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The meats of rehabilitation of existing irrigation systems versus construction of new projects have been debated for the last two decades. International financing agencies have increasingly favored rehabilitation. This is not an opinion which has always been shared by developing countries. In an earlier conference of the International commission on Irrigation and Drainage (ICID), there was a strong plea that priority be given to new construction, as "cultivators served by an existing project already have some supply of water, however deficient, while those in areas as yet unirrigated have none.. The view was also expressed that rehabilitation was a slow and difficult task, and less likely to have a major impact on agricultural production than new projects. It was also not considered a vehicle for substantial aid inflows.
The increased recent emphasis on rehabilitation and improvement has been due to a number of factors, including the diminishing number of sites available for new projects and the greater recognition of ecological constraints on new work, particularly on reservoir construction. Projections of future demand for agricultural production can now no longer be matched by projections of area to come under irrigation from new projects. Increased productivity from existing irrigated areas is essential and that implies extensive rehabilitation.
Judged by crop production per unit of water used, there is certainly room for improvement in most South Asian schemes, and it is commonly asked to what extent could this situation be improved by adoption of the new technologies in water distribution and irrigation being practiced elsewhere. While there is indeed opportunity for injection of new technology, this would unfortunately address only part of the problem. The causes of the low productivity are to some extent inherent in the variable nature of the monsoonal climate and the limited possibilities for regulation by storage. The consequent uncertainty of irrigation supply has given rise to a number of social and management problems which do not have easy solutions.
One approach has been to attack the management problem first, making better use of the existing irrigation infrastructure. Notable increases in production have been achieved in some situations through such an approach and at low cost. However, a study of water management problems usually also discloses deficiencies in infrastructure either at the lower end of the system (the tertiary level) or further upstream. These must be remedied before improvements in management can be effective. Current World Bank practice (e.g the National Water Management Project in India) is to carry out a thorough review of the performance of a project including effectiveness of water use (selection of crops to be irrigated, seasonal use of stored water, etc), evaluate infrastructure and operational procedures, and analyze cultivator attitudes and problems before embarking upon any improvement program. Solutions developed are very much project-specific. However, there are a number of areas which can be discussed in general terms.
Net storage capacity, rate of siltation, and dam safety are of interest. Storage capacity is of premium value in a monsoonal climate. The net capacity available at a particular site is influenced by the amount which must be reserved for flood rise, above normal retention level, during passage of a flood. This is determined by the magnitude of the design flood and the nature of the spillway, particularly its crest length and whether or not it is gated. Public safety is involved, as over-topping of an earth-fill dam can be disastrous. Safety against over-topping runs counter to the interests of irrigation as far as allocating storage against flood rise is concerned. With increased attention to safety (particularly by international agencies), review of a dam in conjunction with a project rehabilitation study frequently leads to an increase in estimated design floods and increased flood rise provision, diminishing the net storage capacity available for irrigation purposes. Modification of the spillway to increase its effective length, in some cases by installation of simple automatic or tilting gates, or possibly raising the crest of the dam may be desirable. The addition of protection to the downstream slope of embankment dams to control erosion in the event of overtopping is a new development being widely practiced elsewhere, which could be relevant to this question. Dam safety also includes the subject of dam stability and a project review could result in reduction of normal storage level unless remedial works are undertaken.
The geography of the reservoir site and of the associated canal service area usually limits the level to which the reservoir may be drawn down by gravity flow through the main canal. Below that level is dead storage. Some of the latter (at the upstream end of the reservoir) may function as silt storage, but for the most part it remains unutilized. Where there is normally more than sufficient seasonal inflow to fill the live storage and spill occurs, there may be a case for utilizing part of the dead storage either by releasing the water through low-level outlets in the dam and rediverting at some point downstream, or by pumped-lift from the reservoir into the main canal. The latter arrangement, while not normally incorporated in original construction, could well be attractive as part of an up-grading program, particularly where the shape of the reservoir basin makes the volume of otherwise dead storage very large.
Siltation has grossly reduced the storage capacity of many small reservoirs in much shorter time than anticipated. Reduction in rate of siltation is a difficult problem, but as discussed earlier, catchment erosion is often largely focused on particular locations and project rehabilitation could well include attention to such areas.
Summarizing, any rehabilitation study for a project which incorporates a reservoir should include re-examination of dam safety, including flood discharge capacity and dam stability. The possibility of increasing net storage capacity by spillway modification or other means should also be considered. Reduction of sedimentation rate by selective treatment of the catchment may also be feasible, and could be relevant to future performance of the project.
Canals are frequently in dire need of maintenance, including silt removal and clearing of vegetation, and canal linings may be in various stages of deterioration. However, improvement works may need to go beyond simply reinstating the original canal condition. Two factors should be reviewed before any such work is undertaken. These are the desirable canal capacity and the method of operation of the canal system.
Developments in cropping within the command since the design of the project, or now contemplated, may indicate the need for greater canal capacity in the peak season than originally provided. This can be accommodated during canal rehabilitation by a change in the canal section or in some situations by lining.
The desirable method of operation of the system is the key question in project rehabilitation or improvement. It is not a simple question to address. The basic alternative methods of operation of an irrigation system, either regulated or run-of-river, have been reviewed in Chapter 8, in the context of a new project. The situation in a rehabilitation project is complicated by the existing canal system and the present status of water distribution. It is not possible, or desirable, to wipe the slate clean and to begin afresh with a new system. Adaptation and compromise are likely to be necessary.
New hydraulic structures may be required on primary and secondary canals, their nature depending upon the choice of operational system aimed at and the nature of the existing structures. In the case of the tertiary canal system, a change in basic layout may also be required. This can present problems as the tertiary comes close to the cultivator. A principal deficiency in many older irrigation systems is the undesirably large area served by a tertiary (or watercourse), and the considerable distance between turn-outs from the tertiary to the typical farm boundary, without the benefit of formal field channels. Supply to the outer perimeter of the tertiary command is then poor, and water use tends to be focussed on the more fortunate headend cultivators.
The design of tertiary distribution systems has been discussed in Chapter 7. The question in project rehabilitation or improvement is to what extent the existing tertiary system should be modified or up-graded to bring it more in line with the current design approach. Such modification could include reduction in size of the tertiary command by construction of additional tertiary channels or leaving the tertiary system virtually unchanged but constructing additional quaternary (field channels) with control structures at branches from the tertiary. The choice of modification, if any, may depend on the degree of self-management expected to be exercised by the water user group and the views of the cultivators.
Where there is good cooperation within the group and a desire for self-management, modification of the existing tertiary system may be minimal, consisting largely of removing particular deficiencies pointed out by the cultivators such as more favorable location of certain tertiary intakes, addition of control structures, provision of additional crossings over tertiaries, and lining of selected reaches with particularly troublesome seepage. A more comprehensive tertiary lining program may be justified in some situations. On the other hand, where cultivators have shown little capacity or inclination for group management within the tertiary command, modification to decrease the command size (by construction of additional tertiaries), and to avoid the need for operation of structures within the command, may be desirable.
As noted earlier, any tertiary modification program aimed at improving equity or efficiency of water distribution is likely to reduce the supply to those cultivators who are using excess water at the headend of the system. If an attempt is made to recover the cost of the work directly from the cultivators, resistance to payment is to be expected.
Generally in an irrigation project, poor drainage is evident as high watertable, as areas of sustained inundation after heavy precipitation, and as very wet conditions in low-lying areas throughout the monsoon season. Problems of maintenance of primary and secondary drainage have been discussed in Chapter 11, and also cultivator attitude to tertiary drainage.
As far as drainage is concerned, many irrigation projects have been designed largely on the philosophy of wait and see, with minimal initial construction. However, the necessary follow-up action, where drainage indeed proves to be a problem, is not generally taken. Inadequate drainage can make cultivation impossible in the wet season and, incidentally, can also make living conditions within the village intolerable. Consequently, construction of additional primary or secondary drainage may be a priority item in project improvement. It may also be strongly urged by the cultivators.
Rehabilitation of a drainage system is unfortunately not a one-time operation. Desilting and clearing need to be repeated at intervals. As emphasized in Chapter 11, an essential feature of a drainage improvement program should consequently be the provision of permanent access for equipment beside each major drain, to facilitate future maintenance. The other essential item is the provision of permanent crossings over the drainage channels, to avoid obstruction through partial in-filling by cultivators constructing informal access-ways.
As discussed in Chapter 3, the most commonly practiced form of water-application by smallholders utilizes the level basin. The basin is of course essential to the cultivation of paddy, also where paddy occurs in mixed cropping. It is also frequently the preferred arrangement for purely non-paddy crops, with or without furrows within the basin.
With well prepared basins and reasonably timely water application, the irrigation efficiency obtainable within the basin is relatively good. Inefficiencies are more likely to be within the conveyance and distribution system. However, with water undoubtedly being the limiting factor in future agricultural production, the question of the possible role of much more efficient, more sophisticated, irrigation systems in South Asian agriculture is of interest. The systems referred to include sprinkler, drip, and micro-sprayer. Low-pressure buried-pipe distribution to outlets supplying hand-held or roll-out plastic hose serving individual orchard trees, quick-coupled portable pipe systems operating in the same manner, and portable gated pipe are in similar category.
Such systems are in fact already in use in South Asia, and are being actively promoted by equipment and pipe suppliers. So far, however, their application has been largely confined to production of specialty crops by individual large cultivators. The water supply is usually from tubewells, which can provide the near continuous service necessary for economic utilization of such systems. Further expansion of their use is likely to be in the same direction, rather than using canal supply which is commonly rotational at the farm level. Where tubewell supply is not available small pondages periodically filled from canals can provide supply to the systems discussed. This is being practiced for highvalue specialty crops in some area. However, the use of such systems for field crops is unlikely in the foreseeable future. Lining of tertiaries, possibly in conjunction with the use of roll-out portable plastic sheet linings for quaternaries (field channels) is more probably the next step in improvement in efficiency of water use for field crops.
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