, Tiruchirapalli, Tiruvarur and Thanjavur
and some parts of Dindigul, Nagapattinam, Pudukottai, Theni, Madurai and Thoothukudi districts
Heavy to extremely heavy rainfall ( daily rainfall >=6.5mm) days lies in the range of 1.8 to 3 days
in most parts of kanyakumari and Tirunelveli, some parts of Nilgiris, Coimbatore and western parts
of Erode, Theni and Thoothukudi districts during the SW monsoon season. While minimum
number of Heavy rainfall days lies in the range of 0.4 to 0.8 day mainly in central part of the state
comprising of Nagapattinam, Tiruvarur, Tanjavur, Pudukottai, Tiruchirapalli, Karur, Dindigul,
Tirupur, Erode, Namakkal, Ariyalur and Cuddalore districts.
Maximum number of dry days lies in the range of 91 to 98 days out of 122 days during the SW
monsoon season in most parts of Nagapattinam, Cuddalore, Thiruvarur, Tanjavur, Ariyalur,
Tiruchirapalli, Karur, Tirupur and Pudukottai and some parts of Perambalur, Dindigul, Madurai
and Namakkal districts. During the entire year the maximum number of dry days lies in the range
of 284 to 293 days out of 365 days in most parts of Tirupur and Tiruchirapalli and some parts of
Erode, Namakkal, Ariyalur, Thanjavur, Pudukottai and Madurai districts.
During the period June to September there is a significant increase in the frequency of Rainy days
in stations in Tiruvallur, Kanchipuram, Viluppuram, Tiruvarur, Tanjavur, western Sivaganga and

27
 Karur districts.
During the entire year there is a significant increase in Rainy days in Karur, western part of Theni,
east Nagapattinam, east Cuddalore, Chennai, Thiruvannamalai, Viluppuram and central part of
Salem districts.
During the period June to September there is a significant increase in Heavy rainfall days in
Tirupur, Dharmapuri, Vellore, Kanchipuram, Thiruvannamalai, Viluppuram, Pudukottai and Theni
districts. Whereas there is a significant decrease in Heavy rainfall days in Nilgiris, Coimbatore,
Tiruvallur, Cuddalore, Tiruvarur, Sivaganga, Madurai, Kanyakumari and Tirunelveli districts.
During the entire year there is a significant increase in Heavy rainfall days in Krishnagiri,
Tiruvannamalai, Viluppuram, Pudukottai. Sivaganga and Thoothukudi districts. Whereas there is
a significant decrease in Heavy rainfall days in Nilgiris, Coimbatore, Tirupur, Vellore, Tiruvallur,
Cuddalore, Tiruvarur, Kanyakumari, Tirunelveli, Madurai, Dindigul, Karur and Coimbatore
districts.
During June to September there is a significant decrease in dry days in Nilgiris, western part of
Coimbatore, Theni, Tiruvallur, Tiruvannamalai and Cuddalore districts.
During the entire year there is a significant decrease in dry days in western part of Pudukottai and
Tiruvarur districts

Acknowledgement:

The authors acknowledge Secretary, MOES, DGM, India Meteorological Department and Head,
Climate Research and Services for guidance, suggestions and encouragement to carry out the works.
Acknowledge also to Hydrology section and National Data Centre of India Meteorological department Pune for
making availability of the data.

References:

1. Guhathakurta P, Rajeevan M, Sikka D R and Tyagi A, 2015 “Observed changes in southwest monsoon
rainfall over India during 1901–2011”, Int. J. Climatol, 35, 1881-1898.
2. Guhathakurta P, Sreejith O P and Menon P A, 2011 “Impact of climate changes on extreme rainfall
events and flood risk in India”, J. Earth System Science. 120, No. 3, 359-373.
3. Guhathakurta P and Rajeevan M, 2008 “Trends in rainfall pattern over India”, International J. of
Climatology, 28: 1453–1469.

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 The report brings out observed rainfall variability
and trends over the state as an impact of climate change based
on recent 30 years of data ( 1981 - 2018 )

Rainfall pattern of monsoon months, south west monsoon season

and annual of the state and it’s districts as well as extreme
rainfall event of different intensity of stations are
analysed.

Observed Rainfall Variability and Changes over

Andhra Pradesh State

PREPARED AND DESIGNED AT

HYDROMETEOROLOGY SECTION
CLIMATE APPLICATION AND USER INTERFACE GROUP
CLIMATE RESEARCH AND SERVICES, PUNE