Rainfall over Bangladesh

ASIA-PACIFIC JOURNAL OF ATMOSPHERIC SCIENCES, 45, 3, 2009, p. 375-389

1. Introduction

Climatic change due to global worming is a major concern in the recent years. It has been indicated that rainfall is changing due to global warming on both the global (Hulme et al., 1998; Lambert et al., 2003; Dore, 2005) and the regional scales (Rodriguez- Puebla et al., 1998; Gemmer et al., 2004; Kayano and Sansigolo, 2008). Future climate changes may in-volve modifications in climatic variability as well as changes in averages (Mearns et al., 1996). The im-plications of these changes are particularly significant for areas already under stress, such as in Bangladesh where hydrological disasters of one kind or another is a common phenomenon. The country is one of the most flood prone countries in the world due to its geo-

graphic position. Severe floods in the years of 1974, 1984, 1987, 1988, 1998, 2004 and 2007 ravaged the country. Drought in the northern part of the country has also become a growing concern in the recent years. The country experienced eight droughts of severe magnitude in the years of 1973, 1977, 1979, 1982, 1989, 1992, 1994-1995 and 1999 (Shahid, 2008; Shahid and Behrawan, 2008).

Rainfall variability in space and time is one of the most relevant characteristics of the climate of Bangladesh. Climatic modelers forecast that as the world warm, the monsoon rains in Bangladesh will be increased and at the same time winter precipitation will be decreased. This will cause a cruel combination of more extreme floods and longer periods of droughts. Bangladesh has been termed as one of the most vulnerable countries in the world due to climatic change (IPCC, 2007). Therefore, for disaster mitigation, agriculture and water resources planning and management in the context of global cli-matic change it is essential to study the characteristics and trends of rainfall in Bangladesh. In the present pa-per, spatial and temporal variation of rainfall during the

Spatio-Temporal Variability of Rainfall over Bangladesh

During the Time Period 1969-2003

Shamsuddin Shahid and Osman Salleh Khairulmaini

Department of Geography, University of Malaya 50603 Kuala Lumpur, Malaysia(Manuscript received 10 February 2009; in final form 2 April 2009)

Abstract Spatial and temporal variability of rainfall in Bangladesh has been studied in this paper from thirty-five years

(1969-2003) of rainfall data recorded at 24 rain gauges distributed over the country. Long-term annual average rainfall, coefficient of variation of annual rainfall, precipitation concentration and aridity indices at each station have been com-puted and then interpolated using kriging method within a geographic information system to show the temporal and spatial variability of rainfall. Mann-Kendall test has been used to analyze the trend in rainfall data in different recording stations and the Sen’s slope method has been used to determine the magnitude of change. A moderate variation in in-ter-annual rainfall and high variation in intra-annual rainfall in Bangladesh have been observed. Non-significant positive trend of annual, monsoon and pre-monsoon rainfall, and a negative trend in winter rainfall are found in Bangladesh. Spatial distribution of rainfall trends shows that rainfall is increasing in the coastal zone and northern Bangladesh, and decreasing in the central part of the country. A declining trend of precipitation concentration is also observed in most of the stations. These results may be a first indication of the precipitation response to global warming – a hypothesis which needs to be further investigated by means of climate model projections.

Key words: Aridity, rainfall trend analysis, precipitation concentration index, climate change, GIS

Corresponding Author: Shamsuddin Shahid, Department ofGeography, University of Malaya, 50603 Kuala Lumpur, Malaysia.Phone : +603-7967-7375E-mail: [email protected]

376 ASIA-PACIFIC JOURNAL OF ATMOSPHERIC SCIENCES

period 1969-2003 has been analyzed.Several studies have been carried out on temporal

variability of rainfall in Bangladesh (Ahmed, 1989; Ahmed and Karmaker, 1993; Ahmed, 1994; Ahmed et al., 1996, Hussain and Sultana, 1996; Kripalni et al., 1996; Rahman et al., 1997; Ahmed and Kim, 2003). Ahmed and Karmakar (1993) studied the variability of the arrival and withdrawal dates of summer monsoon in Bangladesh, Ahmed (1994) studied the variability of summer monsoon and its relation with the monsoon onset dates, Ahmed et al. (1996) showed the relation-ship between the annual rainfall and the ENSO, Rahman et al. (1997) studied the trend of summer mon-soon rainfall in Bangladesh, Ahmed and Kim (2003) investigated the patterns of daily rainfall in Bangladesh during the summer monsoon. All the studies were car-ried out to show the seasonal and temporal pattern of rainfall in Bangladesh. However, no studies have been carried out so far on spatial distribution of temporal rainfall characteristics of Bangladesh. The main ob-jectives of this paper are to show the spatial patterns of inter-annual as well as intra-annual variation of rainfall over Bangladesh during the time period 1969–2003. Aridity assessment using De Martonne’s (De Martonne, 1926) and Thornthwait’s (Thornthwaite, 1931) methods has been carried out for climate zoning of Bangladesh. Trend analysis has been carried out to show the long- term change in annual, monsoon, pre-monsoon and win-ter precipitation over Bangladesh as well as in each cli-mate zone. Mann-Kendall test has been used to analyze the trends in rainfall data in different recording stations and the Sen’s slope method (Sen, 1968) has been used to determine the magnitude of change. Spatial dis-tribution, temporal variation and trend of Precipitation Concentration Index (PCI) have also been assessed. GIS has been used for the development of historic rain-fall database, and development of raster maps of rainfall properties.

2. Climate of Bangladesh

Bangladesh, occupies an area of 143,998 km2, cli-matically belongs to sub-tropical regions where mon-soon weather prevails throughout the year. Geographically, it extends from 20°34'N to 26°38'N latitude and from

88°01'E to 92°41'E longitude. Except the hilly south-east, most of the country is a low-lying plain land. Three distinct seasons can be recognized in Bangladesh from climatic point of view: (i) the dry winter season from November to February, (ii) the pre-monsoon hot summ-er season from March to May, and (iii) the rainy mon-soon season which lasts from June to October (Rashid, 1991). Analysis of temperature data for the time period 1969-2003 shows that the average temperature of the country ranges from 7.2°C to 12.8°C in winter and 23.9 to 31.1°C in summer. The average relative humidity for the whole year ranges from 70.5% to 78.1% with a max-imum in September and a minimum in March (Banglapedia, 2003). About 78% rainfall in Bangladesh occurs in monsoon, caused by weak tropical depressions that are brought from the Bay of Bengal into Bangladesh by the wet monsoon winds.

3. Data and Methodology

Daily recorded rainfall data at 24 rainfall measuring stations during the period 1969–2003 has been used in the present study. The location of rainfall measuring stations is shown in Fig. 1. The homogeneity of the pre-cipitation records are analyzed by calculating the von Neumann ratio (Von Neumann, 1941; Suhaila et al., 2008) and the Buishand range test (Buishand, 1982). The data sets of all stations which have been used in the present study are found homogeneous.

Different statistical characteristics of rainfall like mean rainfall over the year or different seasons, coefficients of inter-annual rainfall variation, precipitation concen-tration index (PCI), coefficient of variation of PCI are calculated for all the stations. The spatial distributions of these rainfall properties as well as trends of rainfall and PCI are mapped to analyze the spatio-temporal pat-tern of rainfall over Bangladesh. Methods used for the processing of rainfall data to find the spatial and tempo-ral trends and their mapping are discussed below.

a. Rainfall characteristics analysis

Precipitation Concentration Index (PCI) proposed by Oliver (1980) has been used to define temporal as-pects of the rainfall distribution within a year. PCI is

31 August 2009 Shamsuddin Shahid and Osman Salleh Khairulmain 377

expressed as percentage in according to following for-mula:

2

12

1

2

100P

pPCI i

i∑=×= (1)

where Pi precipitation of i -th month, and P = annual precipitation.

PCI is very useful to evaluate the degree of seasonal concentration of precipitation. It provides information to compare different climates in terms of seasonality of precipitation regime. The more concentrated is pre-cipitation, the more difficult is water management, irri-gation control, soil erosion prevention and rainfed agriculture.

In order to show the spatial distribution of the intra- and inter- annual variability of rainfall in Bangladesh, PCI and the coefficient of variation of the annual rain-fall(R) at each station are calculated and then interpo-lated using Kriging method (Isaaks and Srivastava, 1989).

b. Aridity indices

Aridity index of an area gives an idea about its cli-mate, bio-environment, soil moisture, drought vulner-ability and erosion susceptibility. For aridity mapping of Bangladesh, De Martonne’s aridity index and Thornthwait’s precipitation effectiveness index are used. De Martonne (1926) proposed a method for cal-culating aridity index (AI) of an area using following equation:

2/])10(/12)10(/[ +++= tpTPAI (2)

where P is the mean annual precipitation in mm, T is the mean annual temperature in °C, p the precipitation of the driest month in mm, and t the mean temperature of the driest month in °C.

Thornthwaite (1931) classified the climatic regions into different classes based on the precipitation effec-tiveness index (PE), which is computed from the monthly values of precipitation and temperature by us-ing following equation:

(3)

where P is the monthly precipitation in inches, T is the temperature in °F, and n is the months(=12).

c. Trend Analysis

Trend of rainfall (1969-2003) is calculated to envis-age the temporal pattern of rainfall in Bangladesh. There exist numerous parametric methods, such as moving average or running mean (Sneyers, 1992, Salinger et al., 1995), linear regression (Gregory, 1978; Lanzante, 1996), etc. and non-parametric method, such as Mann–Kendall’s test, Spearman’s test, etc. (Sneyers, 1992) for the trend analysis. As mentioned by several authors (Yu and Neil, 1993; Suppiah and Hennessy, 1998), complementary information can be obtained by using both techniques. In the present study, the Mann–Kendall trend test (Mann 1945; Kendall 1975) has been used to analyze the trends of rainfall.

Fig. 1. Location of rain-gauge stations in the map of Bangladesh.


In Mann-Kendall (MK) test the data are evaluated as an ordered time series. Each data value is compared to all subsequent data values in order to calculate the Mann-Kendall statistic. The probability associated with Mann-Kendall statistic is then computed to statistically quantify the significance of the trend. Details of Mann- Kendall test can be obtained in Sneyers (1992).

Some trends may not be evaluated to be statistically significant while they might be of practical interest (Yue and Hashino, 2003; Basistha et al., 2007). Even if climate change component is present, it may not be detected by statistical tests at a satisfactory significance level (Radziejewski and Kundzewicz, 2004). In the present study, linear trend analysis is also carried out and the magnitude of trend is estimated by Sen’s Slope method (Sen, 1968). Sen’s Slope method gives a robust estimation of trend (Yue et al., 2002). Sen's method cal-culates the slope as a change in measurement per change in time. Sen's estimator of slope is simply given by the median slope. Details of Sen’s slope method can be found in Sen (1968).

d. Mapping using GIS

For mapping of spatial characteristics of rainfall, mean annual rainfall, rainfall concentration index, their variation coefficients and trends are calculated for each station. GIS is used for the development of historic rain-fall database and calculation of rainfall characteristics at each rain gauge station. Raster maps of rainfall char-acteristics are prepared from point data using kriging interpolation method.

Historical rainfall database within a GIS is devel-oped using the concept proposed by Goodall et al. (2004). In the present study, a data model consists of one spatial component called Rainfall and one temporal component called TimeSeries is developed. The spatial component contains feature classes relevant to that component. For example, Rainfall component con-tains the RainGaugePoint feature class. Every feature within RainGaugePoint class is assigned a unique identifier. This identifier is used to relate features and objects internal to the geodatabase. Therefore, the tem-poral component is linked to the spatial components through the Feature ID. This allows one to query the

database using both spatial and temporal descriptions (Goodall et al., 2004).

Geostatistical analysis tool of ArcGIS 9.0 (ESRI, 2003) has been used for the preparation of raster map from point data using kriging interpolation method. Kriging is a stochastic interpolation method (Isaaks and Srivastava, 1989), which is widely recognized as stand-ard approach for surface interpolation based on scalar measurements at different points. Study showed that Kriging gives better global predictions than other meth-ods such inverse distance or weighted inverse distance techniques (van Beers and Kleijnen, 2004). Therefore, kriging is used in this study for the interpolation of point data to prepare the raster maps of various rainfall parameters. Kriging is an optimal surface interpolation method based on spatially dependent variance, which is generally expressed as a semi-variogram. Surface in-terpolation using kriging depends on the selected semi- variogram model and the semi-variogram must be fit-ted with a mathematical function or model. Depending on the shape of semi-variograms, spherical and Guassian models are used in the present study for their fitting. Spherical and Gaussian models are bounded variogram functions. The spherical model employs a progressively decreasing spatial autocorrelation for the fitting of data. On the other hand Gaussian model uses the maximum likelihood approach for this purpose.

4. Results and Discussion

Precipitation climatology over Bangladesh or the mean of annual precipitation for the period 1969–2003 is shown in Fig. 2a. Rainfall in Bangladesh varies from 1527 mm in the west to 4197 mm in the east. The gradient of rainfall from west to east is approximately 7 mm km-1. The monthly distribution of rainfall over the country is shown by a graph in Fig. 2b. The left verti-cal axis of the graph represents rainfall in millimeter and the right vertical axis represents .the rainfall as a percentage of annual total rainfall. The graph shows that the rainfall is very much seasonal in Bangladesh, more than 89% of rainfall occurs during May to October.

The main mechanism of the rainfall in Bangladesh during the summer monsoon season is caused by trop-ical depressions known as monsoon depression in the


Bay of Bengal (Ahmed and Kim, 2003). The monsoon depressions enter Bangladesh from the Bay of Bengal with south-to-north trajectory and then turn toward the northwest and west being deflected by the Meghalaya Plateau. As these depressions move farther and farther inland, their moisture content decreases, resulting in decreasing rainfall toward the northwest and west of Bangladesh (Ahmed and Kim, 2003). On the other hand, the additional uplifting effect of the Meghalaya

plateau increased the rainfall in northeast of Bangladesh.De Martonne’s aridity index and Thornthwait’s pre-

cipitation effectiveness index maps of Bangladesh are shown in Figs. 3(a) and 3(b) respectively. The aridity maps reveal three climate zones in Bangladesh viz. moist sub-humid, humid and wet. The climate of Bangladesh is mostly humid type. The northeastern side of the country belongs to wet climate and the central western part of the country belongs to moist sub-humid

(a)

(b)

Fig. 2. (a) Spatial variation of mean annual rainfall over Bangladesh; (b) Monthly distribution of rainfall computed from35 years (1969-2003) rainfall data.


climate. Low rainfall and high variation between winter and summer temperature is the characteristics of moist sub-humid zone of Bangladesh. Mean annual rainfall in this zone is less than 2000 mm and the mean temper-ature varies between 20°C in winter and 32°C in summer. In summer, some of the hottest days experi-ence a temperature of about 45°C or even more and in winter it falls to about 5°C in some places of this zone (Banglapedia, 2003). Therefore, the region experi-ences the two extremities that clearly contrast with the climatic condition of rest of the country. The lowest in-

dex values obtained by De Martonne and Thornthwaite methods are 20.89 and 64.04 respectively in the north-western side of this zone. As the dryness index values in northwestern Bangladesh is close to that of a ‘dry zone’, the climate of this region of Bangladesh is termed as ‘dry climate’. The total annual potential evapotranspi-ration in this part of Bangladesh is also lower than or equal to annual rainfall in some years. High rainfall and low variation between winter and summer temperature is the characteristics of humid zone of Bangladesh. Mean annual rainfall in this zone varies between 2000

(a)

(b)

Fig. 3. Dryness map of Bangladesh obtained by (a) De Martonne and (b) Thornwait models.


mm and 4000 mm. The mean annual rainfall is around two times more than the mean annual potential evapo-transpiration in most parts of this zone. The mean tem-perature in humid zone is 22°C in winter and 28°C in summer. On the other hand, the wet zone is charac-terized by huge rainfall and moderate variation of temperature. The mean annual rainfall in the wet zone of Bangladesh is higher than 4000 mm which is four-times more than the mean annual potential evapotranspiration. Maximum aridity index values obtained in this zone by De Martonne and Thornthwait methods are 161.25 and 874.24 respectively.

Spatial pattern of the inter-annual variability of rain-fall over Bangladesh is shown in Fig. 4. There exists no defined method for the classification of rainfall variability. Classification of rainfall variability de-pends on the rainfall characteristics of a region. In the present paper, if the coefficient of variation (CV) of an-nual rainfall is more than 24% then the rainfall is termed as highly variable, if CV is within the range of 16% to 24% then the rainfall is termed as moderately variable and if it is less than 16% then the rainfall is termed as less variable. According to that classification, a moder-ate inter-annual variability of rainfall is observed in most parts of Bangladesh. The value gradually de-creases in all directions except in northeastern hilly region. High variability of rainfall in the northwester part of the country has made the region highly prone to droughts. The region experienced more than eight droughts of major magnitude in last forty years (Paul, 1998). The coefficient of variation of rainfall is com-paratively low in the coastal region of Bangladesh which is highly prone to cyclones and storm surges.

The time series of annual average rainfall over Bangladesh for the period 1969-2003 is shown in Fig. 5a. The mean annual rainfall over Bangladesh is 2488 mm. The deviation of annual precipitation from mean precipitation is found to vary from +408 mm to -586 mm during the time period 1969 to 2003. The magni-tude of change of annual rainfall computed by Sen’s slope method shows an increase of annual rainfall with a rate of +4.94 mm y-1. However, the Mann-Kendall test shows that the trend is not statistically significant.

The time series of monsoon (June–October), pre- monsoon (March–May) and winter (November –

February) rainfall over Bangladesh are shown in Figs. 5(b), 5(c) and 5(d), respectively. The means of mon-soon, pre-monsoon and winter rainfall over Bangladesh are found 1946 mm, 230 mm and 88 mm, respectively. The trend analysis of monsoon, pre-monsoon and win-ter rainfall by Sen’s slope method shows increase of monsoon and pre-monsoon rainfall with a rate of +3.33 mm y-1 and +2.43 mm y-1 respectively and decrease of winter rainfall with a rate of -0.47 mm y-1. However, none of the trends is found statistically significant at 95% level of confidence by Mann-Kendall test.

Trend analysis is also carried out in each climate zone identified by De Martonne’s aridity index and Thornthwait’s precipitation effectiveness index meth-ods to explore the existence of any significant trends of rainfall in a particular climate zone. Statistically sig-nificant change is observed only in annual rainfall in humid zone. The annual average rainfall time series in humid zone is shown in Fig. 6. The study shows that annual average rainfall in humid zone of Bangladesh is increasing by 9.88 mm y-1 at 95% level of confidence.

Fig. 4. Rainfall variability map of Bangladesh.


Trend analysis of rainfall at each station is also car-ried out to study the spatial variation of rainfall trends in Bangladesh. The obtained result is given in Table 1. Out of 24 stations, significant change of annual rainfall is observed in five stations. Among them four stations show positive change of rainfall and one station shows negative change of rainfall. Among the rest nineteen stations, eleven stations show positive change of rain-fall and eight stations show negative change of rainfall. Statistically significant change of monsoon rainfall is observed in seven stations in Bangladesh. Positive change is observed at four stations and negative change is observed at three stations. Out of 24 stations, eleven stations show increase of monsoon rainfall and thirteen

stations show decrease of monsoon rainfall over the study period. Positive change in pre-monsoon rainfall is observed at most of the stations of Bangladesh. However, significant positive change is observed only at six stations. Change in winter rainfall is negligible in most of the stations. Significant change is observed only is two stations.

The spatial distribution of annual, monsoon, pre- monsoon and winter rainfall trend over the country are shown in Figs. 7(a), 7(b), 7(c) and 7(d) respectively. Plus (+) signs in the figures indicates an increase of rain-fall, minus signs (-) means a decrease of rainfall and zero (0) means no or negligible change of rainfall during the time period 1969-2003. The signs in white color denote

Table 1. Trend of rainfall in 24 stations of Bangladesh during the period 1969-2003

Station AnnualRainfall Trend

Monsoon Rainfall Trend

Pre-MonsoonRainfall Trend

WinterRainfall Trend PCI Trend

Sylhet -15.71 -9.10* -6.65 -1.12 0.0229Srimongal 7.41 9.00 2.52 -0.59 -0.0418Comilla 6.60 3.12 1.62 0.86 0.0088Rangamati 13.50 5.00 11.13* 0.33 -0.0586Chittagong 4.40 -1.26 7.42** -0.41 -0.0489CoxBaz 14.21 10.82 7.00** 0.71 -0.0716Hatya 7.15 1.90 2.41 -0.43 -0.0043M.Cort -15.36 -18.79** 7.44** -2.22* 0.0089Sandip 9.77 8.25 2.56 0.88 0.0248Teknaf 33.23* 33.19* 8.00** 0.77 -0.0119Faridpur -10.45 -9.72 -2.25 1.13 -0.0652**

Dhaka -7.01 -8.89 -1.51 0.19 -0.0182Mymen 1.31 -6.41 4.62 -0.64 -0.1009*

Khulna -8.48 -12.10* 0.87 2.46* -0.1509*

Barishal -5.64 -2.48 0.33 -1.71 0.0324Satkhira 0.57 -2.03 4.80* 0.44 -0.1200*

Khepupara 20.73* 14.78* 3.86 1.44 -0.0186Bhola -17.54** -9.86 -1.95 -1.15 -0.0285Jessore 3.22 3.17 1.48 0.63 -0.0043Ishurdi -15.70 -12.37 -2.10 -0.35 -0.0318Bogra 2.56 -0.67 1.76 0.38 -0.0934*

Rajshahi -5.95 -7.23 0.75 -0.52 -0.0337Dinajpur 16.00* 16.44** 2.60 0.29 -0.1166**

Rangpur 15.27* 11.17* 2.33 0.20 -0.0639**

AverageRainfall 4.94 3.33 2.43 -0.47 -0.037

* 5% level of significance ** 1% level of significance


(a)

(b)

(c)

(d)

Fig. 5. Trend of (a) annual; (b) monsoon; (c) pre-monsoon; and (d) winter rainfall of Bangladesh during 1969-2003.


significant change at 95% level of confidence. Spatial distribution of annual rainfall trend reveals that annual mean, monsoon and pre-monsoon rainfall have in-creased in north Bangladesh and south-southeastern coastal zones, and decreased in the central part of the country. Maximum increase of annual rainfall is ob-served at Teknaf (southeast corner of Bangladesh) with a rate of +33.23 mm y-1 at 95% level of confidence. Significant increase of annual rainfall is also observed in the drought vulnerable zones of northern Bangladesh. Maximum negative change of annual rainfall is ob-served at Bhola station situated in central-south Bangladesh. Spatial distribution of monsoon rainfall trend (Fig. 7b) reveals almost similar pattern of annual rainfall trend. Significant decrease of monsoon rainfall (-9.1 mm y-1) is observed in the northeastern region. The region is highly prone to flash flood due to heavy rainfall in monsoon. Pre-monsoon rainfall is very important for agriculture of Bangladesh. Increase pre-monsoon pre-cipitation can reduce the irrigation water demand in paddy field grown during dry season which shares al-most 70% of total rice production in Bangladesh. Spatial distribution of winter rainfall shows that change is negli-gible in most parts of the country.

The monsoon of Bangladesh flows in two branches, one of which strikes western India and the other travels up the Bay of Bengal and over eastern India and Bangladesh. The monsoon from the Bay of Bengal crosses the plain to the north and northeast before being turned to the west and northwest by the foothills of the Himalayas. It is anticipated that increase in sea surface temperature altered the wind patterns, leading to an ac-

cumulation of moisture in the northern region near the foothills of the Himalayas and higher rainfall during the summer monsoon season. The pre-monsoon is a transi-tional season between the northerly circulation of win-ter and southerly circulation of the summer monsoon. The thunderstorms are the sources of pre-monsoon rainfall in Bangladesh. The activity of the thunderstorms during the pre-monsoon season depends upon the sup-ply of moist air from the Bay of Bengal. Stronger and more continuous winds from the Bay of Bengal during pre-monsoon months in the recent year due to the in-crease of sea surface temperature (Mandke and Bhide, 2003) are the causes of increased pre-monsoon rainfall in Bangladesh.

Map of intra-annual distribution of rainfall or PCI in Bangladesh is shown in Fig. 8a. PCI values in most part of the country belong in the range of 13.4 to 16.0. According the classification proposed by Michiels and Gabriels (1996), this means that the rainfall of Bangladesh is moderately seasonal. Higher PCI values (PCI>16) in northern and southeastern part of the coun-try means that rainfall in these region are seasonal. PCI only allow us to examine the variability of rainfall with-in a year. In order to measure the year-to-year variations of rainfall concentration, coefficient of variation of PCI is also calculated. The map of coefficient of variation of PCI of Bangladesh is shown in Fig. 8b. The map shows that irregular intra-annual rainfall distribution is mainly concentrated in the low rainfall zone. Higher values of coefficient of variation of PCI in western part of Bangladesh mean that seasonal variability of rainfall in the region is high.

No significant change in average PCI over Bangladesh is observed by Mann-Kendall test. Trend analysis of average PCI in each climate zone reveals statistically significant change only in sub-humid zone. The aver-age PCI time series in sub-humid zone is shown in Fig. 9. Trend analysis shows that the average PCI in sub-hu-mid zone of Bangladesh is decreasing by -0.077% at 95% level of confidence.

The trend analysis of PCI at each station is given in Table 1. PCI is found negative in 19 stations out of 24 stations of Bangladesh. Signification negative change of PCI is found in five stations. Spatial distribution of PCI trend over Bangladesh is shown in Fig. 10. The fig-

Fig. 6. Trend of average annual rainfall in humid zone of Bangladesh shows an increase of rainfall by 9.88 mm/yearat 95% level of confidence.


(a) (b)

(c) (d)

Fig. 7. Spatial distribution of (a) annual; (b) monsoon; (c) pre-monsoon; and (d) winter rainfall trend over Bangladeshduring 1969-2003.


ure shows a negative trend of PCI in all over Bangladesh except in northeastern corner.

A positive trend in annual rainfall and negative trend in winter rainfall means that the rainfall of Bangladesh is concentrating in the monsoon and pre-monsoon months. Increasing annual and monsoon rainfall has been reported in other parts of Southeast Asia and East Asia (Chang and Kwon, 2007; Zhang et al., 2009). Chang and Kwon (2007) investigated the spatial pat-terns of trends in summer precipitation for South Korea and found increasing trends in precipitation amount during the summer months. Zhang et al. (2009) studied the precipitation trends of China and observed increas-ing annual and summer precipitation. Most of the re-gional climate models also predict similar results-in-creasing annual rainfall and decreasing winter rainfall in south Asia due to global climate change (IPCC, 2007). It has been predicted by using climate models that rainfall in Bangladesh will increase in monsoon and pre-monsoon, and decrease in winter in 21st cen-tury (Organization for Economic Co-operation and Development, 2003). However, a negative trend of PCI means concentration of rainfall during monsoon in some parts of Bangladesh is decreasing. This may be due to the increase of rainfall during pre-monsoon months. Though it is not possible to come to a concrete decision about climate change impact on rainfall in Bangladesh by analyzing the data presented, the results obtained in this study may be a first indication of the precipitation response to global warming–a hypoth-esis which needs to be further investigated by means of climate model projections.

5. Summary

A study on spatio-temporal variability of rainfall over Bangladesh for the time period 1969-2003 has been carried out in this paper. The results show a high spatial and temporal variation of rainfall in Bangladesh. The gradient of rainfall from west to east of Bangladesh is approximately 7 mm km-1. The aridity maps prepared by using De Martonne’s and Thornthwait’s models re-veal three climate zones in Bangladesh namely moist sub-humid, humid and wet. Spatial pattern of the in-ter-annual variability of rainfall shows a moderate to

(a)

(b)

Fig. 8. Spatial distribution of (a) precipitation concen-tration index; and (b) coefficient of variation of precip-itation concentration index.


high variability in Bangladesh. The maximum in-ter-annual variation is observed in northwestern part of

the county. Trend analysis of annual average rainfall over Bangladesh shows an increase of annual rainfall at a rate of +4.94 mm y-1 for the time period 1969-2003. Trend analysis of seasonal rainfall shows increasing monsoon and pre-monsoon rainfall and decreasing winter rainfall in Bangladesh for the same time period. However, none of the trends is statistically significant at 95% level of confidence. Spatial distribution of rain-fall trends reveal that annual, monsoon and pre-mon-soon rainfalls have increased in north Bangladesh and south-southeastern coastal zones, and decreased in the central part of the country. Maximum increase of annual rainfall is observed in southeast corner of Bangladesh at a rate of +33.23 mm y-1 at 95% level of confidence. Significant increase of annual and monsoon rainfall is also observed in drought vulnerable north Bangladesh. Significant decrease of monsoon rainfall is observed in flood prone northeastern region. Pre-monsoon rain-fall which is very important for agriculture of Bangladesh is found to increase in the south and southeastern Bangladesh at 95% level of confidence. Intra-annual distribution of rainfall or PCI shows that the rainfall of Bangladesh is moderately seasonal. Higher PCI values in northern and southeastern part of the country mean that rainfall in these regions is seasonal. The coefficient of variation of PCI reveals that irregular intra-annual rainfall distribution is mainly concentrated in the low rainfall zone of Bangladesh. Trend analysis of PCI

Fig. 10. Trend of average PCI in sub-humid zone of Bangladesh shows a decrease of rainfall concentration at95% level of confidence.

Fig. 9. Trend of average PCI in sub-humid zone of Bangladesh shows a decrease of rainfall concentration at 95% levelof confidence.


shows that rainfall concentration has decreased in most part of the country over the time period 1969-2003.

Acknowledgements. The authors acknowledge the Korean Meteorological Society for supporting the publication fee.

REFERENCES

Ahmed, A. S. M. S, A. A. Munim, Q. N. Begum, and Chowdhury, A. M., 1996: El-Niño southern oscillation and the rainfall variation over Bangladesh. Mausam, 47, 157-162.

______, 1989: Probabilistic estimates of rainfall extremes in Bangladesh during the pre-monsoon season. Indian Geogr. J., 64, 39-53.

______, 1994: Variabilities of rainfall and duration of the summer monsoon, and their relationships with mon-soon onset dates in Bangladesh. Proc. Intl. Conf. on Monsoon Prediction and Variability, Trieste, Italy, 320-327.

______, and S. Karmakar, 1993: Arrival and withdrawal dates of the summer monsoon in Bangladesh. Int. J. Climatol., 13, 727-740.

______, and I.-K. Kim, 2003: Patterns of daily rainfall in Bangladesh during the summer monsoon season: case studies at three stations. Phys. Geogr., 24, 295-318.

Banglapedia, 2003: National Encyclopaedia of Bangladesh. Asiatic Society of Bangladesh, 500 pp.

Basistha, A., N. K. Goel, D. S. Arya, and S. K. Gangwar, 2007: Spatial pattern of trends in Indian sub-divisional rainfall. Jalvigyan Sameeksha, 22, 47-57.

Buishand, T. A., 1982: Some methods for testing the homo-geneity of rainfall records. J. Hydrol., 58, 11-27.

Chang, H., and W.-T. Kwon, 2007: Spatial variations of summer precipitation trends in South Korea, 1973- 2005. Environ. Res. Lett., 2, 045012.

De Martonne, E., 1926: Aérisme et indice d’aridité. Comptes rendus de l’Académie des Sciences, 182, 1395-1398.

Dore, M. H. I., 2005: Climate change and changes in global precipitation patterns: What do we know? Environ. Int., 31, 1167-1181

ESRI, 2004: ArcGIS9: Getting Started with ArcGIS. Environmental System Research Institute, 272 pp.

Gemmer, M., S. Becker, and T. Jiang, 2004: Observed monthly precipitation trends in China 1951-2002. Theor. Appl. Climatol., 77, 39-45

Goodall, J. L., D. R. Maidment, and J. Sorenson, 2004: Representation of spatial and temporal data in ArcGIS, AWRA GIS and Water Resources III Conference, Nashville,

TN.Gregory, S., 1978: Statistical Methods and the Geographer.

Longman, 256 pp.Hulme, M., T. J. Osborn, and T. C. Johns, 1998:

Precipitation sensitivity to global warming: compar-ison of observations with HADCM2 simulations. Geophy. Res. Lett., 25, 3379-3382.

Hussain, A. M., and N. Sultana, 1996: Rainfall distribution over Bangladesh stations during the monsoon months in the absence of depressions and cyclonic storms. Mausam, 47, 339-348.

IPCC, 2007: Climate Change 2007: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Parry, M. L., Canziani, O. F., Palutikof, J. P., van der Linden, P. J., and Hanson, C. E., Eds, Cambridge University Press, 976 pp.

Isaaks, H. E., and R. M. Srivastava, 1989: An Introduction to Applied Geostatisitics. Oxford University Press, 592 pp.

Kayano, M. T., and C. Sansígolo, 2008: Interannual to dec-adal variations of precipitation and daily maximum and daily minimum temperatures in southern Brazil. Theor. Appl. Climatol., DOI 10.1007/s00704-008-0050-4

Kendall, M. and J.D. Gibbons, 1990: Rank Correlation Methods. Griffin, 272 pp.

Kripalini, R. H., S. Inamdar, and N. A. Sontakke, 1996: Rainfall variability over Bangladesh and Nepal: Comparison and connections with features over India. Int. J. Climatol., 16, 689-703.

Lambert, F., P. Stott, and M. Allen, 2003: Detection and attribution of changes in global terrestrial precipitation. Geophys. Res. Abs., 5, 06140

Lanzante, J. R., 1996: Resistant, robust and non-para-metric techniques for the analysis of climate data: theo-ry and examples including applications to historical ra-diosonde station data. Int. J. Climatol., 16, 1197-1226.

Mandke, S. K., and U. V. Bhide, 2003: A study of decreas-ing storm frequency over Bay of Bengal. J. India Geophys. Union, 7, 53-58

Mann, H. B., 1945: Nonparametric tests against trend. Economet., 13, 245-259

Mearns, L. O., C. Rosenzweig, and R. Goldberg, 1996: The effect of changes in daily and interannual climatic varia-bility on CERES-wheat: a sensitivity study. Climatic Change, 32, 257-292.

Michiels, P., and D. Gabriels, 1996. Rain variability in-dices for the assessment of rainfall erosivity in the Mediterranean region. In: Soil Degradation and Desertification in Mediterranean Environments, Rubio, J. L., and Calva, A., Eds., Spain/Logrono, 49-70.


Oliver, J. E., 1980: Monthly precipitation distribution: a comparative index. Prof. Geogr., 32, 300-309.

Organization for Economic Co-operation and Development, 2003: Development and climate change in Bangladesh: Focus on coastal flooding and the Sundarbans. In: Organization for Economic Co-operation and Development (OECD) Report COM/ENV/EPOC/ DCD/DAC(2003)3/FINAL, Agrawala, S., Ota, T., Ahmed, A.U., Eds.. [Available online at http://www. oecd.org/dataoecd/46/55/ 21055658.pdf]

Paul, B. K., 1998: Coping mechanisms practiced by drought victims (1994/5) in North Bengal, Bangladesh. Appl. Geogr., 18, 355-373.

Radziejewski, M., and Z. W. Kundzewicz, 2004: Detectability of changes in hydrological records. Hydro. Sci. J., 49, 39-51.

Rahman, M.R., M. Salehin, and J. Matsumoto, 1997: Trends of monsoon rainfall pattern in Bangladesh. Bangladesh J. Water Resour., 14-18, 121-138.

Rashid, H. E., 1991: Geography of Bangladesh. University Press Ltd, 529 pp.

Rodriguez-Puebla, C., A. H. Encinas, S. Nieto, and J. Garmendia, 1998: Spatial and temporal patterns of an-nual precipitation variability over the Iberian Peninsula. Int. J. Climatol., 18, 299-316.

Salinger, M. J., R. E. Basher, B. B. Fitzharris, J. E. Hay, P. D. Jones, J. P. Macveigh, and I. Schmidely-Leleu, 1995: Climate trends in the south-west Pacific. Int. J. Climatol., 15, 285-302.

Sen, P. K., 1968: Estimates of the regression coefficient based on Kendall's tau. J. Amer. Stat. Assoc., 63, 1379- 1389

Shahid, S., 2008: Spatial and temporal characteristics of droughts in the western part of Bangladesh. Hydrol. Process, 22, 2235-2247.

______, and H. Behrawan, 2008: Drought risk assessment in the western part of Bangladesh. Nat. Hazards, 46, 391- 413.

Sneyers, R., 1992: Use and misuse of statistical methods for detection of climatic change. Climate Change Detection Project. Report on the Informal Planning Meeting on Statistical Procedures for Climate Change Detection, WCDMP, 20, J76-J81.

Suhaila, J., S. M. Deni, and A. A. Jemain, 2008: Detecting Inhomogeneity of Rainfall Series in Peninsular Malaysia, Asia-Pacific J. Atmos. Sci., 44, 369-380.

Suppiah, R., and K. J. Hennessy, 1998: Trends in total rain-fall, heavy rain events and number of dry days in Australia, 1910-1990. Int. J. Climatol., 10, 1141-1164.

Thornthwaite, C. W., 1931: The climate of North America according to a new classification. Geogr. Rev., 21, 633- 55.

van Beers, W. C. M., and J. P. C. Kleijnen, 2004: Kriging Interpolation in Simulation: A Survey. Proc. 2004 Winter Simulation Conference Washington, Ingalls, R.G., M. D. Rossetti, J. S. Smith, and B. A. Peters, Eds., 113-121.

Von Neumann, J., 1941: Distribution of the ratio of the mean square successive difference to the variance. Ann. Math. Stat., 13, 367-395.

Wilson, E.M., 1990: Engineering Hydrology. Palgrave Macmillan, 320 pp.

Yu, B., and D. T. Neil, 1993: Long-term variations in re-gional rainfall in the south-west of Western Australia and the difference between average and high intensity rainfalls. Int. J. Climatol., 13, 77-88.

Yue, S., and M. Hashino, 2003: Long term trends of annual and monthly precipitation in Japan. J. Amer. Water Resour. Assoc., 39, 587-596.

Yue, S., P. Pilon, and G. Cavadias,2002: Power of the Mann-Kendall and Spearman’s Rho tests for detecting monotonic trends in hydrologic series. J. Hydrol., 259, 254-271.

Zhang, Q., C. Y. Xu, Z. Zhang, Y. D. Chen, and C. L. Liu, 2009: Spatial and temporal variability of precipitation over China, 1951-2005. Theor. Appl. Climatol., 95, 53-68.

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Rainfall over Bangladesh