Analysis of Historical Pattern of Rainfall in the Western Region of Bangladesh

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1 24 25 April 214, Asian University for Women, Bangladesh Analysis of Historical Pattern of Rainfall in the Western Region of Bangladesh Md. Tanvir Alam 1*, Tanni Sarker 2 1,2 Department of Civil Engineering, Ahsanullah University of Science and Technology (AUST), Dhaka-128, Bangladesh. Abstract: *Corresponding author: tanviralam_ce@yahoo.com North-western region of Bangladesh is generally very low-lying and crisscrossed by river systems; south-western region is the part of inactive delta of large Himalayan Rivers and located just behind the mangrove forest Sunderban and Bay of Bengal. In this study attempts were taken to reveal the monthly trend, seasonal and annual variation of rainfall. Different statistical analysis was done to asses the changing behavior of climate. Monthly rainfall in northwestern region experiences negative trend line in January, February, July, August, September, November and December while all other month shows positive slope of trend line. South-western region is experienced with negative trend line in April, May, August, November and December while other month experienced with positive slope of trend line. The trend from the analysis of previous rainfall data also depicts an apparent seasonal variation as dry months are getting even drier and monsoon periods are having more rain but those are already vulnerable to floods. Annual average rainfall shows that north-western region is the dry part of the country and south-western region is the average wet part of the country. A significant signs of changing behavior of rainfall pattern in these regions of Bangladesh during last forty years were observed which also indicate that more change is imminent; obviously proper preparation should be taken to feel the least impacts of the changes and better adaptation strategies with nature s adverse effects. Keywords: Daily Rainfall, Monthly Rainfall, South-Western Region, North-Western Region, Climate Change. 1. Introduction: According to intergovernmental panel on climate change [1] global mean surface temperatures have increased by.74% since late nineteenth century. This means change in present climate. Recently it is changing fast; more or less every part in the world is experiencing adverse effect of it. Climate changes usually have strong effect on the people and their behavior as well as on agriculture resources and the availability of water and especially is important in regions where economic activity depend on agriculture [2,3]. Developing country like Bangladesh is feeling its worst impacts in the form of flood, cyclone, drought, landslide and many other natural disasters which are related to rainfall [4]. Rainfall is changing due to global warming on both global [5,6,7] and regional scales[8,9,1]. Rainfall is the most important parameter in understanding global warming & climate changes. Mainly the change of rainfall pattern and also temperature are considered as the measure of climate change in a particular region. So to understand the changing behavior of climate, rainfall is widely used in the modern studies of climate change. Understanding the nature and scale of changes are important for the policy makers and people who are working on it; they will get a chance to be prepared for better mitigation and adaptation program. For this a detailed statistical analysis of historical daily rainfall has been done in this study in order to check the extent of the changing behavior of climate.

2 24 25 April 214, Asian University for Women, Bangladesh 2. Location: According to hydrological characteristics Bangladesh is divided into five regions: (1) North-Western (2) South- Western (3) Central (4) North-Eastern (5) South-Eastern. Figure 1 shows the hydrological regions of Bangladesh. Our study areas are North-Western and South-Western region. North-Western region is consisting of Rangpur and Rajshahi division. It has an area of 34,337 square kilometer [11,12]. The area is generally very low-lying, and crisscrossed by river systems. The soil tends to be sandy and water tables deeper than in other regions. The region is vulnerable to frequent natural disasters. People face catastrophe like cyclones during summer, floods and river-bank erosion in the rainy season, drought in summer, spring and cold wave in winter [13]. South-Western Region is consisting of Khulna and Barisal division. It has an area of 35,59 square kilometer [14,15]. It is the part of inactive delta of large Himalayan Rivers and located just behind the mangrove forest Sunderban and Bay of Bengal. Since the South-western region is located in the coastal zone, it possesses a fragile ecosystem and is exposed to a number of calamities like cyclones, floods, tidal surges, repeated water logging, and land erosion, degradation etc that shaped the lives and livelihood patterns of people [16]. 3. Methodology: Figure 1: Hydrological Regions of Bangladesh For this study daily rainfall data for past 4 years from 1971 to 21 for the North-Western and South-Western region of Bangladesh had been collected from Bangladesh Meteorological Department (BMD). The following major analyses were done with the daily records: (a) Monthly average calculation and Construction of trend line: Monthly average of rainfall was estimated from the daily records to assess the changes and trends in monthly rainfall with respect to different year. Linear trends were constructed for every individual month during the period of 1971 to 21. (b) Variation of rainy days as well as rainfall: Any rainfall greater than 1mm in a day was considered as a rainy day. Total number of rainy days were estimated and plotted to assess its yearly variation. ly summation of daily rainfall also estimated over the period of 1971 to 21 in order to check the variation pattern. Moreover seasonal variation of four distinct seasons available in Bangladesh was checked from the

3 24 25 April 214, Asian University for Women, Bangladesh daily rainfall data. The different seasons were considered as Monsoon (June-August), Post-monsoon (Sept- Oct), Winter (Nov-Feb) and Summer (March-May). (c) Statistical analysis: Monthly maximum and minimum rainfall was estimated from the daily records and checked several statistical parameters i.e. mean, standard deviation etc in order to assess the changing behavior of the climate. 4. Result and Discussion: 4.1 Monthly Average Rainfall: From analysis of the daily rainfall data during it is found that the most productive month of rainfall is July 391 mm in north-western region and 449 mm in south-western region respectively. June, August and September are other productive month in these regions. January is the driest of all months 8.38 mm and 9.18 mm in north-western and south-western region respectively. November, December, February and March are other dry month in north-western region. December, February and March are other dry month in south-western region. Figure 2 shows Monthly Average of Rainfall during in north-western and Figure 3 shows Monthly Average of Rainfall during in south-western region. Monthly rainfall data was estimated from the daily data and checked the trend of every month for the past 4 years. From the trend in north-western region, it is observed the month January, February, July, August, September, November and December experience negative trend lines while all other months show positive slope of trend lines. Significant increasing trend is observed in June, and October. In south-western region April, May, August, November and December is experienced with negative trend lines while other month experienced with positive slope of trend lines. Significant increase in trend lines is observed in July, September and October while significant decrease in trend lines observed in April, August and November. Figure 4 and Figure 5 shows the maximum increasing trend in north-western region and south-western region respectively. Figure 6 and Figure 7 shows the maximum decreasing trend in north-western region and south-western region respectively. From the trend analysis we have seen that the months of winter season are showing negative trends in both regions which depicts that dry period is getting drier. The months of monsoon in north-western region is showing decreasing trend whereas southwestern region is showing increasing tendency in monsoon period, which is alarming since this is the most vulnerable period for frequent floods. Rainfall(mm) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Month Rainfall(mm) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Month Figure 2: Monthly average of rainfall during in North-Western Region Figure 3: Monthly average of rainfall during in South-Western Region

4 24 25 April 214, Asian University for Women, Bangladesh Monthly Rainfall(mm) y = 2.414x R² = Monthly Rainfall(mm) y = x R² = Figure 4: Rainfall trend during the month of June in North-Western Region Figure 5: Rainfall trend during the month of October in South-Western Region Monthly Rainfall(mm) y =.4154x R² = Figure 6: Rainfall trend during the month of July in North-Western Region Monthly Rainfall(mm) y = x R² = Figure 7: Rainfall trend during the month of August in South-Western Region 4.2 Variation of Rainfall and Rainy Days: Annual average rainfall varies every year Figure 8 shows in north-western region the lowest annual average rainfall of 189 mm occurs in 1972 and the highest annual average rainfall of 2732 mm occurs in Average annual rainfall during the period ( ) is 1833 mm per year indicated by a horizontal line parallel to the x-axis which is less than the Bangladesh standard of average rainfall of 22mm to 28mm [17].Which depicts that northwestern region is the driest part of the country. For south-western region Figure 9 shows the lowest annual average rainfall of 168 mm occurs in 1972 and the highest annual average rainfall of 271 mm occurs in Average annual rainfall during the period ( ) is 2134 mm indicated by a horizontal line parallel to the x-axis which is close to the Bangladesh standard value.which depicts that south-eastern region is average wet part of the country. In north-western region number of rainy days varies between 81 to 123 days per year. Average number of rainy days is 12 days per year shown in Figure 1. In south-eastern region number of rainy days varies between 92 to137 days per year. Average number of rainy days is 116 days per year shown in Figure 11.

5 24 25 April 214, Asian University for Women, Bangladesh Monsoon, Post-monsoon and Summer are showing increasing trends in terms of seasonal rainfall whereas winter shows decreasing trend in term of seasonal rainfall in north-western region shown in Figure 12. In south-eastern region Monsoon and Post-monsoon are showing increasing trends in terms of seasonal rainfall whereas Winter and Summer are showing decreasing trend shown in Figure 13. Rainfall(mm) Rainfall(mm) Figure 8: Annual rainfall variation in North-Western Region from Figure 9: Annual rainfall variation in South-Western Region from Rainy Days Figure1: Variation in number of rainy days estimated from the history of in North- Western Region Rainy Days Figure11: Variation in number of rainy days estimated from the history of in South- Western Region

6 24 25 April 214, Asian University for Women, Bangladesh (a) Monsoon (c) Winter y = x R² = y =.5117x R² = y = x R² = (b) Post-monsoon y = 1.451x R² = (d) Summer Figure 12: Seasonal rainfall trend during in North-Western Region (b) Monsoon y =.5668x R² = y = 7.469x R² = (b) Post-monsoon

7 24 25 April 214, Asian University for Women, Bangladesh (c) Winter y = 1.273x R² = y = 1.593x R² = (d) Summer Figure 13: Seasonal rainfall trend during in South-Western Region 4.3 Statistical Analysis of Monthly Rainfall: Statistical parameters i.e. mean, maximum, minimum, standard deviation of the collected rainfall records were estimated and summarize in Table 1. Minimum rainfall in different months varies from to mm in northwestern region and to 17.5 mm in south-western region. Maximum rainfall varies from 36 mm to mm in north-western region and mm to mm in south-western region. Standard deviation (SD) is highest in July ( mm) for north-western region and in June ( mm) for south-western region. Which depicts that amount of rainfall varies most in the month of July for north-western region and in June for south-western region. Coefficient of Variation (CV) is highest in December, for north-western region (18.47%) and for south-western region (24.78%).Which depicts that amount of rainfall varies most based on percentage in December for both regions. Table 1: Statistical analysis of the monthly rainfall from Region Month Monthly Rainfall(mm) SD(mm) CV (%) Minimum Maximum Mean January February March April North-Western May June July August September October November December January February March April May South-Western June July August September October November December

8 24 25 April 214, Asian University for Women, Bangladesh 5. Conclusion: The study reveals that climate change has a significant impact in the occurrences of rainfall pattern during the recent decades due to global warming. Dry-winter is getting drier and wet monsoon is getting more wet during last 4 years in the north-western and south-western region of Bangladesh resulting adverse impacts of drought and flood respectively. Monthly rainfall in north-western region experiences negative trend line in January, February, July, August, September, November and December while all other month shows positive slope of trend line. In southwestern region April, May, August, November and December is experienced with negative trend line while other month experienced with positive slope of trend line. Annual rainfall varies every year, in north-western region the lowest annual rainfall is 189 mm the highest annual rainfall is 2732 mm; average annual rainfall during the period ( ) is 1833 mm. For south-western region the lowest annual rainfall is 168 mm and the highest annual rainfall is 271 mm; average annual rainfall during the period ( ) is 2134 mm. In north-western region number of rainy days varies between 81 to 123 days per year and in south-western region number of rainy days varies between 92 to137 days per year. Standard deviation (SD) is highest in July for north-western region and in June for south-western region. Coefficient of Variation (CV) is highest in December for both regions. Acknowledgement: Authors are indebted to the Bangladesh Meteorological Department (BMD) for providing the necessary meteorological data. References: 1. IPCC, Climate Change: The Intergovernmental Panel on Climate Change Scientific Assesment, (Cambridge University Press, Cambridge,27.) 2. T.B.Altin, B. Barak, B.C. Altin, Change in precipitation and temperature amounts over three decades in Central Anatolia, Turkey Atmospheric and Climate Science,212, 2, Alam,M.T., Iskander,S.M.,(213), Climate Change Impact: Climate Type, Vegetation Type, Rainfall Intensity Over Three Decades in Bangladesh IOSR Journal of Environmental Science, Toxicology And Food technology, Vol.4, Issue.6,pp Islam,M.S., Hasan,G.M.J., Chowdhury,M.A..I.,Ahmed,S.,(21), Historical pattern of daily rainfall in the north-eastern part of Bangladesh CERIE Hulme, M., T. J. Osborn, and T. C. Johns, 1998: Precipitation sensitivity to global warming: comparisonof observations with HADCM2 simulations. Geophy. Res. Lett., 25, Lambert, F., P. Stott, and M. Allen, 23: Detection and attribution of changes in global terrestrial precipitation. Geophys. Res. Abs., 5, Dore, M. H. I., 25: Climate change and changes in global precipitation patterns: What do we know? Environ. Int., 31, Rodriguez-Puebla, C., A. H. Encinas, S. Nieto, and J. Garmendia, 1998: Spatial and temporal patterns of annual precipitation variability over the Iberian Peninsula. Int. J. Climatol., 18, Gemmer, M., S. Becker, and T. Jiang, 24: Observed monthly precipitation trends in China Theor. Appl. Climatol., 77, Kayano, M. T., and C. Sansígolo, 28: Interannual to decadal variations of precipitation and daily maximum and daily minimum temperatures in southern Brazil. Theor. Appl. Climatol., DOI 1.17/s Ahmed,M.F., Rahman, M.M.,(23) : Water Supply and Sanitation. ITN-Bangladesh pp.319