TRENDS IN MADRAS RAINFALL

Jn

his

valuable paper on “Secular Changes of Tropical Rainfall Régimes” Kraus

(1955)

discussed the changing seasons of south-east India and particularly based s o m e of his conclusions on a n analysis of the rainfall

of

Madras.

H e remarked that whilst the annual rainfall of Madras

during

the period examined never fluctuated sígnifi- cantly the picture changes when the annual records are broken down. Kraus considered residual mass curves for the months of “earlyrain” i.e., May-July and for the main rainy period October-December. T h e change revealed in the m e a n rainfall is m u c h larger in

May- July during

the south-west monsoon and the curves (Figs.

4

and

5

of his paper) for the two periods show opposing trends. Kraus therefore suggested the possi-

bility

that the same influence which caused the south- west monsoon rain to decrease

may

have caused the north-east monsoon rains to increase.

Rainfall data are available for Madras from

1813

(Fig.

1).

Recently, a detailed statistical study of the monthly and annual rainfall

of

Madras has been completed (see

Fig. 1).

O n e of the items examined was the fluctuation in annual/seasonal rainfall

during

the last

140

years. T h e method used for the study of trends was M e r e n t from that employed

by

Kraus.

M a y has been treated

by

Kraus as a month of the south-west monsoon, but the normal date of onset of the south-west monsoon in Madras is about June.

Whalever the amount of rainfall

in

the month

of May

(it is only about

4

per cent of the annual rainfall), it cannot really be regarded as a month of the south-west monsoon. M a y is a “pre-monsoon” month and storms from the B a y of Bengal give rainfall

in

this area. Table

1

gives the averages, extremes, etc., for M a y to

July.

The

period of the data considered

is 1813

to

1955.

50

Climatic changes in India

interest.

A

comparison of the

14

decadal means from

1813

to

1952

is shown

in Table 3.

T A B L E

3. Comparison of 14 decadalmeansfrom 1813 to 1952

M a y to July June to July

Highest decadal average (in.) 11.38 5.95

yo

of mean 152 142

Lowest decadal average (in.) 4m.86 4.30

yo

of mean 65 70

June plus

July

has less variation than

May-July.

Moreover, June and

July

are only two of the south-west monsoon months (which comprise June to September).

T h e period from October to November is regarded as the north-east monsoon season. T h e average, extremes, etc., of rainfall are given

in

Table

4.

T h e frequency distribution of October-December seasonal rainfall is not significantly M e r e n t from normal. T h e decadal averages for this season have been compared

with

the m e a n €or the entire period

1813

to

1955.

N o n e of the

14

decadal means is signiñcantly m e r e n t from the m e a n for the entire period. Fitting of polynomials

up

to the third degree shows no trend

in

the series.

The

variations of the decadal averages are from

77

per cent to

13

per cent of the mean. T h e varia- tions are m u c h smaller than

in

May-July and the rainfall of May-July is only about

25

per cent of that of October- December.

In Table

1

above, the rainfall of June and

July

has been given. Averages and extremes for the remaining months and for the south-west monsoon season are given inTable

5.

A

comparison of Tables

4

and

5

is of interest. T h e variations

in

the south-west monsoon period are m u c h smaller than

during

October to December. T h e coeffi- cient of variation for the south-west monsoon season (June-September) is only

75

per cent of that for October- December although

in

the latter period the rainfall is nearly double that in the former. T h e combined rainfall for May-July and October-December accounts for only

75

per cent of the annual amount. These points are mentioned

in

order to emphasize that it would b e more appropriate, for detecting trends, to consider the two

T A B L E

4. October to December

October

-

December

October November December

Mean (in.)

Changes of climate

I

Les changements de climat

90 80 70 60

5o Normal 49.92 40

30 20

1810 1825 1850 1875 1900 1925 1950

60 50 40 30 20 IO

Normal 31.45

T A B L E

5. June to September of formulating tests for

highly

skew series.

In

view

of

the above

the

residual mass curve as a tool for the study A'mst c~$I;~w of secular variations of rainfall becomes of doubtful utility and, remembering Professor Barnard's comments, M e a n (in.) 4.65 4.74 14.98 it should

be

used with the greatest caution.

A

detailed M e a n as

yo

of annual 9.4 9.6 30.3 paper on Madras rainfall is being published separately.

T h e annual and seasonal rainfall of Madras are shown (Mean

-

median)

x

100 9 15 5

T h e above study also raised the question whether Highest (as

Yo

of mean) 285 314 226

26 each of the monthly values of rainfall should be

indi-

Mdually examined for trends and changes in a country like India where rainfall

is

so characteristically seasonal.

M a n y of the months receive hardly any rainfall. The following is further illustrative of the very

highly

variable character of monthly rainfall

in

the less rainy months of the year.

Great care is therefore necessary

in

drawing conclu- sions about changes

and

trends

in

rainfall, w h e n only monthly values, particularly for non-monsoon months, are considered.

M e a n

in Fig. 1.

Lowest (as

yo

^{of mean)} 15 10

Coefficient of variation

(yo)

53 51 33 important seasons, the south-west monsoon (June- September) and the north-east monsoon season (October- December), separately and together rather than to compare a period like May-July with October-December.

This study of trends and of changes in the rainfall of Madras does not support the conclusions of Kraus regardhg the significance of the variations in M a y -

July

and October-December. It shows the difficulty

T A B L E

6. Coefficient of variation

(%)

January February March April May June July A u y s t September October November December

Assam a7 52 59 30 30 19 ia 18 24 41 80 105

Gujarat 271 250 250 150 226 62 47 59 74 155 150 200

Madras Deccan 207 241 221 81 61 41 52 63 45 56 83 172

52

Climatic changes in India

II. R A I N F A L L

bY

K. N. R A O

and

P.

J A G A N N A T H A N

INTRODUCTION

In

Part I, a brief review was made of the different methods which are most generally employed in studies on climatic changes and trends.

The

results obtained

by

Kraus

(1955)

for Madras rainfall by using the method of residual mass curves were compared with those obtained

by

other methods and some of the limitations of the former method were referred to. In this part, the results of studies on climatic changes

in

Indian

rainfall

with

particular reference to the a n d zones of the country are presented.

India is a vast country and is fortunate

in

having more than

900

stations

with

rainfall records for

80

years or more.

The

series of seasonal and annual rainfall from these stations can be considered to be broadly h o m o - geneous. T h e total number of rain gauges

in

the country exceeds

3,000. A

very large proportion of these have records for more than

50

years. It is possible there- fore to present a seasonally reliable picture of the

I _..

FIG.

1. Mean annual rainfall (in inchea) of India.

53

Changes of climate

1

Les changements de climat

FIG.

2. Coefficient of variation of annual rainfall Standard deviation

Mean

x

100

distribution of rainfall in the country

in

space and time.

T h e average or m e a n annual rainfall varies from less than

5

in.

in

parts

of

western Rajasthan to over

400 in.

inthe Khasi and Jaintia

H i l l s ,

Chenapunji

in

the latter having an average of

423 in.

T h e Western Ghats, the source of m a n y south Indian rivers, receives the

full

force of the south-west monsoon and has a n average of

200-300 in.

Considering stations

with

elevations less than

3,000

ft. above sea level, the annual average rainfall for the country is about

42 in.

Not only is Indian rainfall characterized

by

wide variations

in

spatial distribution but the variations from year to year are also very large and can be most marked over large tracts of the country.

Taking the coefficient of variation as a measure of

I

Standard deviation

x loo),

it exceeds

(

^{M e a n}

variability

30

per cent 'over large parts of the country. Its value exceeds

50

to

60

per cent

in

parts of Saurashtra and

Kutch and West Rajasthan.

It

m a y

be

interesting to remark that many of these areas get

very

heavy falls of rain of

15

in. or more in 24 hours. Dharampur, a plain station in Surat district of Gujarat recorded

39 in.

in 24

hours, probably a record for a plain station any- where

in the

world. T h e principal features

of

the rainfall of India are shown

in

Figs.

1, 2

and 3.

Except over certain southern areas, most of the country receives more than

70

per cent of the annual rainfall during the south-west monsoon season (reckoned from June to September). T h e west coasts north

of

latitude

WN.,

Gujarat and Rajasthan, get

80

to

90

per cent of the annual rainfall during the south- west monsoon season.

West Rajasthan, Kutch and the western

half

of Saurashtra

get

less than

20 in.

rainfall annually. These

20 20

o

I

1 .

FIG.

3 (u).

Rainfall

during south-west monsoon (June to

Sep-

tember) in each of the sub-divisions and its variability.

L V U

FIG.

3 (b). South-west moneoon (June to September) rainfall aa pcrcentage of annual.

54

Climatic changes in India

a

FIG.

4.

Rainfall

in Rajasthan. (u) Normal annual rainfall. (b) Coefficient of variation

(%).

b

areas m a y be regarded as the main arid zones of the country. There are small pockets

in

the Deccan Plateau which have an annual rainfall of

18-20 in.

In fact, a good portion of the Deccan Plateau gets less than

25 in.

annually. These areas could

be

classed as semi- arid.

With

this very

brief

introduction of Indian rainfall, w e

will

now present the main results of studies on climatic changes (rainfall)

in

Rajasthan, Gujarat and other areas of the country.

Dans le document CHANGEMENTS DE CLIMAT OF CLIMATE (Page 59-64)