31-03-2012, 01:45 PM
GROUND WATER AND WELLS
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GROUND WATER RESOURCES
The amount of water stored in the earth’s crust may be of the order of 8 billion cubic kilometres,
half of which is at depths less than 800 m (1). This water inside the earth is about 35 times the
combined storage of all the world’s rivers, fresh water lakes, reservoirs, and inland seas, and is
about one-third the volume of water stored in the arctic and antarctic ice fields, the glaciers of
Greenland, and the great mountain systems of the world (2). All of this ground water, however,
cannot be utilised because of physiographic limitations.
The estimate of the present ground water resource in India (3) is of the order of 650
cubic kilometres (as against 1880 cubic km for surface water resources), out of which utilisable
ground water is assessed at around 420 cubic km (as against 690 cubic km for surface water
resources); see Table 1.4. Ground water is that part of the subsurface water which occurs
within the saturated zone of the earth’s crust where all pores are filled with water (2). Ground
water has also been referred to as that part of the subsurface water which can be lifted or
which flows naturally to the earth’s surface. A hole or shaft, usually vertical, is excavated in
the earth to lift ground water to the earth’s surface and is termed a well. A well can also be
used for disposal of water, artificial recharge, draining out agricultural lands, and relieving
pressures under hydraulic structures. The Chinese are known to be the first to have drilled
deep wells using bamboo rods tipped with iron (2).
GROUND WATER AND WELLS
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122 IRRIGATION AND WATER RESOURCES ENGINEERING
(vi) Underground reservoirs storing ground water are more advantageous than surface
reservoirs.
(a) There is no construction cost involved in underground reservoirs. But, well construction,
pumps and energy for pumping water, and maintenance of pumps and wells
require money.
(b) Underground reservoirs do not silt up, but surface siltation of recharge areas may
appreciably reduce recharge rates.
© The evaporation from underground reservoirs is much less.
(d) Underground reservoirs do not occupy the land surface which may be useful for some
other purposes.
(vii) Ground water is generally of uniform temperature and mineral quality and is free of
suspended impurities.
(viii) Ground water source has indefinite life, if properly managed.
WELL IRRIGATION
In view of the large amount of utilisable ground water, higher agricultural yield of tubewellirrigated
lands in comparison to that of canal-irrigated lands (see Table 1.3), and favourable
impact of its use on waterlogging, it is only logical to develop ground water resources for
irrigation and other activities. Most of the existing canal systems in India are of a protective
nature, i.e., they provide protection against famine. They were not designed to promote intensive
farming. Well irrigation ensures more reliable irrigation and, therefore, enables the farmers
to grow more remunerative crops with improved yield. The following are the main requirements
for the success of well irrigation: