Bangladesh is primarily a low-lying plain of about
144 000 km2, situated on deltas of large rivers flowing
from the Himalayas. Geographically, it extends from
20°34N to 26°38N latitude and from 88°01E to 92°41E
longitude. Bangladesh has a sub-tropical humid climate
characterised by wide seasonal variations in rainfall,
moderately warm temperatures and high humidity
(Rashid, 1991). Four distinct seasons can be recognised
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. Spatial distribution of rainfall
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. Monthly distribution of rainfall
over Bangladesh is shown in Figure 1(b). More than
75% rainfall in Bangladesh occurs in 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 from
70.5% 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
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 terrace land can be found in the northwestern
and central regions of the country. 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 covers 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.