Geoscience Reference
In-Depth Information
Unlike the Ganga sub-basin in India, the Padma sub-basin in Bangladesh receives
abundant rains over various regions throughout the year. The climate is marked by
high temperatures and humidity. The northern and north-eastern parts, comprising
Sylhet, Mymensingh and Coomilla districts, receive 230-300 cm of rain and south-
eastern Chittagong region receives about 250 cm. Rangpur and Dinajpur districts
in the north-west are drier and receive 180-250 cm of rain. Rajshahi district in the
west is the driest with less than 150 cm in a year. Southern Bangladesh, compris-
ing Pabna, Kushtia, Faridpur, Jessore and Khulna districts gets abundance of rains,
between 200 and 230 cm. Seasonal distribution of rains over Bangladesh is more
uniform than over India. Except in 5 months, from November to March, the country
gets good rains in other seven months, from April to October, with high intensity in
three monsoon months, from June to August.
Low rainfall regions in India, where annual downpour is less than 80 cm, are
scattered all over; the total area is also quite large. Most parts of Rajasthan, a part
of Gujarat, Punjab, Uttar Pradesh, Andhra Pradesh, Karnataka, Tamilnadu, Orissa
and West Bengal are arid or semi-arid regions, spread over about a million square
kilometres, and receive only 40-80 cm of rains on an average in a year. About one-
third of the total cultivated area in India suffers from low rainfall. On the contrary,
minimum average rainfall over Bangladesh is about 120 cm.
The ground-water storage capacity in India is also limited and scattered all
over the country. Arid and semi-arid regions in north, west and south have meagre
reserves. Except in some parts of west Bengal, Orissa and Kerala, the ground-water
level is at about 100 m below the surface. The total ground-water reserve in India
may be about 0.25 million million (1,000 billion) cubic metres, of which about
0.01 million million cubic metres have been tapped and utilised for irrigation and
drinking. Efforts are on to exploit more reserves for the burgeoning population.
On the other hand, the normal sub-soil reserve of water in Bangladesh is compara-
tively higher because of favourable soil structure and topography in most parts of the
country. The highest ground-water table in Bangladesh in monsoon months varies
from 0.5 to 2 m below the surface in the delta, including the Padma sub-basin, and
about 15 m in dry months, the average ranging from 1.5 to 3 m. Total quantity of
ground-water reserve in Bangladesh is not known.
The north Indian Gangetic plain above Haridwar is composed of largely older
alluvial high land ( banjar in local parlance), through which the river carved out its
valley between 15 and 30 m below the land surface. Its entire middle course, of about
1,500 km, from Haridwar to Farakka is through an extensive flood-prone valley,
about 10 km wide, on either flank. The course is yearly visited by floods which
deposit sand and clay of varying thickness. Much of the flood-plain is marshy and
stretched on extinct river channels. The courses often change with either braided
or meandering reaches. The bed is 1 km or more in width but much less above
Haridwar. In the Himalayan part of the river, average elevation between the source
and Haridwar where the Ganga descends on the plain is 13 m/km, whereas, down
below on the plains in eastern Uttar Pradesh it is hardly 40 cm/km. This is even less
in Farakka and Nabadweep, about 5 cm/km. The longitudinal section of the river is
shown in Fig. 4.2.
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