Meteorological drought occurrences at Shillong, Meghalaya

· Articles
Authors

LALA I.P. RAY*, P.K. BORA, A.K. SINGH, RAM SINGH, N.J. SINGH & S.M. FEROZE

College of Postgraduate Studies, Umroi Road, Barapani-793103, Meghalaya
*Corresponding Author: Lala I.P. Ray, email: lalaiswariprasadray@yahoo.co.in

Abstract

Shillong, the capital of Meghalaya receives an annual average rainfall of 2,267 mm. The state is mostly agrarian with mostly rice-potato and potato-potato based cropping system. Rainfed rice mostly practiced in and around this belt. Rainfall analysis was done based on thirty five years (1973-2007) daily rainfall data to study monthly, seasonal and yearly occurrence of drought at Shillong based on India Meteorological Department (IMD) protocols. There was an incident of severe drought in the year 2000; eight years with moderate and eleven years with mild drought at Shillong. The more frequency of drought incidents were recorded for the January, November and December compared to other months.

Keywords Drought year, meteorological drought, rainfall analysis.

Introduction

Monsoon mostly decides the agrarian economy of the Meghalaya where more than 60% of the cultivable area is rain-fed. The tribal farmers of this area mostly adhere to rice based mono-cropping. A major share of the rainfall (more than 70%) occurs during June to September with an uneven distribution (Dhar et al., 1979).

Extreme conditions of rainfall are also observed in certain years. Floods and droughts are the two extremes of rainfall distribution. Deficiency of rainfall is the basic cause of drought. As such no general method is available which can be applied for the drought prediction (Salas, 1986). Depending on the climate, drought varies from place to place. Several workers have done meteorological analyses based on rainfall data (Kumar & Kumar, 1989; Dabral, 1996; Satapathy et al., 1998 and 1999; Marathe, 2001; Ray et al., 2012a, 2012b, 2013a, 2013b; Sahoo, 2013; Singh, 2013). Sharma et al. (1979, 1987a and 1987b) analyzed the rainfall using the definition of drought month as a month in which the actual rainfall is less than 50% of the average monthly rainfall. Drought year is the year receiving rainfall less than or equal to the average rainfall minus twice standard deviation of the series. Shrivastava et al. (2008) used this definition to assess meteorological droughts in North Lakhimpur district of Assam. Sinha (1986) and Ray et al. (1987) used this definition to study the drought at Gopalpur, Odisha. Kumar and Kumar (1989), Dabral (1996) and Ray et al. (2012a) analyzed the weekly, monthly, seasonal and yearly rainfall of Pantnagar, Ranchi and Barapani respectively for drought; as per the procedure followed by Sharma et al. (1979) and Dhar et al. (1979). Tiwari et al. (2007) characterize the meteorological drought indices using the data of Hazaribagh station. Similar analysis has been done by various researchers for meteorological drought analysis at various places in India. In the present paper an attempt has been made to study the frequency of drought occurrence at Shillong in terms of rainfall deficiency. Shillong and its surrounding belts is the major potato contributing region of Meghalaya.

Materials and Methods

The study place, Shillong is located at 91º 88? E Longitude and 25º 57? N Latitude with an altitude of 1,496 m above mean sea level. The behavioral pattern of rainfall with reference to the amount of rainfall and number of rainy days at Shillong were analyzed using probabilistic approach from historic daily rainfall records (1973-2007).

The monthly rainfall, seasonal rainfall (i.e. June to September – monsoon; October to December – post monsoon; and January to May – pre monsoon) and yearly rainfall were analyzed. The average monthly, seasonal and yearly rainfall values were worked out. The variation of rainfall in each month, season and year from the mean was determined and the mean deviation for the seasons was calculated. Total numbers of drought months, seasons and year were determined using the following definition:

Drought month: if the actual rainfall is less than 50% of the average monthly rainfall (Sharma et al., 1979). Drought season: if the annual rainfall is deficient by more than twice the mean deviation of the season (Marathe et al., 2001). Drought year: if the annual rainfall is deficient by 20-60% of the average yearly rainfall and if the deficient is more than 60% of the average yearly rainfall it is known as scanty drought year (Dhar et al.,1979). The yearly intensity of drought was also determined using the criteria suggested by IMD (1971) which is based on the percentage deviation of rainfall from its long term mean and it is given by (Eq.1).

                        Di = ((Pi – µ ) / µ ) x 100                   …(1)

Where Di is the percentage deviation from the longterm mean,
Pi is the annual rainfall, mm and
µ is the long term mean of the annual rainfall, mm

Drought codification based on percentage departure of rainfall from normal is presented in Table 1. The percentage of deviation (Di) is then used to categorise the drought.

Result and Discussion

Descriptive statistical analysis of monthly rainfall was done for Shillong station. The coefficient of variation is more than 100% for the month of January, November and December. Standard deviation was maximum for the month May to October and minimum for the rest of the year. The monthly minimum, maximum, normal, standard deviation and coefficient of variation (CV) is presented in Table 2. The highest normal rainfall of 506.20 mm was observed in the month of July and the lowest 20.38 mm occurred in the month of January. The frequency of drought was observed to be the highest at a magnitude of 16 times in 35 years in December; while it is 15, 12, 9 and 9 times in 35 years during January, November, February and March month respectively (Table 3). It indicates that, there is a need for assured irrigation in the above months. From the seasonal analysis of the rainfall, it is evident that 22.7% of rainfall was received during pre-monsoon, 67.7% during monsoon and 9.6% during post monsoon session. So for growing winter season crops during post monsoon season assured irrigation facilities need to be provided, simultaneously ample emphasis may be given to rainwater harvesting during the monsoon season as a high quantum of runoff is anticipated during this seasons. No drought was observed during monsoon, pre-monsoon and post monsoon period. The yearly intensity of drought for Shillong, Meghalaya is presented in Table 4. The years are codified according to IMD specification as described in the Table 1. It is found from the Table 4 that, the year 2000 faced severe drought. However, for the years 1975; 1976; 1978; 1989; 1991; 2004; 2006 and 2007 there was moderate drought occurrence.

The average annual rainfall of Shillong is 2,266.56 mm with a maximum of 7,425.3 mm corresponding to the year 1980 and a minimum of 1,019.7 mm corresponding to the year 2006. The average monthly rainfall of the place is 1031.8, 1481.9, 2010.6, 2127.8, 604.4 and 528.8 mm for the month of May, June, July, August, September and October respectively. The maximum average rainfall is received during the month of July to a tune of 506.20 mm and the minimum average rainfall is received during the month of December to a tune of 20.38 mm.

Table 1: Drought codification based on percentage departure of rainfall from normal value

% departure of rainfall from normal

Intensity of drought

Code

0.0 or above

No drought

M0

0.0 to -25.0

Mild drought

M1

-25.0 to -50.0

Moderate drought

M2

-50.0 to -75.0

Severe drought

M3

-75.0 or less

Extreme drought

M4

Table 2: Monthly Normal and Extreme Rainfall along with SD, CV and Percentage contributio

Month

Normal

(mm)

Extreme Value

Standard Deviation

(mm)

Coefficient of

Variation (%)

Percentage

contribution

(%)

Minimum

(mm)

Maximum

(mm)

January

20.38

0

92.5

24.68

121.07

0.83

February

32.56

0.6

94.8

29.33

90.08

1.33

March

61.92

3.9

195.1

49.12

79.33

2.53

April

139.91

0

286.8

70.50

50.39

5.71

May

294.94

53

1031.8

189.47

64.24

12.03

June

461.27

0

1481.9

271.97

58.96

18.82

July

506.20

114.2

2010.6

394.89

78.01

20.65

August

367.81

159

2127.8

358.71

97.52

15.01

September

313.64

129.7

604.4

124.81

39.79

12.80

October

180.19

4

528.8

140.36

77.90

7.35

November

48.87

0.0

266.6

59.26

121.25

1.99

December

23.56

0.0

99.1

29.10

123.52

0.96

Table 3: Analysis of monthly, seasonal and yearly rainfall for drought

Month/

Season

Month/season

Average

rainfall

(mm)

Half of the

average rainfall

(mm)

No. of drought,

month/season/

year

Percentage of

drought months

Month

Jan

20.38

10.19

15

15.46

Feb

32.56

16.28

9

9.28

Mar

61.92

30.96

9

9.28

Apr

139.91

69.95

4

4.12

May

294.94

147.47

4

4.12

Jun

461.27

230.63

5

5.15

Jul

506.20

253.10

8

8.25

Aug

367.81

183.91

4

4.12

Sep

313.64

156.82

2

2.06

Oct

180.19

90.09

9

9.28

Nov

48.87

24.44

12

12.37

Dec

23.56

11.78

6

16.49

                                                                     (Twice the mean dev value, mm)
Season Pre monsoon

555.95

430.90

Monsoon

1659.53

1963.34

Post monsoon

235.77

332.13

(80% of average rainfall)1961

8

(40% of average rainfall)980.5(Scanty)

0


Table 4: Yearly intensity of drought

Year

Annual rainfall

(mm)

Mean rainfall

(mm)

% deviation

Category

Intensity of Drought

1973

2499.9

2451.255

0.98

M0

No Drought

1974

2837.0

2451.255

15.74

M0

No Drought

1975

1413.5

2451.255

-42.34

M2

Moderate Drought

1976

2270.1

2451.255

-7.39

M1

Mild Drought

1977

1562.9

2451.255

-36.24

M2

Moderate Drought

1978

1808.8

2451.255

-26.21

M2

Moderate Drought

1979

7425.3

2451.255

202.92

M0

No Drought

1980

2164.6

2451.255

-11.69

M1

Mild Drought

1981

2274.8

2451.255

-7.20

M1

Mild Drought

1982

2457.7

2451.255

0.26

M0

No Drought

1983

2318.5

2451.255

-5.42

M1

Mild Drought

1984

1840.8

2451.255

-24.90

M0

Mild Drought

1985

1919.1

2451.255

-21.71

M0

Mild Drought

1986

2940.2

2451.255

19.95

M0

No Drought

1987

3265.0

2451.255

33.20

M0

No Drought

1988

2775.6

2451.255

3.23

M0

No Drought

1989

1691.9

2451.255

-30.98

M2

Moderate Drought

1990

2945.3

2451.255

20.15

M0

No Drought

1991

1808.8

2451.255

-26.21

M2

Moderate Drought

1992

2591.1

2451.255

5.70

M0

No Drought

1993

2070.2

2451.255

-15.55

M1

Mild Drought

1994

2846.6

2451.255

16.13

M0

No Drought

1995

2420.5

2451.255

-1.25

M1

Mild Drought

1996

2121.8

2451.255

-13.44

M1

Mild Drought

1997

2130.4

2451.255

-13.09

M0

No Drought

1998

2464.8

2451.255

0.55

M0

No Drought

1999

2245.6

2451.255

-8.39

M1

Mild Drought

2000

1019.7

2451.255

-58.40

M0

Severe Drought

2001

2955.9

2451.255

20.59

M0

No Drought

2002

2499.9

2451.255

1.98

M0

No Drought

2003

2837.0

2451.255

15.74

M0

No Drought

2004

1413.5

2451.255

-42.34

M2

Moderate Drought

2005

2270.1

2451.255

-7.39

M1

Mild Drought

2006

1562.9

2451.255

-36.24

M2

Moderate Drought

2007

1808.8

2451.255

-6.88

M2

Moderate Drought

Conclusion

The drought analysis of Shillong made according to deficiency of rainfall shows that out of thirty five years there was no severe drought occurrence in this region, except for the year 2000. However, for the year 1976; 1980-81; 1983-85; 1993; 1995-96; 1999 and 2005 there was mild drought occurrence. For growing rain-fed rice during monsoon farmers of this region may depend on monsoon as there was hardly any drought occurrence. Since, the post monsoon seasonal rainfall is very less, for growing winter season crops arrangement may be done for assured irrigation with proper rainwater harvesting methods.

Acknowledgement

The financial assistance received from Central Agricultural University (CAU, Imphal) vides Code No. PG.IRP-VI/2010-11; dated, 30th November 2010; for conducting the experiment is duly acknowledged.

References

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