• Storage reservoirs are vital to the exploitation of water resources for sustained development of the country. Beginning from Krishnarajsagar in Karnataka and Mettur in Tamil Nadu, a large number of storage reservoirs have been built in India which have contributed substantially to irrigation and power sectors besides affording protection from floods to millions of people. Storage reservoirs are also necessary for providing water supplies to large cities and towns and to high water consuming industries.
• In general, large storage reservoirs are constructed under major/medium irrigation and multi purpose schemes and small storage are constructed through surface water minor irrigation schemes. For some time there has been a controversy on the adverse effects of large dams. Environmentalists vehemently oppose the construction of large dams because of the adverse impacts caused by the large dams on the environment. It is often asserted that developmental activities, which change the existing natural balance, are not advisable and hence irrigation development should hereafter be through small dams, ground water utilization etc , which have comparatively less adverse impacts.
• Large dams provide assured supplies for irrigation, municipal and industrial requirements. If the dams are designed to have a carry over capacity, they also cater for the needs of crops in the following year if the rainfall becomes erratic. Large irrigation systems with a network of canals under large dams are more amenable to the introduction of a rotational supply of water and better management policies, enhancing the irrigation efficiency. Large dams can generate substantial firm power because small storage cannot effectively regulate the erratic nature of river flows for sustained Dower development. Besides the cost per KW of installed capacity is much higher in the case of small dams.
• Large dams can also be designed for specific releases for maintaining the quality of the river water. The scope for developing recreation centers is also greater in large reservoirs.
• To replace a single large dam, a large number of small dams would need to be constructed either in upper or lower reaches of a watershed. Such a large number of alternative sites are rarely available in practice necessitating curtailment in the envisaged development
• Large storage dams are required even to meet the drinking and industrial water needs. For example Kishau, Renuka and Lakhwarvyassi dam in Jamuna basin and Tehri Dam are absolutely necessary to meet the drinking water needs of Delhi, Rajasthan, U.P. and Haryana. Similarly construction of the Pairi High Dam in Mahanadi basin is inevitable to meet the municipal and industrial water requirements of the Chhattisgarh.

Thus the future needs of the country in water resources sector cannot be achieved without taking up the construction of large storage reservoirs.

Storage v/s Run-Of-The River (Irrigation ) :

• As Indian rivers carry most of their flow during the monsoon months, storage reservoirs are necessary to hold it for monsoon and help regulated releases for irrigation and hydropower generation through the year. Many storage dams have been constructed on our rivers over the years. The Bhakra, Hirakund, Nagarjuna Sagar, Koyna, Pong, Rihand, Srisailam, Idukki dams are some of the landmarks created through our dam building activity. Besides, the muiti purpose storage schemes, the run-of-the river schemes play an important role in irrigation and hydroelectric power generation. The storage dams are generally major projects while the run-of-the river schemes generally come under minor irrigation projects. An economic comparison of two is of course not very simple. In the first place some addition to the investment cost in the minor irrigation may have to be made to cover elements which it does not take into account. Secondly, the cost of operation may be much higher in the case of minor irrigation and there may be need for capital repairs of renewals and replacements every few years. Assets created under major irrigation project have a much longer life. There are some advantages in the case of larger projects. In a capital scarce situation where the resources to undertake a complete large projects are not easy to find, we may necessarily have to give some preference to option which call for limited investment. Much greater control over small projects can be exercised. These are not plagued by time and cost over-runs. The area covered is limited and so there should be no time lag in utilization; the gap between potential created and potential utilized is essentially a problem connected with major irrigation projects, though its effect is being felt now in case of minor irrigation also. The question of a conflict between head reach farmers and tail-enders are also one, which does not arise in case of minor irrigation. Minor irrigation is also unlikely to create problems of water logging and salinity. As the farmer ultimately bears both the capital and the operating cost though with some subsidization under minor irrigation, water-use is likely to be much more careful and economical than under major irrigation. Major irrigation project has a long gestation period whereas minor irrigation yields minimum returns. In view of these considerations, much greater emphasis is being placed on minor irrigation and major irrigation is to be more selective. This does not mean a ruling out of major projects. Minor irrigation is not a substitute for storage dams but it is only to supplement the farmer.

Option for Hydropower Generation

The World View

Knowing that influential advocacy groups oppose any further World Bank financing for large dams, many governments of developing countries are now ambivalent about seeking the Bank's support for this purpose. Yet they are reluctant to hold back the buildings of dams and the cheaper energy and more reliable food supplies that flow from them

To take an objective look at the issues underlying the controversy and to help set policy directions for future decision making, Operation Evaluation Department (OED) of the World Bank examined the rationale, likely impacts, and economic justification of 50 large dams supported, completed and evaluated by the Bank. The prime finding of this study is as under :

" The large majority of the dams reviewed are yielding benefits that far outweigh their cost, including the costs of adequate measures to mitigate their adverse impacts. Among the 50 dams as built, 45 are yielding acceptable benefit/cost ratios. Adding the probable cost of measures that the projects could have taken to satisfy today's guidelines on resettlement and environment makes little difference to the estimates of economic returns.

In almost all other cases, mitigation measures, including appropriate resettlement programs, could have been financed without jeopardizing the dams' economic returns. Thus, by today's standards, and based on the available information, 30 of the 50 dams can be regarded as acceptable; 24 as potentially acceptable; and 13, a/I appraised before 1985. as unacceptable. "

It can be summarized, in conclusion, that :

• Emphasis may be placed on micro watershed development to be a part of the future strategy but it should be understood that it cannot replace other forms of water development.
• The future needs of the country in Water Resources sector can be met with only through judicious mix of small and large storage reservoirs and ground water development.
• The Song-term strategy in irrigation development should be to expedite surface storage simultaneously along with groundwater development and take up inter- basin transfer of water.
• The development of minor irrigation is not a substitute for storage dams but it is only a supplement to the former. Development of surface water complements the ground water development through recharge and conjunctive use.