RAINFALL OF RAJASTHAN

The

area of Rajasthan is

130,000

square miles. T h e average annual rainfall is

21 in.;

East Rajasthan has an average of

27 in.

and West Rajasthan has

11 in.

Over

90

per cent of the annual rainfall is received

in

the four months, June to September and varies from less than

5

in. in the extreme west of Rajasthan to more than

30 in. in

the east. The highest values of the coesi- cient of variation are

in

Rajasthan; it varies from

30

per cent

in

the extreme south-east to over

70

per cent

in

Jaisalmer district. Both the rainfall and its variability are shown

in Fig. 4. For

a few stations the standard deviation is as

high

as the average rainfall itself. T h e coefficient of variation of south-west monsoon rainfall is

28

per cent.for East Rajasthan and

49

per cent for West Rajasthan.

The

monthly values are given

in

Table

1.

The

question of trends and changes of climate

with

reference to rainfall has been examined

by

a number of methods referred to below.

C

(c) Differences between 1940 and 1920 normals of districts

(= +

^.0002).

C O M P A R I S O N O F DISTRICT N O R M A L S

This method has been used

by

s o m e workers as a test for climatic change. For comparison, it is essential that the same set of stations should be used.

The

comparison given

in

Table

2

is between averages based on data

up

to

1920

and

up

to

1940.

55

Changes of climate

I

^Les changements de climat

TABLE

1. Monthly values

January February March April M a y June July August September October November December

W e s t Rajasthan

Average (in.) 0.13 0.21 0.14 0.10 0.28 1.13 3.43 3.93 1.44 0.17 0.06 0.10 Coefficient of

variation(%) 223 152 157 170 182 96 59 76 147 253 333 180 East Rajasthan

Average (in.) 0.29 0.25 0.21 0.13 0.39 3.07 8.94 8.45 4.13 0.56 0.20 0.21 Coefficient of

variation(%) 107 152 162 231 139 63 38 50 78 150 145 171

TABLE

2. Comparison of 1920 and 1940 normals

Numerical m e r e n c e Frequency Numerical difference

(%

of mean) Frequency

0.01-0.10 0.11-0.20 0.21-0.30 0.31-0.41 0.41-0.50 0.51-0.75 0.76-1.00

1

7 0.1-1 13

4 2 4

5 3 4

O 4 2

3 5 O

2 5 2

2 2

T h e average difference taking sign into account is

0.002 in.

whilst the average numerical value of the differ- ence is

0.37 in.

Fifty per cent of the differences are less than

0.23 in.

Although the number ,of negative differ- ences is

16,

most of them are small. T h e m e a n difference as percentage of the normal is

only 0.7

per cent. T h e district Merences are shown

in Fig. 4.

T h e differences in rainfall normals of

1920

and

1940

can be regarded as small and negligible.

C O M P A R I S O N O F

1901

T O

1930

A V E R A G E S WITH T H O S E F O R

1901

T O

1950

Uniform averages for the period

1901-50

were prepared for all the rain-gauge stations

in

Rajasthan. For each station, the difference

1901-30

minus

1901-50

w a s examined

by

the test mentioned in the section ‘ T o m - parison of means of different periods” of Part

I.

It is interesting to observe that for only

6

of the

87

stations was the difference statistically significant. Out of these, for only one w a s the clifference significant at the

1

per cent level of significance. Similar results were obtained for differences between averages for

1901-40

and for

1901-50.

These analyses do not suggest any significant change

in

the annual rainfall of the stations in Rajasthan during

1901

to

1950.

T h e results of analysis for t w o districts are shown in Table!

3.

S T U D Y O F T R E N D

Linear regression equations have been fitted to the data of one or two stations

in

each district with records extending to over

45

years, the equation being

R =

by

+

Const., where

R

is rainfall,

y =

year and

b

is the regression coefficient.

Of

the

50

stations for which this method w a s tried, only t w o showed signifi- cance at

5

per cent level.

This

method was also tried for seven stations with records for longer periods (see Table

4).

Only

the regression coefficient of Partabsar was found to be significant.

For,

Jaipur

(1871-1950),

orthogonal polynomials have been fitted

up

to the

fifth

degree (Pramanik and Jagannathan,

1953).

T h e results are as follows: Mean:

24.412 in.;

Regression coefficients:

-2.48 x loF2, 1.90 X 0.90 x 3.37 x lo-) 0.12 x lo-‘ a;

square roots of variance contributed

by

different degrees of polynomial:

6.11, 2.54, 0.060, 5.86, 1.48

E

;

polynomial as a whole,

3.45,

residual

9.68.

None of the regression coefficients is significant.

The analysis of the data of individual stations also suggests that there isno trendinthe rainfall of Rajasthan.

In

this connexion, the monsoon seasonal rainfall of the two sub-divisions East and West Rajasthan has also been examined. T h e frequency distributions of the series were tested for normality

by

Fisher’s g1 and

gz

statistics and the homogeneity of the decadal variances

by

the

M

statistic (see Table

5).

None of these figures indicates any statistically signi- ficant trend and the frequency distributions of seasonal rainfall of East and W e s t Rajasthan are not significantly different from normal. The means for different decades were also compared.

For East Rajasthan, the decade

with

the highest mean,

115.9

per cent of normal, is

1941-50.

Comparing it

with

the mean for

1901-50,

w e get t

= 1.53

with

78

degrees of freedom.

This

is not significant even at

5

per cent level of significance.

For West Rajasthan, the decade with the highest mean,

116

per cent of normal, is

1926-35.

T h e lowest decadal mean is

83

per cent of normal, for

1896-1905.

56

Climatic changes in India

TABLE

4. Trend study of seven stations

deviation Regression coefficient F Mean

Changes of climate

/ Les

changements de climat

T A B L E

5. Frequency distribution of seasonal rainfall

East Rajasthan West Rajasthan

g, with standard error 0.02

&

0.27 0.16

&

0.27 g, with standard error 0.64

&

^{0.53 0.90}

&

^0.53

M

9.7 11.7

Comparing these

with

the mean for

1875

to

1955,

w e get t

= 1.656

and t

= 1.5

which are not significant.

T h e difference between the means for the two periods

1875-1924

and

1925-46

is

1.27 in.;

t

= 1.002

and is not significant even at

5

per cent level.

T h e linear regression coe5cients are

ó = 0.029

and b

= 0.0007

for East and West Rajasthan respectively.

Neither of these is significant.

A

brief discussion of the time intervals between years w h e n the monsoon rainfall of East Rajasthan was less than

80

per cent of the normal is n o w given. There were less. Three of the intervals are greater than

9

years,

the

highest being

15.

Assuming that

the

intervals follow an exponential law and following the test used

by

Maguire and others

in 1952,

w e

find

the ratio of the largest of time intervals (tn) to

t (= ^{! ! = 0.197)}

is less than the significant value at

5

per cent. T h e longest interval

of 15

years is thus not significant.

To

find

out

if

there is any significant tendency for intervals to succeed one another

in

groups, the statistic

M

cent and hence not significant.

Thus a critital examhation of the rainfall of Rajas- than,

by

considering the data of districts, of individual stations and of the two divisions, East and West Rajasthan, does not show that there has been any significant change or trend

in

the annual/seasonal rainfali of Rajasthan during the past

80

years for which data are available. Table

7

gives the monsoon seasonal rainfall for East and West Rajasthan from

1875

to

1955.

Dans le document CHANGEMENTS DE CLIMAT OF CLIMATE (Page 64-67)