Journal of Indian Water Resources Society,
Vol 34, No.2, April, 2014
RAINFALL CHARACTERISTICS, PATTERN AND DISTRIBUTION OF
CENTRAL MEGHALAYA
Lala I. P. Ray1, P. K. Bora2, A. K. Singh2, N. J. Singh2, Ram Singh3 and S. M. Feroze3
ABSTRACT
Indian agriculture is mostly rainfed (around 60% of arable land) and monsoon plays a major role not only in agriculture but also in allied
day to day activities. The knowledge amount of rainfall, number of rainy days and its distribution over the cropping season are important for
timely preparation of seed bed, selection of crop varieties, choice of cropping pattern. Rainfall analysis with advanced statistical methods
using computer programming and software bring out many features which can be directly used for crop planning, landwater management,
aquaculture and floriculture planning etc. The analysis of 28 years (1983-2010) daily rainfall data of Nongstoin station, of central
Meghalaya has been done for determining the characteristics of rainfall and probability of occurrence of normal weekly rainfall.
Keywords: Rainfed agriculture; normal weekly rainfall; probability of occurrence; rainy days.
INTRODUCTION
The temporal and spatial distribution of rainfall is important
for crop planning especially for rainfed agriculture (Reddi and
Reddy, 2004). The importance of this distribution is realised in
agricultural and allied sectors. In most part of our country,
rainfall is uneven, uncertain and erratic. The knowledge of
distribution of dry spells and amount of rainfall during wet
spells is very much essential for successful management of
agriculture. The information of amount of rainfall during wet
spell is useful for storage purpose based on the magnitude of
dry spells and drought severity. Also the crop development is
severely affected if dry spells coincide with the sensitive
phonological stage of the crop and it is sometimes beneficial, if
it coincide with ripening stage. An attempt has been made in
this paper to analyse the rainfall in respect of standard week
wise as well as monthly rainfall distribution at different
probability levels for Nongstoin station, of central Meghalaya
by using suitable techniques. Nongstoin station, of central
Meghalaya occupies the remotest position of the state and is
mostly dominated by khasi tribals with agriculture their prime
occupation. The distribution of weekly, monthly and
seasonally rainfall is discussed in this paper. The probability of
occurrence of a quantum of normal weekly rainfall is also
analysed. Various scientific studies have been carried out in
the past by various investigators on rainfall analysis
(Chakraborty and Mandal, 2008; Jakhar et al., 2011; Harrison,
1983; Mohanty et al., 2001; Satapathy et al., 1998; Sharda and
Bhushan, 1985; Verma and Sharma, 1989; Ray et al., 2012a;
Ray et al., 2012b; Rodda et al., 2009). The criteria set by
Raman (1979) for rainfall of 1 mm for defining a rainy day are
not suitable for agriculture purpose. Ashokraj (1979) used the
criteria fixed by India Meteorological Department (IMD) for
defining the rainy day i.e. the day with at least 2.5 mm rain is
1.
2.
3.
Assistant Professor (Water Resource Management) School of
Natural Resource Management, College of Postgraduate
Studies, (Central Agricultural University),
Umiam, Barapani- 793103 Meghalaya
Email: [email protected]
School of Natural Resource Management, College of
Postgraduate Studies, (Central Agricultural University),
Umiam, Barapani- 793103, Meghalaya.
called rain day. When probability of occurrence of different
dry spell length in a week bounded by wet weeks is known;
adequate steps may be taken by shifting the sowing time or
arranging minimal irrigation to get optimum yield.
MATERIALS AND METHODS
The West Khasi Hills district is the largest district in the state
with a geographical area of 5,247 km2. The district was carved
out of Khasi Hills District on 28th October 1976. The district
headquarter is located at Nongstoin. The district map of
Meghalaya showing the study area is shown in Fig-1. It has a
population of 3, 85,601with a population density of 73 persons
per km2 (Census, 2011). It is located at 250 10` to 250 51` N
latitude and 900 44` to 910 49` E longitude with an elevation of
1,200 m. Rainfed agriculture is most prevelant in this
region.The amount of rainfall and number of rainy days in a
week at Nongstoin, Meghalaya from historic daily rainfall
records (1983-2010) collected from IMD, Pune were
calculated using probabilistic approach.
Fig. 1: District map of Meghalaya showing the study area
Weibull’s Method of Probability Analysis
The weekly rainfall data have been analysed at different levels
of probability by using Weibull’s method (Rosin and
Rammler, 1933). In this method, the weekly rainfall arranged
in descending order of magnitude was given rank, 1 next
magnitude was given rank 2 and so on. The probability ‘p’ of
the week having rainfall exceeding or equaling normal value
was calculated by using Weibull’s formula:
P =
m
n +1
School of Social Sciences, College of Postgraduate Studies,
(Central Agricultural University), Umiam, Barapani- 793103,
Meghalaya.
where ,
Manuscript No.: 1342
P = probability of occurrence
(1)
9
J. Indian Water Resour. Soc., Vol. 34, No. 2, April, 2014
m = rank number
n = number of data used
RESULTS AND DISCUSSION
Twenty eight years (1983-2010) of daily point rainfall data
were analysed for the Nongstoin station. The average annual
rainfall of Nongstoin is 3,529.4 mm with 118 number of rainy
days. The maximum annual rainfall recorded was 6,189.2 mm
corresponding to the year 1988; and the minimum recorded
was 1,825.0 mm corresponding to the year 1998. The average
number of rainy days per week is more than thee from 15th to
36th Standard Meteorological Week (SMW). Total weekly
rainy days and quantum of weekly rainfall are presented in
Fig-2 and 3, respectively.
The probability of getting weekly rainy days and amount of
rainfall is presented in Fig- 4 and 5, respectively. The
probability analysis was made at 50%, 60%, 70%, 8 0% and
6.00
Number of rainy days
5.00
4.00
3.00
2.00
1.00
0.00
1
4
7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52
Standard week
Fig. 2: Number of rainy days in a week
300.00
Rainfall, mm
250.00
200.00
150.00
100.00
50.00
0.00
1
4
7
10 13 16 19 22 25 28 31 34 37 40 43 46 49 52
Sta nda rd week
Fig. 3: Depth of rainfall in a week 10
J. Indian Water Resour. Soc., Vol. 34, No. 2, April, 2014
5152
6
4950
48
5
47
46
4
45
3
44
43
2
42
1
41
40
0
39
38
37
36
35
34
33
32
3130
2928
1
2 3
4 5
27
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
2625
50% Prob
60% Prob
70% Prob
80% Prob
90% Prob
Fig. 4: Probability of number of rainy days in a week
Amount of ra infa ll, mm
240
200
160
120
80
40
0
1
4
7
10 13 16 19 22 25 28 31 34 37 40 43 46 49 52
Standard week
50% Prob
60% Prob
70% Prob
80% Prob
90% Prob
Fig. 5: Probability of rainfall in a week 11
J. Indian Water Resour. Soc., Vol. 34, No. 2, April, 2014
0%
1%
1%
0% 2%
6%
6%
12%
10%
16%
18%
28%
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Fig. 6: Monthly distribution of rainfall
Number of rainy days
25.0
20.0
15.0
10.0
5.0
0.0
Ja n
Feb Ma r Apr Ma y Jun
Jul
Aug Sep
Oct Nov Dec
Month
Fig. 7: Average number of rainy days in a month 12
J. Indian Water Resour. Soc., Vol. 34, No. 2, April, 2014
3%
29%
68%
Pre monsoon
Monsoon
Post monsoon
Fig. 8: Yearly distribution of rainy day
Post monsoon
1%
Pre monsoon
19%
Monsoon
80%
Fig. 9: Yearly distribution of rainfall 13
J. Indian Water Resour. Soc., Vol. 34, No. 2, April, 2014
90% probability level. Monthly analysis of point rainfall
revealed that July and August are the wettest month
contributing more than 40% of annual rainfall (Fig-6).
However, the annual rainy days are more than 10 for April to
September month (Fig-7). Seasonal rainfall, i.e., pre monsoon
(February to May), monsoon (June to September) and post
monsoon (October to January) were analyzed figures (Fig-8
and Fig-9) of the Nongstoin station show that monsoon season
contributing around 75% of total rainfall with 67% of rainy
days.
The number of analysed years categorized into three blocks
based on annual number of rainy days prevailed in a water year
are presented in Table-1. It was found that the annual number
of rainy days is mostly between 100-150 days in a monsoon
year, i.e., around 96.4%. The percentage rainydays is around
3.6 for years with annual rainy days less than 100. The average
monthly rainfall and number of rainy days is presented in
Table-2. The weekly extreme and normal rainfall, standard
deviation (SD), coefficient of variation (CV) and percentage of
contribution of rainfall at Nongstoin station is presented in
Table- 3. The chances of getting 2, 3 and 4 dry days in a SMW
is more than 50% from 1st to 12th SMW. Similarly, the
Table 1: Number of years under different magnitude of rainy days
Annual rainy day
<100
100-150
>150
Number of years
1
27
0
Percentage of rainy day (%)
3.6
96.4
0
Table 2: Average monthly rainfall and rainy days at Nongstoin, Meghalaya
Month
January
February
March
April
May
June
July
August
September
October
November
December
Rainfall, mm
16.4
25.5
76.1
196.1
350.4
641.4
983.4
570.7
409.2
212.0
31.0
17.1
Rainy days
1.2
2.3
4.8
10.4
15.3
18.9
20.9
18.4
15.3
7.3
1.7
1.3
Table 3: Weekly Extreme and Normal rainfall SD, CV and Percentage of contribution at Nongstoin station
Standard
Met Week
(SMW)
Extreme Value
Normal
( mm)
Standard
Deviation
(mm)
Coefficient of
Variation
(CV, %)
Percentage of
Contribution
(%)
3.6
14.6
405.2
0.10
1.
Min.
(mm)
0
Max.
(mm)
77.2
2.
0
75.5
4.0
14.3
355.9
0.11
3.
0
40.6
4.1
10.3
248.4
0.12
4.
0
26.0
2.9
6.5
219.0
0.08
5.
0
30.5
4.8
8.4
176.7
0.13
6.
0
63.4
4.5
12.5
281.8
0.13
7.
0
43.0
7.2
11.2
156.7
0.20
8.
0
33.2
7.7
9.8
127.5
0.22
9.
0
61.4
5.6
13.2
234.5
0.16
10.
0
103.8
7.5
20.5
275.2
0.21
11.
0
63.2
12.7
15.5
122.2
0.36
12.
0
132.0
21.3
26.9
126.4
0.60
13.
0
117.2
40.7
33.1
81.3
1.15
14.
0
171.0
31.0
44.9
145.0
0.88
14
J. Indian Water Resour. Soc., Vol. 34, No. 2, April, 2014
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
35.
36.
37.
38.
39.
40.
41.
42.
43.
44.
45.
46.
47.
48.
49.
50.
51.
52.
0
0
5.0
8.0
7.0
0
0
0
4.0
18.3
13.0
3.0
37.0
17.2
0
2.0
12.6
11
13.8
11
0
0
0
0
0
0
6.2
0
0
0
0
0
0
0
0
0
0
0
103.0
148.1
237.4
213.4
143.9
213.8
581.0
395.0
442.1
699.2
530.0
739.1
859.2
798.2
909.5
1058.0
779.1
360.4
780.4
1330.6
322.0
750.0
444.0
391.4
256.1
612.0
521.7
155.4
173.8
48.0
96.8
105.0
37.0
204.0
10.0
46.2
34.2
62.0
30.8
41.9
71.4
74.8
66.2
78.2
75.2
107.6
87.9
194.7
133.2
227.9
284.0
180.9
200.9
252.9
141.8
107.4
124.8
161.2
113.5
120.4
84.7
75.1
73.9
71.8
90.8
31.2
14.7
5.2
7.0
10.9
4.2
8.6
0.4
6.3
2.5
6.6
25.6
44.6
62.4
54.8
43.3
65.9
109.2
107.7
88.6
203.3
124.4
211.2
241.8
176.3
233.9
278.3
165.3
91.8
141.4
244.6
96.8
148.6
83.7
79.6
66.2
117.6
137.2
44.1
36.1
10.6
18.6
28.6
8.9
38.6
1.9
11.4
7.1
16.8
83.2
106.6
87.3
73.2
65.5
84.4
145.2
100.1
100.7
104.4
93.4
92.6
85.1
97.4
116.4
110.1
116.6
85.4
113.3
151.7
85.3
123.4
98.8
106.1
89.4
163.9
151.1
141.3
245.5
204.9
264.9
262.9
211.9
450.7
446.5
180.4
284.8
253.7
0.87
1.19
2.02
2.12
1.88
2.21
2.13
3.05
2.49
5.52
3.77
6.46
8.05
5.13
5.69
7.16
4.02
3.04
3.54
4.57
3.22
3.41
2.40
2.13
2.10
2.03
2.57
0.88
0.42
0.15
0.20
0.31
0.12
0.24
0.01
0.18
0.07
0.19
7000
y = ‐21.15x + 45756
R² = 0.029
6000
Rainfall, mm
5000
4000
3000
2000
1000
0
1980
1985
1990
1995
2000
2005
2010
2015
Yea r
Fig. 10: Trend of yearly rainfall 15
J. Indian Water Resour. Soc., Vol. 34, No. 2, April, 2014
consecutive wet spells prevails for 18th to 33th week with a
probability of more than 50%. The annual variation of rainfall
at Nongstoin station is erratic. The yearly rainfall showed more
or less linear fashion, but a decreasing trend (Fig-10).
4
Jakhar, P., Hombe Gowda, H. C., Naik, B. S. and
Barman, D. 2011. Probability analysis of rainfall
characteristics of Semiliguda in Koraput, Orissa. Indian
J. Soil Cons., 39(1): 9-13.
CONCLUSION
5
Mohanty, S., Marathe, R. A. and Singh, S. 2001.
Rainfall Characteristics of Vidarbha Region. Indian J.
of Soil Cons., 29 (1): 18-21.
6
Raman, C. R. V. 1979. Analysis of commencement of
monsoon rains over Maharastrha state for agricultural
planning. Scientific Report No -216, IMD, Pune.
7
Satapathy, K. K., Jena, S. K., Das Choudhury, D. 1998.
Characteristics of monsoon and rainfall pattern at
Umiam, Meghalaya. J. of Soil and Water Cons.,
(42):155-161.
8
Sharda, V. N. and Bhushan, L. S. 1985. Probability
analysis of annual maximum daily rainfall for Agra.
Indian J. Soil Cons., 13(1): 16-20.
9
Ray, Lala I. P., Bora, P. K., Ram, V., Singh, A. K.,
Singh, R., Feroze, S. M. 2012a. Probable Annual
Maximum Rainfall for Barapani, Meghalaya. J. of
Progressive Agriculture 3 (1):16-18.
10
Ray, Lala I. P., Bora, P. K., Ram, V., Singh, A. K.,
Singh, R., Feroze, S. M. 2012b. Meteorological drought
assessment in Barapani. J. of Indian Water Res. Soc.
(IWRS) 32 (1-2):56-61.
11
Reddi, T. Y. and Reddy, G. H .S. 2004. Text Book of
Agronomy. Kalyani Publishers, Ludhiana, India. pp 0608.
12
Rodda, H. J. E., Little, M. A., Wood, R. G., MacDougall,
N. and McSharry, P.E. 2009. A digital archive of
extreme rainfalls in the British Isles from 1866 to 1968
based on British Rainfall. J. of Weather 64(3): 71-75.
13
Rosin, P. and Rammler, E. 1933. The Laws Governing
the Fineness of Powdered Coal. J. of the Institute of
Fuel (7): 29-36.
14
Verma, H. N. and Sharma, P. B. S. 1989. Critical dry
spells and supplemental irrigation to rainfed crops. J. of
Indian Society of Water Res., 9(4): 12-16. Rainfall is the most important component in agriculture
production and its spatial and temporal distribution is uneven,
uncertain and erratic in nature. Nongstoin, Meghalaya has an
average annual rainfall of 3,529.4 mm and its trend is
decreasing. The average annual rainy days are 145 days and
number of rainy days in a month varies from 1 to 20 days.
More than 80% rainfall occurs during 17th to 42nd week. There
is a good amount of rainfall (more than 150 mm) both in the
months of April and May, which is considered as pre-monsoon
showers, help in seed bed preparation. Hence the length of
monsoon is about 145 days which helps in growing paddy and
other cereal crops in the central Meghalaya. Since winter
season gets only four percent of total rainfall, it is necessary to
construct water harvesting systems, to store excess water
during rainy season, which will be utilized as life saving
irrigation for fruit crops, vegetables and other cash crops
during winter season. A good amount of rainfall during
monsoon season helps the farmer to go for fish cum paddy
culture and pisciculture in the water harvesting ponds. In
Meghalaya the temperature is low, humidity is high and almost
all weeks get some rainfall, whatever small the amount may
be; which helps this place as a centre for Orchids and other
floriculture activities.
ACKNOWLEDGEMENTS
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
1
Ashokraj, P. C., 1979. Onset of effective monsoon and
critical dry spell. IARI Research Bulletin No. 11, WTC
New Delhi, pp: 6-18.
2
Chakraborty, P. B. and Mandal, A. P. N. 2008. Rainfall
characteristics of Sagar island in Sunderban, West
Bengal. Indian J. Soil Cons., 36(3): 125-128.
3
Harrison, M. S. J. 1983. Rain day frequency and mean
daily rainfall intensity as determinants of total rainfall
over the eastern Orange free state. Int. J. of
Climatology 3(1): 35-45.
16