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Observed Changes in Rainfall Pattern

Trend and Variability in All India Rainfall

Descriptive statistics along with linear trend (in mm/decade) of all India area weighted rainfall during the SW monsoon season (June-September) for the period 1901-2010 are given in Table 1. All India mean south-west monsoon rainfall is 887 mm which is 75 % of the annual rainfall. Around 11 and 10 % of the annual rainfall is received during the pre-monsoon and post-monsoon seasons, respectively. Coefficient of variability is about 10 % for the south-west monsoon whereas maximum variability is about 34 % during the winter season. No significant trend in all India rainfall is observed in any of the four seasons as well as for the annual rainfall. However, increasing trend in the all India south-west monsoon (June- September) rainfall is seen during the period 1901-50 and insignificant decreasing trend during the period 1951-2011 as shown by Guhathakurta et al. (2015).

Figure 1 shows the 36 meteorological subdivisions of India along with the four homogeneous regions. Figure 2 shows the all India south-west monsoon rainfall during the period 1901-2010 and also the all India south-west monsoon rainfall of IITM based on the fixed network of 306 stations. The variations in the IMD and IITM time series are almost the same. The 31-year moving average of the all India south-west monsoon rainfall is shown in Fig. 3. The all India monsoon seasonal rainfall shows significant multi-decadal variation with epochs of above normal and below normal rainfall. Out of the four homogeneous regions, only north-east India

Table 1 Descriptive statistics of all India rainfall

Winter

Pre-monsoon

Monsoon

Post-monsoon

Annual

Months

JF

MAM

JJAS

OND

Mean (mm)

42.2

128.5

887.0

121.9

1179.6

% of annual rainfall

3.6

10.9

75.2

10.3

100

Standard dev (mm)

14.2

21.9

87.1

32.2

107.1

Coeff. var (%)

33.7

17.1

9.8

26.4

9.1

Maximum (mm)

  • 89.2
  • (2005)
  • 210.7
  • (2005)
  • 1084.3
  • (1917)
  • 206.1
  • (1956)
  • 1463.9
  • (1917)

Minimum (mm)

  • 11.6
  • (1902)
  • 83.5
  • (1910)
  • 674.3
  • (1972)
  • 52.7
  • (1908)
  • 933.8
  • (2002)

Figures in bracket indicate year of occurrence for maximum in green colour and minimum rainfall value in red colour.

meteorological subdivisions of India and the four homogeneous regions, viz

Fig. 1 36 meteorological subdivisions of India and the four homogeneous regions, viz. north-west India, central India, north-east India and peninsular India considered by the India Meteorological Department (IMD)

All India south-west monsoon rainfall (as % departure from mean) time series for the period, 1901-2010 with IMD data (red in colour) and IITM data (blue in colour) (color figure online)

Fig. 2 All India south-west monsoon rainfall (as % departure from mean) time series for the period, 1901-2010 with IMD data (red in colour) and IITM data (blue in colour) (color figure online)

year moving average of all India south-west monsoon rainfall as per cent departure from the long-term mean. The year shown is the central year of the 31-year period

Fig. 3 31-year moving average of all India south-west monsoon rainfall as per cent departure from the long-term mean. The year shown is the central year of the 31-year period

(NEI) has shown significant decreasing trends in SW monsoon rainfall during the period, 1901-2010 as well as during the period, 1951-2010 (not shown).

Epochal average rainfall for 30 years and trends in each of the 30 years epoch of the monsoon rainfall averaged over the country as a whole are shown in Table 2. The epoch 1901-1930 was found to be drier than normal. The decadal means of all India and the four homogenous regions of India are shown in Fig. 4. All India SW monsoon rainfall was in dry epoch during the first two decades (1901-10, 1911— 20). However, the next five decades (1921-30, 1931-40, 1941-50, 1951-60 and 1961-70) were in epochs with above normal rainfall. The present dry epoch started from the period 1971-1980. The presence of multi-decadal variation was also seen in the monsoon rainfall of the four homogeneous regions. Rainfall deficiency was the largest during the decade 2001-2010 for NE India.

Table 2 All India SW monsoon rainfall during 30 years epoch

All India south-west monsoon rainfall in mm

30-year period

  • 1901—
  • 30
  • 1931
  • 60
  • 1961
  • 90
  • 1991
  • 2010

Epochal average in mm

868.4

929.6

881.0

861.9

Trends (slope in mm per year) in 30 years epoch

4.03*

0.23

-0.32

-2.21

The mark * denotes statistically significant trends

Decadal means of south-west monsoon rainfall (percent departure from mean) for all India and the four homogeneous regions of India

Fig. 4 Decadal means of south-west monsoon rainfall (percent departure from mean) for all India and the four homogeneous regions of India

 
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