SPATIO-TEMPORAL VARIATION OF PRE-MONSOON RAINFALL AND RAINY DAYS OVER BANGLADESH

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Journal of Engineering Science 09(1), 2018, 21-27

SPATIO-TEMPORAL VARIATION OF PRE-MONSOON RAINFALL AND RAINY DAYS OVER BANGLADESH

M. K. H. Reza^1*, M. M. Alam¹ and M. M. Rahman²

1Department of Physics, Khulna University of Engineering & Technology, Khulna-9203, Bangladesh

2Director, Save Earth Climate Services Ltd., Mirpur DOHS, Dhaka, Bangladesh Received: 22 November 2017 Accepted: 10 February 2018 ABSTRACT

In the present study sixty eight years (1948-2015) daily rainfall data of 30 meteorological stations of Bangladesh Meteorological Department (BMD) have been used to understand the rainfall characteristics during pre-monsoon season over Bangladesh. Yearly variations of rainfall and rainy days (if there is any rainfall in a day is considered as rainy day) have been studied to see the long term variability. The study period has been divided into six decades starting from 1950 to investigate the decadal and interdecadal, spatial and temporal variations. Average rainfall has been found 4.66 mm/day with standard deviation (SD) 1.50 mm/day and coefficient of variation (CV) 32.25%

during 68 years in the pre-monsoon season. Average rainy days have been found 21.56 days with SD 5.39 days and CV 25.02%. Both rainfall and rainy days have been found to have an annual increasing trend. The amount of rainfall has been found highly correlated with rainy days with coefficient of correlation 0.82 with 99.5% level of significance. Maximum amount of Rainfall observed in the fourth decade and rainy days in the sixth decade.

Keywords: Rainfall, Rainy day, pre-monsoon, decadal variation.

1. INTRODUCTION

Precipitation is a natural process and is related to the amount of latent heat transported from the surface to the atmosphere. It is an essential component of scientific investigation of the hydrologic cycle, the global water balance and large scale global atmospheric modeling. But it is one of the most difficult atmospheric parameters to measure because of the large variations in space and time (Kummerow et al., 2000).

Indian subcontinent is well known as a southwest monsoon region (Gadgil et al. 2005; Goswami et al. 1994;

Hastenrath 1995; Kumar et al. 1995; Pattanaik et al. 2015; Rajeevan 2001; Thapliyal and Kulshreshtha 1992) and Bangladesh is a heavy rainfall area in it. Bangladesh is a narrow flat low land, the maritime continent Bay of Bengal is located in the south and the highly elevated Himalayas and Tibetan Plateau are situated in the north. The climate of this country is comprised of four seasons: pre-monsoon (March–May), monsoon (June–September), post- monsoon (October and November) and winter (December–February).

Kripalani and Kumar, 2004 studied rainfall variability over south peninsular India during northeast monsoon (NEM) and found interannual and decadal variabilities in NEM with alternate epochs of above- and below-normal rainfall.

Shahid and Khairulmaini (2009) studied over Bangladesh during 1969-2003 and found a negative trend in winter rainfall. Kiguchi et al. (2016) studied on the pre-monsoon rainfall over the Indochina Peninsula and found that the passage of the upper trough and moisture convergence in the lower troposphere produce intermittent rainfall events during this season. Alam et al. (2010) studied on temporal variation of rainfall over southwestern part of Bangladesh and found increasing trend of seasonal rainfall. Ahmed et al. (1996) and Reza et al. (2004) worked on seasonal rainfall variation over Bangladesh and found negative correlation with ENSO and southern oscillation index (SOI) respectively. Reza et al. (2015) studied on the spatial and temporal variation of rainfall and rainy days during post- monsoon season and found increasing trend of both rainfall and rainy days. Reza et al. (2017) also studied on winter rainfall and got similar results. In this study an attempt has been made to study spatial and temporal variation of rainfall and rainy days over Bangladesh during pre-monsoon season.

2. DATA AND METHODOLOGY

Daily rainfall data of 30 meteorological stations of Bangladesh Meteorological Department (BMD) from 1948 to 2015 is used in this study. Data have been collected from the Climate Division of BMD, Dhaka. It is important to note that some of the stations started its operation after 1948 and some data were not available in some stations for a few years. These data have been considered as missing and have not been used in statistical computations.

The daily rainfall data is used to obtain monthly and seasonal mean. The station wise rainy days (if there is any rainfall in a day) is identified and accumulated for the season. Yearly variations of the rainfall and rainy days along with the trend lines, standard deviation, coefficient of variation and time variant coefficient of correlation are studied

JES

an international Journal

* Corresponding Author: [email protected] KUET@JES, ISSN 2075-4914/09(1), 2018

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with the standard statistical technique. Statistical significance is calculated with student’s t distribution: 𝑡 = 𝑟 , where r is the coefficient of correlation and n is the number of events. Thirty years moving average is computed for both rainfall and rainy days and their variations along with the trend lines have also been studied to understand the climatic swing.

To study the decadal variations of the studied parameters the study period is divided into six decades starting from 1950. 1950-1959 has been designated as Decade1 (D1), 1960-1969 as Decade2 (D2), 1970-1979 as Decade3 (D3), 1980-1989 as Decade4 (D4), 1990-1999 as Decade5 (D5) and 2000-2009 as Decade6 (D6). Here after different decades will be used as D1, D2, D3, D4, D5 and D6. From seasonal average rainfall decadal average is made for each station for each decade during pre-monsoon season (March-May). Spatial distribution of rainfall is studied for each decade. To understand the climatological variation of rainfall over the country 60 years average (1950-2009) are made for each station. From this 60 years average rainfall; anomaly (Decade average-60 years station average) are calculated for each decade during pre-monsoon for individual station. Distributions of rainfall anomaly are studied for each decade over the country during pre-monsoon season. Similar works are done with respect to 60 years country average rainfall. Rainfall anomaly (60 years average for each station - 60 years average for all stations) is calculated. Distribution of this rainfall anomaly has also been studied during this season. Decadal average is also computed to study the interdecadal variation of rainfall and rainy days over the country.

3. RESULTS AND DISCUSSION

Spatial and temporal variations of rainfall and rainy days of thirty meteorological stations over Bangladesh have been studied and are presented in Figures 1-7.

Figure 1: Distribution of average rainfall (mm/day) for a) decade1, b) decade2, c) decade3, d) decade 4, e) decade5 and f) decade6 during pre-monsoon season.

Decadal distributions of average rainfall of decade1 to decade6 (D1-D6) during pre-monsoon season have been presented in Figure1(a-f). Rainfall is found minimum in the western part of the country in D1 [Figure1 (a)] and maximum in the northeast part of the country. 4.0 mm/day isopleth of rainfall contour passed over Mymensingh, Dhaka, Bhola and Chittagong Region. Rainfall minimum found in the northwestern part of the country [Figure1 (b)]

and rainfall increased from southwest to northeast direction over the country. Rainfall in the eastern part was higher

88 E 89 E 90 E 91 E 92 E 93 E 21 N

22 N 23 N 24 N 25 N 26 N 27 N

Sitakundu

Teknaf Dhaka

Dinajpur

Tangail

Barisal

Srimongal Sylhet

Satkhira

Sandwip Chandpur

Chittagonj Comilla

Cox's Bazar Rangamati Rangpur

Rajshahi

Khepupara Khulna Bogra

Jessore Faridpur

Feni Hatiya Ishurdi

Maijdi court Mymensingh

Patuakhali Bhola

Kutubdia

88 E 89 E 90 E 91 E 92 E 93 E 21 N

22 N 23 N 24 N 25 N 26 N 27 N

Sitakundu

Teknaf Dhaka

Dinajpur

Tangail

Barisal

Srimongal Sylhet

Satkhira

Sandwip Chandpur

Chittagonj Comilla

Rajshahi

Jessore Faridpur

Feni Hatiya Ishurdi

Patuakhali Bhola

Kutubdia

88 E 89 E 90 E 91 E 92 E 93 E 21 N

22 N 23 N 24 N 25 N 26 N 27 N

Sitakundu

Teknaf Dhaka

Dinajpur

Tangail

Barisal

Srimongal Sylhet

Satkhira

Sandwip Chandpur

Chittagonj Comilla

Rajshahi

Jessore Faridpur

Feni Hatiya Ishurdi

Patuakhali Bhola

Kutubdia

88 E 89 E 90 E 91 E 92 E 93 E 21 N

22 N 23 N 24 N 25 N 26 N 27 N

Sitakundu

Teknaf Dhaka

Dinajpur

Tangail

Barisal

Srimongal Sylhet

Satkhira

Sandwip Chandpur

Chittagonj Comilla

Rajshahi

Jessore Faridpur

Feni Hatiya Ishurdi

Patuakhali Bhola

Kutubdia

88 E 89 E 90 E 91 E 92 E 93 E 21 N

22 N 23 N 24 N 25 N 26 N 27 N

Sitakundu

Teknaf Dhaka

Dinajpur

Tangail

Barisal

Srimongal Sylhet

Satkhira

Sandwip Chandpur

Chittagonj Comilla

Rajshahi

Jessore Faridpur

Feni Hatiya Ishurdi

Patuakhali Bhola

Kutubdia

88 E 89 E 90 E 91 E 92 E 93 E 21 N

22 N 23 N 24 N 25 N 26 N 27 N

Sitakundu

Teknaf Dhaka

Dinajpur

Tangail

Barisal

Srimongal Sylhet

Satkhira

Sandwip Chandpur

Chittagonj Comilla

Rajshahi

Jessore Faridpur

Feni Hatiya Ishurdi

Patuakhali Bhola

Kutubdia

(f) Decade6 (d) Decade4

(a) Decade1 (b) Decade2 (c) Decade3

(e) Decade5

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23

than the western part and found maximum in the northeast part of Bangladesh [Figure1 (a-f)]. Rainfall is found to increase from west to eastward in the western part of the country and then it increased towards northeastward direction. In D4 rainfall is found to increase from southwest to northeast part of the country [Figure1 (d)]. However in D6 [Figure1 (f)] northwestern part of the country is found to have the minimum amount of rainfall.

Figure 2: Rainfall anomaly (mm/day) for a) decade1, b) decade2, c) decade3, d) decade 4, e) decade5 and f) decade6 with respect to decadal country average during pre-monsoon season.

Rainfall anomalies with respect to decadal country average have been presented in Figure 2(a-f). Western part of the country has been found to be deficit in rain while excess rain is found in the northeastern part of the country [Figure 2 (a & b)]. The zero anomaly line is found to pass over Mymensing, Dhaka, Chandpur, Bhola, Kutubdia and Chittagong region in D1. In many places over the central and southeastern part of the country there was no rainfall anomaly [Figure 2(c)]. Similar rainfall anomaly pattern has been observed in D4 and D6 [Figures 2(d & f))] with negative anomaly in the western and southwestern part of the country and positive anomaly in the eastern and northeastern part of the country.

Rainfall anomalies with respect to sixty years station average have been presented in Figures 3(a-f). Negative anomaly prevails almost all over the country in D1 [Figure 3(a)]. -0.5 mm/day anomaly line passes over Rajshahi, Bogra, Tangail, Faridpur, Khulna and Satkhira region as well as Khepupara, Bhola, Chandpur and Comilla region.

D2 is found to be deficit of rain all over the country [Figure 3(b)] with a minimum of -3.5 mm/day over Khepupara.

In D3 [Figure 3(c)] there is no anomaly in the central and southwestern part of the country while in the northeastern part the anomaly is negative. Positive anomaly prevails over the country except in a few places over the southern coastal regions of the country in D4 [Figure 3(d)]. Rainfall has been found to be static (no anomaly) over the country in D5 [Figure 3(e)] except in the southeast part of the country where it is positive. Rainfall anomaly has been found to be positive in the north, northeastern and southeastern regions of the country in D6 [Figure 3(f)]. -0.5 mm/day anomaly line is found to pass over Khepupara, Chandpur and Faridpur region.

88 E 89 E 90 E 91 E 92 E 93 E 21 N

22 N 23 N 24 N 25 N 26 N 27 N

Sitakundu

Teknaf Dhaka

Dinajpur

Tangail

Barisal

Srimongal Sylhet

Satkhira

Sandwip Chandpur

Chittagonj Comilla

Rajshahi

Jessore Faridpur

Feni Hatiya Ishurdi

Patuakhali Bhola

Kutubdia

88 E 89 E 90 E 91 E 92 E 93 E 21 N

22 N 23 N 24 N 25 N 26 N 27 N

Sitakundu

Teknaf Dhaka

Dinajpur

Tangail

Barisal

Srimongal Sylhet

Satkhira

Sandwip Chandpur

Chittagonj Comilla

Rajshahi

Jessore Faridpur

Feni Hatiya Ishurdi

Patuakhali Bhola

Kutubdia

88 E 89 E 90 E 91 E 92 E 93 E 21 N

22 N 23 N 24 N 25 N 26 N 27 N

Sitakundu

Teknaf Dhaka

Dinajpur

Tangail

Barisal

Srimongal Sylhet

Satkhira

Sandwip Chandpur

Chittagonj Comilla

Rajshahi

Jessore Faridpur

Feni Hatiya Ishurdi

Patuakhali Bhola

Kutubdia

88 E 89 E 90 E 91 E 92 E 93 E 21 N

22 N 23 N 24 N 25 N 26 N 27 N

Sitakundu

Teknaf Dhaka

Dinajpur

Tangail

Barisal

Srimongal Sylhet

Satkhira

Sandwip Chandpur

Chittagonj Comilla

Rajshahi

Jessore Faridpur

Feni Hatiya Ishurdi

Patuakhali Bhola

Kutubdia

88 E 89 E 90 E 91 E 92 E 93 E 21 N

22 N 23 N 24 N 25 N 26 N 27 N

Sitakundu

Teknaf Dhaka

Dinajpur

Tangail

Barisal

Srimongal Sylhet

Satkhira

Sandwip Chandpur

Chittagonj Comilla

Rajshahi

Jessore Faridpur

Feni Hatiya Ishurdi

Patuakhali Bhola

Kutubdia

88 E 89 E 90 E 91 E 92 E 93 E 21 N

22 N 23 N 24 N 25 N 26 N 27 N

Sitakundu

Teknaf Dhaka

Dinajpur

Tangail

Barisal

Srimongal Sylhet

Satkhira

Sandwip Chandpur

Chittagonj Comilla

Rajshahi

Jessore Faridpur

Feni Hatiya Ishurdi

Patuakhali Bhola

Kutubdia

(f) Decade6 (d) Decade4

(a) Decade1 (b) Decade2 (c) Decade3

(e) Decade5

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Figure 3: Rainfall anomaly (mm/day) for a) decade1, b) decade2, c) decade3, d) decade 4, e) decade5 and f) decade6 with respect to 60 years station average during pre-monsoon season.

Figure 4: Distribution of (a) 60 years average rainfall (mm/day), (b) Rainfall anomaly (mm/day) with respect to 60 years country average and c) Average rainfall (mm/day) and rainy days over the country in each decade during pre-monsoon season.

It is observed that 60 years average rainfall minimum is in the western part and maximum in the northeastern part of Bangladesh [Figure 4(a)]. Over a large part of the country including Rangpur, Faridpur, Barisal, Patuakhali, Khepupara and Kutubdia the average rainfall is found 4 mm/day. The rainfall is found to increase from west to east in the western part and then in the northeast direction through central part of the country with maximum 6 mm/day anomaly over Sylhet [Figure 4(b)].

Decadal average rainfall and rainy days along with their polynomial fittings are presented in Figure 4(c). Both rainfall and rainy days are found to have similar pattern with minimum in D2 while maximum rain in D4 and rainy days in D6. The maximum rainfall and rainy days are 5.21 mm/day & 23.72 days and minimum are 3.63 mm/day and 18.16 days respectively. Polynomial curves are fitted to the decadal rainfall and rainy days. The third order time variant polynomials are:

88 E 89 E 90 E 91 E 92 E 93 E 21 N

22 N 23 N 24 N 25 N 26 N 27 N

Sitakundu

Teknaf Dhaka

Dinajpur

Tangail

Barisal

Srimongal Sylhet

Satkhira

Sandwip Chandpur

Chittagonj Comilla

Rajshahi

Jessore Faridpur

Feni Hatiya Ishurdi

Patuakhali Bhola

Kutubdia

88 E 89 E 90 E 91 E 92 E 93 E 21 N

22 N 23 N 24 N 25 N 26 N 27 N

Sitakundu

Teknaf Dhaka

Dinajpur

Tangail

Barisal

Srimongal Sylhet

Satkhira

Sandwip Chandpur

Chittagonj Comilla

Rajshahi

Jessore Faridpur

Feni Hatiya Ishurdi

Patuakhali Bhola

Kutubdia

88 E 89 E 90 E 91 E 92 E 93 E 21 N

22 N 23 N 24 N 25 N 26 N 27 N

Sitakundu

Teknaf Dhaka

Dinajpur

Tangail

Barisal

Srimongal Sylhet

Satkhira

Sandwip Chandpur

Chittagonj Comilla

Rajshahi

Jessore Faridpur

Feni Hatiya Ishurdi

Patuakhali Bhola

Kutubdia

88 E 89 E 90 E 91 E 92 E 93 E 21 N

22 N 23 N 24 N 25 N 26 N 27 N

Sitakundu

Teknaf Dhaka

Dinajpur

Tangail

Barisal

Srimongal Sylhet

Satkhira

Sandwip Chandpur

Chittagonj Comilla

Rajshahi

Jessore Faridpur

Feni Hatiya Ishurdi

Patuakhali Bhola

Kutubdia

88 E 89 E 90 E 91 E 92 E 93 E 21 N

22 N 23 N 24 N 25 N 26 N 27 N

Sitakundu

Teknaf Dhaka

Dinajpur

Tangail

Barisal

Srimongal Sylhet

Satkhira

Sandwip Chandpur

Chittagonj Comilla

Rajshahi

Jessore Faridpur

Feni Hatiya Ishurdi

Patuakhali Bhola

Kutubdia

88 E 89 E 90 E 91 E 92 E 93 E 21 N

22 N 23 N 24 N 25 N 26 N 27 N

Sitakundu

Teknaf Dhaka

Dinajpur

Tangail

Barisal

Srimongal Sylhet

Satkhira

Sandwip Chandpur

Chittagonj Comilla

Rajshahi

Jessore Faridpur

Feni Hatiya Ishurdi

Patuakhali Bhola

Kutubdia

88 E 89 E 90 E 91 E 92 E 93 E 21 N

22 N 23 N 24 N 25 N 26 N 27 N

Sitakundu

Teknaf Dhaka

Dinajpur

Tangail

Barisal

Srimongal Sylhet

Satkhira

Sandwip Chandpur

Chittagonj Comilla

Rajshahi

Jessore Faridpur

Feni Hatiya Ishurdi

Patuakhali Bhola

Kutubdia

88 E 89 E 90 E 91 E 92 E 93 E 21 N

22 N 23 N 24 N 25 N 26 N 27 N

Sitakundu

Teknaf Dhaka

Dinajpur

Tangail

Barisal

Srimongal Sylhet

Satkhira

Sandwip Chandpur

Chittagonj Comilla

Rajshahi

Jessore Faridpur

Feni Hatiya Ishurdi

Patuakhali Bhola

Kutubdia

0 5 10 15 20 25 30 35

D1 D2 D3 D4 D5 D6 Rainfall (mm/day)/Rainy days ^Rainfall_{Rainy days}

Poly. (Rainfall) Poly. (Rainy days)

(f) Decade6 (d) Decade4

(a) Decade1 (b) Decade2 (c) Decade3

(e) Decade5

(a) Average (b) Anomaly (c)

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25

y = -0.089x³ + 0.906x² - 2.361x + 5.538 for rainfall and y = -0.364x³ + 4.008x² - 11.74x + 28.40 rainy days respectively.

Anomaly of rainy days with respect to sixty years station average is presented in Figure 5(a-f). Anomaly of rainy days in D1 is negative in the northwestern, southwestern and southeastern part of the country [Figure 5(a)]. There is no anomaly in the central part of the country. In D2 negative anomaly prevails in the south and southeastern parts of the country [Figure 5(b)]. Mainly negative anomaly prevails all over the country in D3 [Figure 5(c)]. -4 days anomaly line passed through Rajshahi and Jessore in the western part and also passed through Rangpur, Bogra, Tangail, M.Court and Comilla region. Anomaly of rainy days is found positive almost all over the country in D4, D5 and D6 [Figures 5(d-f)].

Figure 5: Anomaly of rainy days for decade (a-f) D1-D6 respectively with respect to 60 years station average during pre-monsoon season over Bangladesh.

Figure 6: Annual variation of (a) rainfall (mm/day) and b) rainy days along with thirty years moving average and the linear trend line.

88 E 89 E 90 E 91 E 92 E 93 E 21 N

22 N 23 N 24 N 25 N 26 N 27 N

Sitakundu

Teknaf Dhaka

Dinajpur

Tangail

Barisal

Srimongal Sylhet

Satkhira

Sandwip Chandpur

Chittagonj Comilla

Rajshahi

Jessore Faridpur

Feni Hatiya Ishurdi

Patuakhali Bhola

Kutubdia

88 E 89 E 90 E 91 E 92 E 93 E 21 N

22 N 23 N 24 N 25 N 26 N 27 N

Sitakundu

Teknaf Dhaka

Dinajpur

Tangail

Barisal

Srimongal Sylhet

Satkhira

Sandwip Chandpur

Chittagonj Comilla

Rajshahi

Jessore Faridpur

Feni Hatiya Ishurdi

Patuakhali Bhola

Kutubdia

88 E 89 E 90 E 91 E 92 E 93 E 21 N

22 N 23 N 24 N 25 N 26 N 27 N

Sitakundu

Teknaf Dhaka

Dinajpur

Tangail

Barisal

Srimongal Sylhet

Satkhira

Sandwip Chandpur

Chittagonj Comilla

Rajshahi

Jessore Faridpur

Feni Hatiya Ishurdi

Patuakhali Bhola

Kutubdia

88 E 89 E 90 E 91 E 92 E 93 E 21 N

22 N 23 N 24 N 25 N 26 N 27 N

Sitakundu

Teknaf Dhaka

Dinajpur

Tangail

Barisal

Srimongal Sylhet

Satkhira

Sandwip Chandpur

Chittagonj Comilla

Rajshahi

Jessore Faridpur

Feni Hatiya Ishurdi

Patuakhali Bhola

Kutubdia

88 E 89 E 90 E 91 E 92 E 93 E 21 N

22 N 23 N 24 N 25 N 26 N 27 N

Sitakundu

Teknaf Dhaka

Dinajpur

Tangail

Barisal

Srimongal Sylhet

Satkhira

Sandwip Chandpur

Chittagonj Comilla

Rajshahi

Jessore Faridpur

Feni Hatiya Ishurdi

Patuakhali Bhola

Kutubdia

88 E 89 E 90 E 91 E 92 E 93 E 21 N

22 N 23 N 24 N 25 N 26 N 27 N

Sitakundu

Teknaf Dhaka

Dinajpur

Tangail

Barisal

Srimongal Sylhet

Satkhira

Sandwip Chandpur

Chittagonj Comilla

Rajshahi

Jessore Faridpur

Feni Hatiya Ishurdi

Patuakhali Bhola

Kutubdia

y = 0.022x + 3.996 R² = 0.679 1

2 3 4 5 6 7 8 9 10 11

1948 1952 1956 1960 1964 1968 1972 1976 1980 1984 1988 1992 1996 2000 2004 2008 2012 2016

R ai n fa ll (m m /d ay )

Rainfall Moving average Linear (Moving average)

y = 0.146x + 16.30 R² = 0.937

5 10 15 20 25 30 35

1948 1952 1956 1960 1964 1968 1972 1976 1980 1984 1988 1992 1996 2000 2004 2008 2012 2016

R ai n y d ay s

Rainydays Moving average Linear (Moving average)

(f) (d)

(a) (b) (c)

(e)

(a) (b)

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Rainfall and rainy days of 68 years (1948-2015) during pre-monsoon season over Bangladesh have been analyzed.

Average rainfall is found 4.66 mm/day with standard deviation (SD) 1.50 mm/day and coefficient of variation (CV) 32.25% during 68 years in the pre-monsoon season. Average rainy days are found 21.56 days with SD of 5.39 days and CV 25.02%.

Annual variation of rainfall and rainy days along with thirty years moving average and linear trend lines are presented in Figure 6(a &b). Highest rainfall is found in the year 1981 and lowest in 1979. While highest rainydays is found in the year 1990 and lowest in 1971. Both rainfall and rainy days are found to have an increasing trend with 0.022mm/day/year and 0.146 days/year with time variant coefficient of correlation 0.82 and 0.97 respectively at 99.5% level of significance.

Variability of rainfall and rainy days are presented in Figure 7 (a & b). Rainfall is found to have maximum positive anomaly in the year 1981 with 3.85mm/day and minimum negative anomaly in 1979 with -2.82mm/day. Rainy days is found to have maximum positive anomaly in the year 1990 with 11.54 days and minimum negative anomaly in 1971 with -15.21days. Both the parameters are found to have an annual increasing trend. Although increasing rate of rainfall is not statistically significant (similar result obtained by Shahid and Khairulmaini, 2009) but increasing rate of rainy days is significant with 97.5% level. The relation between rainfall and rainy days are presented in Figure 7(c). Rainfall is found to be highly correlated with rainy days with coefficient of correlation 0.82 at 99.5% level of significance.

Figure 7: Variability of (a) rainfall, (b) rainy days and c) plot of rainfall vs rainy days.

4. CONCLUSIONS

On the basis of the study presented in this paper the following conclusions have been drawn:

i) The rainfall distribution pattern is similar in all the decades and it increased in the northeastern part of Bangladesh. Minimum amount of rainfall found in the Midwestern part of Bangladesh in D2, D5 and D6.

ii) The west and northwestern part of the country is found to be deficit of rain and the northeastern part of the country is found to have excess of rain.

iii) Rainfall and rainy days are found to have similar pattern with minimum in D2. The maximum rainfall and rainy days were 5.21 mm/day in D4 & 23.72 days in D6 and minimum were 3.63 mm/day & 18.16 days in D2 respectively.

iv) Rainy days have been found above normal distribution in D4-D6 and below normal in D1-D3 over the country.

y = 0.009x - 0.325 R² = 0.015 -4-3

-2 -1012345

1948 1952 1956 1960 1964 1968 1972 1976 1980 1984 1988 1992 1996 2000 2004 2008 2012 2016 Rainfall anomaly (mm/day)

Rainfall Linear (Rainfall)

y = 0.074x - 2.580 R² = 0.075 -20

-15 -10 -5 0 5 10 15

1948 1952 1956 1960 1964 1968 1972 1976 1980 1984 1988 1992 1996 2000 2004 2008 2012 2016 Anomaly of rainy days _Rainydays

Linear (Rainydays)

y = 0.229x - 0.280 R² = 0.676 0

1 2 3 4 5 6 7 8 9

0 5 10 15 20 25 30 35

R ai n fa ll (m m /d ay )

Rainy days Rainfall

Linear (Rainfall)

(a) (b)

(c)

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27

v) Average rainfall has been found 4.66 mm/day with SD 1.50 mm/day and CV 32.25% and average rainy days were 21.56 days with SD 5.39 and CV 25.02% during 68 years in the pre-monsoon season.

vi) 30 years moving averaged rainfall and rainy days both have been found to have an increasing trend with 0.022 mm/day/year and 0.146 days/year with time variant CC 0.82 and 0.97 respectively at 99.5% level of significance.

vii) Variability of rainfall and rainy days shows that both the parameters have an annual increasing trend.

Although increasing rate of rainfall is not statistically significant but increasing rate of rainy days is significant with 97.5% level.

viii) Rainfall is found to be highly correlated with rainy days with CC 0.82 with 99.5% level of significance.

ACKNOLEDGEMENT

The authors are thankful to the climate division of Bangladesh Meteorological Department for providing the data.

We are grateful to the editor and two anonymous reviewers for their constructive and insightful suggestions to improve the article.

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