Oriental Geographer Vol. 58, No. 2, 2014 (Published in December 2017) SPATIOTEMPORAL VARIABILITY OF RAINFALL IN BANGLADESH: USING THIESSEN POLYGON AND SPLINE INTERPOLATION Md. Sofi Ullah* Anamik Ani Khan** Md. Amran Hossain*** Abstract: The aim of the study is to analyze the rainfall variability of Bangladesh. Previous research gap has supported to develop the rationale and conceptual framework of the study. Sixty years (1953 to 2012) rainfall data were collected from Bangladesh Meteorological Department and then arranged in the tabular format to analyze rainfall variability using ArcGIS. Rainfall variability has been shown by using Thiessen polygon and Spline interpolation. Ten years interval has been used to shown climatic maps. Total 12 variability maps (6 from Thiessen polygon and 6 from Spline interpolation) have been analyzed. Data shows that north-western zone of Rajshahi division of Bangladesh is dry region due to the occurrence of low rainfall but north-eastern zone of Sylhet division, Bhola district and Coxs-Bazar district are facing high rainfall. The differences of rainfall pattern at spatial context in both Thiessen polygon and Spline interpolation have also been represented in this study. The annual rainfall pattern is irregular, where the increasing or decreasing trends are unique and the rainfall variability is high in Bangladesh. Keywords: GIS, Rainfall, Spline Interpolation, Thiessen Polygon, Zonal Statistics, Bangladesh INTRODUCTION Bangladesh is a small country in South Asia where the tropic of cancer line has just passed through middle portion of the country. The geographic location of Bangladesh has strong influence on climate and leads equatorial monsoon climate (Rashid, 1991; Islam, 2009). This climatic factor is indicating rainfall intensity in rainy season and *Md. Sofi Ullah, Associate Professor, Department of Geography and Environment, University of Dhaka, Dhaka 1000, Bangladesh ** Anamik Ani Khan, Officer-GIS, Helen Keller International, Dhaka 1212, Bangladesh ***Md. Amran Hossain, Cartographer, Department of Geography and Environment, University of Dhaka, Dhaka 1000, Bangladesh 92 ORIENTAL GEOGRAPHER dry winter. The climate of Bangladesh is also influenced by the presence of Himalayan Mountain range (IPCC, 2007). Wind moves two opposite directions in different seasons. There are six seasons found in Bangladesh but this area is identified as hot and humid climatic region (BBS, 2005a). Wind can flow from south to north in summer and rainy season but winter experiences totally opposite movement of wind from north to south (Rashid, 1991). Summer monsoons air contains vast water vapor from Bay of Bengal that leads much rainfall in rainy season (Wang and Ho, 2002; BBS, 2005a). The winter climate is very attractive in Bangladesh, for the cause of its geographical location. The Himalayan Mountain surrounded Bangladesh from its northern part which causes interruption of cold air circulation in Bangladesh. Bangladesh has faced approximately 250 to 350 cm rainfall within a year (Islam, 2009). According to the scientists this amount of rainfall is adequate and it helps to grow plants and crops in this country. In consequence of it, Bangladesh has achieved self-sufficiency in agricultural economy and agricultural production (BBS, 2005b). Moreover, about 80 percent of the people of Bangladesh have directly or indirectly linked with agriculture (Rashid, 1991; BBS, 2005b). Water is the source of all kinds of life; overall Bangladesh has no water scarcity except Rajshahi district. Adequate water supply in this country has made the country well vegetative in the world (IPCC, 2007). The presence of tropical rainforests and mangrove forests in Bangladesh cause adequate supply of water by rainfall and river flows (Rashid, 1991; BBS, 2005a). Geographic Information System (GIS) and remote sensing are sophisticated tools to analyze all kinds of geographical, environmental and climatic conditions of the earth surface. These tools are considered to analyze spatiotemporal variability of rainfall in Bangladesh. The rationale of the study indicates to analyze the rainfall variability of Bangladesh using GIS which has portrayed actual scenarios of rainfall that can easily be understood by its readers. The analysis of rainfall variability using GIS were focused in various studies which have been reviewed by the researchers, but these kinds of analysis were very limited or still not enough (BBS, 2005b; BBS, 2005a; IPCC, 2007; Islam, 2009). Bangladesh Meteorological Department (BMD) maintains all kinds of tabular data related with temperature and rainfall for its stations (Meteorological Stations). Most of the previously conducted researchers have focused only on tabular and graphical relationship of rainfall data (Wang and Ho, 2002; BBS, 2005a). The various studies of spatiotemporal rainfall variability in Bangladesh has been conducted by using different statistical and cartographic techniques, but the use of sophisticated tools like GIS and remote sensing are missing, which indicates a research gap here. On the other hand, most of the previously conducted research used only 30 to 40 years meteorological data. Considering 60 years meteorological data in a study can be a milestone, which was a gap in the previous research. The present SPATIOTEMPORAL VARIABILITY OF RAINFALL IN BANGLADESH: 93 research has tried to consider filling up the gap of the previous research on rainfall variability in Bangladesh by applying GIS tools and techniques appropriately and also temporal aspects. The problem statement indicates to find out proper tools and techniques of GIS to analyze rainfall variability of Bangladesh which is a challenge for the researchers of this study. This research has significance for geographers, environmentalists, national policy makers and so on. LITERATURE REVIEW Previous research findings on rainfall variability of Bangladesh and applications of GIS to analyze rainfall and climatic situation in different studies have also been reviewed. Dore (2005) has analyzed global precipitation pattern by applying GIS where multiple methods were applied to represent different decade’s precipitation maps. Dore (2005) has also tried to correlate precipitation variability climate change. IPCC (2007) indicates that the precipitation pattern at different regions of the world is changing due to climate change. IPCC has used an excellent GIS technique to represent the precipitation maps of different regions of the world (IPCC, 2007). Shaw (2012) has stated that water, communities and disasters are inter-linked where major water sources have represented from rainfall and provided valuable information about disasters of Bangladesh. This study was based on qualitative analysis; basically it is a shallow type analysis of rainfall of Bangladesh without applying any GIS or advanced tools. The linkage of water supply through stream flow and rainfall can influence community and society to mitigate local disasters (Shaw, 2012). Tveito and Schoner (2002) have applied spatial interpolation to analyze climatic data; which ensure advanced application of GIS and represented in form of climatic and meteorological presentations. Anuforom (2004) has used Thiessen polygon method to analyze and represent rainfall variability of Nigeria. Daly et al, (2004) have prepared monthly climate maps of USA using advanced GIS and cartographic techniques, which represent applied climatological features of USA. Ayanlade (2009) has analyzed Guinea Savanna seasonal rainfall variability using GIS Spline interpolation method to represent rainfall variability and climate change. Bollen and Van de Sompel (2006) have developed the mapping structure or layout to ensure map title, legend, north line, proper spatial reference or grid and research target based spatial features by using GIS tools and techniques. Sabir et al, (2010) have used Spline interpolation method for estimating the problem peak with time lags. Liu and Rousseau (2011) have shown GIS Spline technique for discrete data representation in map, they also showed the process of creating dynamic information from spatiotemporal data. Liu and Rousseau (2012) have shown the use of Spline interpolation of GIS to represent functional 94 ORIENTAL GEOGRAPHER fitting of discrete data for climate, regional planning, rainfall and land use management. Rashid (1991) has stated that Bangladesh lies at hot and humid climatic region which indicates adequate rainfall in this country. He also indicates Bangladesh faces 200 to 300 cm rainfall each year which is sufficient for healthy plant growth and effective for land cultivation (Rashid, 1991). Wang and Ho (2002) have analyzed seasonal variability of rain in Asia where summer monsoon periods are given top priority. The authors also argue that Bangladesh and eastern part of India are facing tropical monsoon climate and faces extreme rainfall (Wang and Ho, 2002). BBS (2005a) has represented rainfall is significant compendium of environment of Bangladesh where total agricultural system depends on rainfall. Seasonal rainfall is the determinant for agricultural production like rice, wheat, crops and other main food grain of Bangladesh (BBS, 2005a). BBS (2005b) has stated that average rainfall from 1995 to 2004 was about 300 cm per year and which is very helpful for agricultural production in Bangladesh. About 300 cm rainfall per year also has a continuous trend in Bangladesh, this trend has sustain in 1998, 1999 and 2000 and helps to grow more agricultural production and mitigate