Contents preface (VII) introduction 1—37

121

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 construc-tion, 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.
(c) 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.
Ground water source is replenished through the processes of infiltration and percolation. Infiltration is the process by which the precipitation and surface water move downward into the soil. Percolation is the vertical and lateral movement through the various openings in the geological formations. Natural sources of replenishment include rainwater, melting snow or ice and water in stream channels, and lakes or other natural bodies of water. Rainwater may infiltrate into the ground directly or while flowing over the land enroute to a river, or stream, or other water bodies. Artificial sources of replenishment (or recharge) include the following (2):
(i) Leakage from reservoirs, conduits, septic tanks, and similar water related struc-tures.
(ii) Irrigation, or other water applications including deliberate flooding of a naturally porous area.
(iii) Effluents discharged to evaporation or percolation ponds. (iv) Injection through wells or other similar structures.