Page 3 of 3 Previous For De Martonne’s aridity modeling first the mean annual precipitation and temperature are calculated and the driest month of the year is identified from monthly mean climatic data. Thematic maps of mean annual precipitation and temperatures, and precipitation and temperatures of the driest month are prepared from twenty years average of the climatic data of 50 stations using krigging method. These thematic maps are then integrated using the equation (1). Finally, the AI values of the integrated layer are classified into different classes following the ranges given in Table 1 to prepare the De Martonne’s aridity index map of Bangladesh, which is shown in Figure 3(a).  De Martonne, For Thornthwait’s aridity modeling, first the PE index for all the 50 stations are calculated by using equation (2). The PE values are then krigged and classified following the ranges given in Table 2 to prepare the thematic map of Thornthwait’s precipitation effectiveness index of Bangladesh, which is shown in Figure 3(b).  Thornwait, In UNESCO aridity index modeling, first the potential evapotranspiration (PET) at 50 climatic stations are calculated using Pennman’s formula. The PET and precipitation values are then used to generate the thematic maps of PET and precipitation using krigging method. Finally, the maps are integrated using equation (3) and the generated AI values are classified using the ranges given in Table 3 to develop the UNESCO aridity index map of Bangladesh (shown in Figure 3(c)).  UNESCO models. Result and Discussion The aridity maps prepared by using De Martonne’s, Thornthwait’s and UNESCO models show that there exist on arid, semi-arid or dry sub-humid zone in Bangladesh. Most of the area of Bangladesh is belong to humid class except some parts in the northeastern side which belongs to wet class. The statistics of aridity index values obtained by using three models are given in Table 4. Lest AI values obtained by De Martonne’s, Thornthwait’s and UNESCO models are 20.89, 64.04 and 0.98 respectively in the central-western and northwestern side of Bangladesh. As the AI values in the region is close to that of dry zone, the climate of these regions of Bangladesh can be said very close of dry. The total annual evapotranspiration in this region is also lower than or equal to annual rainfall. The region which is very close to the dry sub-humid climate is shown by shade in the aridity maps.  Table 4: Statistics of aridity index (AI) values obtained by different models. It can be found from the maps that climate in the central western and northwestern side of the country is very close to dry climate. However, the climate of the area was not as like the present. The monsoon rainfall in the region was more intense than at present. Rapid population growth along with modifications of the landforms in the region has been degrading the biophysical environment and consequent changing in climate of the region. The rainfall has decreased and the difference of temperature between the hot daytime and cooler late night has increased. Since the area is free from flood, rainwater is the main source of groundwater recharge. A decrease in rainfall has caused the successive lowering of the groundwater table of the region, which has eventually been greatly affecting the environmental parameters. Besides lowering of the water table, there is a noticeable change in forest area. Almost 70% forestland of the region had been changed into cultivable land during the past few decades (Ahmed, 2001). Due to the dry nature, relatively low rainfall and massive deforestation, the vegetation cover in the area has decreased distinctively. If this trend continued, the area might become almost an arid region. Sustainable measures like rapid forestation and integrated micro-watershed management for the promotion of in situ moisture conservation should be taken in the region to tackle the situation. Programs that have relevance to environmental and sustainable development of natural resources have to be expedited. Local people have to be educated and social awareness should be grown about deforestation and its environmental consequences. On the other hand, the northeastern side of Bangladesh, which is close to Meghalaya of India is belongs to wet climate. The mean annual rainfall in this region is four-times more than the mean annual evapotranspiration. Maximum AI values obtained in this side are 161.25, 874.24 and 4.83 by De Martonne, Thornthwait and UNESCO methods respectively. Area belongs to different AI ranges are given in Table 5. It can be observed from the table that climate of 1.55% to 7.92% area of the Bangladesh has been identified by different aridity index models as near dry sub-humid. 25.14% to 32.5% area is demarcated as moist sub-humid, 55.59% to 90.44% area as humid and 7.82% to 9.34% area as wet. Mean aridity index values obtained by different models (Table 4) are also belongs to humid range. Therefore, it can be said that the climate of country is mostly humid type except few areas in the northeast margin of the country.  Table 5: Percentage of area belongs to different climate zone as demarcated by the models. Conclusions An attempt has been made for the mapping of dryness of Bangladesh using geographic information system. Three well-known and widely used models are used for this purpose. The result shows that the climate of Bangladesh is mostly humid in nature. There exists no climatically dry zone in Bangladesh. However, some parts in the western side of the country have an aridity index close to that of dry zone. Deforestation and other environmentally harmful activities in the region might cause a gradual declination of rainfall and inclination of temperature, and consequent increase of aridity of the region. If the aridity of the area tends to increase, it might lose its productivity and become an arid region. Necessary steps should be taken to manage the situation in a sustainable manner. References
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