Aridity assessment in Bangladesh

Aridity assessment in Bangladesh

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Shamsuddin Shahid
Postdoctoral Fellow
[email protected]


Xiaoyong Chen
Associate Professor
[email protected]


Manzul Kumar Hazarika
Senior Training and Research Specialist
[email protected]

Remote Sensing and GIS Field of Study
School of Advanced Technologies
Asian Institute of Technology
P.O. Box: 4, Klong Luang, Pathumtani 12120
Thailand

Abstract
Rapid growth of population and massive deforestation has caused a noticeable change in climatic condition in some parts of Bangladesh. Increased aridity due to this climatic change is a growing environmental problem of the country. Assessment and monitoring of aridity is essential to combat probable land degradation and desertification. A geographic information system is used in this paper for the assessment of aridity of Bangladesh from long-term climatic data collected from fifty stations situated in and around the country. Three well-known aridity measuring models viz. De Martonne Aridity Index, Thornthwait Precipitation Effectiveness Index and UNESCO Aridity Index are used for this purpose. The result shows that climate of Bangladesh is mostly humid in nature. There exist no climatically dry zones in the country. However, climate of the northwestern and central-western side of Bangladesh, which comprises 1.55% to 7.92% of total area of country, is very close to dry sub-humid type.

Introduction
Increased dryness has long been recognized as a major environmental problem affecting the living conditions of the people in the affected regions in many countries of the world. The studies indicated that over the preceding 20 years, the problem of land degradation due to increased aridity had continued to worsen. In the past, dry lands recovered easily following long droughts and dry periods. Under modern conditions, however, they tend to lose their biological and economic productivity quickly unless they are managed in a sustainable manner (GOI, 2001). Once the aridity or dryness of an area increased beyond a certain level it becomes difficult to recover. Therefore, study the extent of dryness of an area is essential to combat land degradation and desertification. Measurement of dryness is also necessary for evaluating drought vulnerability, measuring drought severity, monitoring climatic change, assessing bio-environment, monitoring soil moisture and planning agricultural. Therefore, it is very important for a country to have an updated dryness maps. Geographic Information System is used in this paper for the mapping of aridity or dryness of Bangladesh from long-term climatic data.

Climate of Bangladesh is characterized by high temperature, heavy rainfall, often-excessive humidity, and fairly marked seasonal variations (Rashid, 1979). There are widespread differences in the intensity of the seasons at different places of the country. On the basis of entire climatic condition Chowdhury (2001) divided Bangladesh into seven distinct climatic zones. However, no standard method was used for the classification of climate. Consequently, the map fails to distinguish the dryness or wetness clearly. Recently, concern has been increased among the scientists on the extent of aridity in the northwestern side of the country. Rapid population growth along with massive deforestation and modifications of the landforms has been degrading the biophysical environment of the region. This has also caused a drastic change in climatic condition of the region. Many researchers believe that the northwestern sides of Bangladesh have become a dry region. Recently, some studies have been carried out to assess the extent of aridity, drought and risk of desertification in the northern and western region of Bangladesh. However, there are lot of differences among the geographers and ecologists regarding dryness and threat of desertification (GOB, 2001). The aim of this paper is to get an accurate and reliable picture of aridity of Bangladesh.

There are number of methods used for the mapping of aridity of an area. The methods are based on precipitation and temperature, precipitation and evaporation, precipitation and relative humidity, number of dry days, and precipitation-evapotranspiration ratio. Among the methods, a precipitation-temperature based method proposed by De Martonne (1926) is widely used for measuring aridity of an area. Another widely used method is Thornthwaite’s method (Thornthwaite, 1931) based on precipitation and temperature. Recently UNESCO (1979) has proposed a rainfall-evapotranspiration ratio based method for the estimation of dryness. In this paper all the three methods are used for the aridity mapping of Bangladesh.

Climate of Bangladesh
Bangladesh occupies about 143,998 km2. Climatically, the country belongs to sub-tropical regions where monsoon 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 southeast, 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 through February, (ii) the pre-monsoon hot summer season from March through May, and (iii) the rainy monsoon season which lasts from June through October. The average temperature of the country ranges from 7.20C to 12.80C during winter and 23.9 to 31.10C during summer. The average relative humidity for the whole year ranges from 78.1% to 70.5% with a maximum in September and a minimum in March. The average annual rainfall varies from 1329 mm in the northwest to 4338 mm in the northeast.

The dry season begins first in the west-central part of the country by December, where its duration is about four months, and it advances toward east and south, reaching the eastern and southern margins of the country by January where its duration is about one and half months. During the dry season in some regions, especially northwestern region, the evapotranspiration exceeds by an amount of more than two times the rainfall. So, for that duration within a year these regions are sometimes considered as dry region.