Contents preface (VII) introduction 1—37

4
GROUND WATER AND WELLS
4.1. 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). The rods were lifted and dropped manually and the method was similar to the method now known as cable tool drilling. Ground water flows to the earth’s surface through naturally discharging springs and streams and rivers which are sustained by ground water itself when overland runoff is not present. Following significant features of ground water should always be kept in mind while managing ground water (2):

(i) Ground water is a huge water resource, but is exhaustible and is unevenly available.

(ii) Ground water and surface water resources are interrelated and, hence, should be considered together.
(iii) Excessive and continued exploitation of ground water must be avoided as natural replenishment of the ground water resource is a very slow process.
(iv) Ground water is generally better than surface water in respect of biological charac-teristics. On the other hand, surface water is generally better than ground water in terms of chemical characteristics.
(v) Ground water may be developed in stages on ‘‘pay-as-you-go’’ or ‘‘pay-as-you-grow’’ basis. Surface water development usually needs large initial capital investment.