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Rainfall Variability and the Trends of Wet and Dry Periods in Bangladesh

Shamsuddin Shahid
Department of Geology, University of Malaya, 50603 Kuala Lumpur, Malaysia

Abstract/ Executive Summary:

Spatial patterns of annual and seasonal rainfall trends of Bangladesh over the time period 1958 – 2007 has been assessed using rainfall data recorded at 17 stations distributed over the country. Mann – Kendall trend test and the Sen’s slope method are used to detect the significance and the magnitude of rainfall change, respectively. Historical dry and wet months are identified by using the standardized precipitation index method and their trends are analyzed to assess the possible change in wet and dry events in Bangladesh. The result shows a significant increase in the average annual and pre-monsoon rainfall of Bangladesh. The number of wet months is found to increase and the dry months to decrease in most parts of the country. Seasonal analysis of wet and dry months shows a significant decrease of dry months in monsoon and pre-monsoon.

Keywords: Rainfall trends; Standardized precipitation index; Extreme events; Climate change; Geographical information system; Bangladesh

Location: Bangladesh

Start Date: 00-00-1958

End Date: 00-00-2007

Program Area: Risk Management in Agriculture

Non-commodity: Rainfall

Objectives:

A study has been carried out in this article to assess the changes in rainfall and extreme events in Bangladesh through the analysis of the spatial patterns of the trends of long-term annual and seasonal rainfall as well as the number of wet and dry months in different seasons.

Methodology:

The climate of Bangladesh: Bangladesh is primarily a low-lying plain of about144 000 km2, situated on deltas of large rivers flowing from the Himalayas. Geographically, it extends from20°34′Nto26°38′N latitude and from 88°01′Eto92°41′Elongitude. Bangladesh has a subtropical humid climate characterized by wide seasonal variations in rainfall, moderately warm temperatures and high humidity (Rashid, 1991). Four distinct seasons can be recognized in Bangladesh from the climatic point of view: (1) the dry winter season from December to February, (2) the pre-monsoon hot summer season from March to May, (3) the rainy monsoon season from June to September and (4) the post-monsoon autumn season which lasts from October to November. Rainfall variability in space and time is one of the most relevant characteristics of the climate of Bangladesh. The spatial distribution of rain-fall in Bangladesh is shown in Figure 1(a). Rainfall in Bangladesh varies from 1400 mm in the west to more than 4300 mm in the east of the country. Higher rainfall in the northeast is caused by the additional uplifting effect of the Meghalaya plateau. Rainfall in Bangladesh mostly occurs in monsoon, caused by weak tropical depressions that are brought from the Bay of Bengal into Bangladesh by the wet monsoon winds. The monthly distribution of rain-fall over Bangladesh is shown in Figure 1(b). More than75% of rainfall in Bangladesh occurs in the monsoon. The average temperature of the country ranges from 17 to 20.6°C during winter and 26.9 to 31.1°C during summer. The average relative humidity for the whole year ranges from70.5% to 78.1% in Bangladesh (Banglapedia, 2003). The topography of Bangladesh is extremely flat(Figure 2(a)) with some upland in the northeast and the southeast. The plain land lies almost at sea level along the southern part of the country and rises gradually towards the north. Land elevation in the plain varies from 1 to 60 m above the sea level from south to north. The hilly areas are located in the southeastern and northeastern regions, and the terraced land can be found in the northwestern and central regions of the country. The land use map of Bangladesh (United Nations Environment Program,1994) is shown in Figure 2(b). Agriculture, being the main economic mainstay of the country, covers almost all cultivable land of Bangladesh. About 68% land of the country is used for agriculture and village settlement. Rivers and standing water bodies cover almost 13% of the area. About 14% area is covered by mangrove and upland forests. Urban area covers about 5% of the land and the rest is used for other purposes.3. Data and methodologyMonthly rainfall records of 17 stations of Bangladesh for fifty years (1958 – 2007) are collected from Bangladesh Meteorological Department for the study. Out of the 17 stations, a complete set of data was available in 12 stations. The rate of missing data in the rest of the stations was less than 2%. If data for one monsoon or pre-monsoon month of a year is found missing then that year is discarded from the trend analysis. The homogeneity of the rainfall records is analyzed by calculating the von Neumann ratio (Von Neumann,1941), standard normal homogeneity test (Alexanders-son, 1986) and the range test (Buishand, 1982). The data sets of all the stations are found homogeneous. Mann – Kendall test is applied to detect the trend in rainfall time series. Confidence levels of 90%, 95% and 99%are taken as thresholds to classify the significance of positive and negative trends. The methods used in the present study are discussed below.3.1.Mann – Kendall trend test In the Mann – Kendall test (Mann, 1945; Kendall, 1975) the data are evaluated as an ordered time series. Each data are compared to all subsequent data. The initial value of the Mann – Kendall statistic, S, is assumed to be 0 (e.g.no trend). If data from a later time period is higher than data from an earlier time period, Sis incremented by 1. On the other hand, if the data from a later time period is lower than data sampled earlier, Sis decremented by1.

Source of Information: Int. J. Climatol.30: 2299 – 2313 (2010) Published online 20 November 2009 in Wiley Online Library(wileyonlinelibrary.com)

Article Link (Online Journal): DOI: 10.1002/joc.2053

Funding Source:

Budget:

Total Budget (Tk.) :

Achievement/Findings:

A study has been carried out in this article to assess the spatial patterns of the trends of long-term annual and seasonal rainfall as well as the number of wet and dry months to detect the changes in rainfall and rain-induced extreme events in Bangladesh. The trend analysis shows a significant increase in the average annual rainfall of Bangladesh at a rate of 5.52 mm/year over the time period 1958 – 2007. The spatial pattern of average annual rainfall shows that rainfall is significantly increasing in the western part of Bangladesh. Increased sea surface temperature might have altered the wind patterns to the west of Bangladesh, leading to an accumulation of moisture and to an increase in rainfall in the region. The trend analysis of seasonal rainfall over Bangladesh shows no significant changes in the monsoon, post-monsoon and winter rainfall of Bangladesh. However, a significant increase of pre-monsoon rainfall by 2.47 mm/year at the95% level of confidence is noted. The trend analysis of wet and dry months in Bangladesh shows that the number of wet months is increasing and the number of dry months is decreasing both in monsoon and pre-monsoon in most parts of the country. Increased pre-monsoon precipitation can reduce the pressure on groundwater for irrigation in Bangladesh. However, it might not help to improve the crop water stress conditions in the north and northwestern region during pre-monsoon as no significant decrease of pre-monsoon dry months is observed in this part of the country. Increasing annual rainfall and pre-monsoon rainfall, and decreasing the number of dry months may help to reduce the drought vulnerability and increase crop productivity in some parts of Bangladesh. Increasing trends of moderate and severe wet months upstream of the river might increase the possibility of floods in some parts of the country. An increase of severe monsoon wet months and maximum SPI-1 in the southeast hill region of Bangladesh may accelerate the landslide events in the area.

Knowledge Management: Journal