|Design and Operation of Smallholder Irrigation in South Asia (WB, 1995, 134 p.)|
|Chapter 13 - Village schemes and small tank projects|
These commonly consist of a low weir, often requiring renewal after each high-flow season, a conveyance canal leading to the area to be irrigated, and distribution channels within that area. Much ingenuity and a great deal of labor have gone into construction of these works, and considerable labor is required in the annual task of reconstructing or repairing the diversion weir. The weir may be made of brush-wood and cobbles, or many rows of wooden stakes driven into the river-bed where it is of soft material, and infilled with mud. More recently wire mesh baskets filled with cobbles (gabions) are being used for weir construction, where stones of suitable size are available.
Such systems are exposed to a number of hazards. The diversion weir may be washed out during the irrigation season by a late flood, or the conveyance canal may be put out of service by slides (particularly in the very steep terrain of the hill schemes). In a dry year the stream-flow at the diversion may be excessively small. Many such schemes are on streams which normally cease flowing entirely early in the dry season (i.e. are non-perennial), in which case irrigation is confined to supplemental watering in the wet season and supply to a limited portion of the service area early in the dry. Others are on perennial streams, but usually with dry-season flow highly variable from year to year. In these circumstances, the extent of the area which can be irrigated in any particular year is uncertain, and farmers at the tailend of the distribution systems are likely to have very precarious supply. However, experience over the years indicates how far the distribution system can usefully be extended, granted that returns from the lower end of the system may be marginal.
Conventional economic analyses did not, of course, enter into the design of the existing farmer constructed schemes. They were constructed at a time when there was little avenue for other employment (low "opportunity cost" labor). In a subsistence-level situation even uncertain irrigation supply was judged to be better than none. This is obviously not an adequate basis for analyses of such a scheme from the viewpoint of an international development agency. The entry of such agencies into the field of the small farmer system poses a number of questions. These include the scope of the assistance, whether improvement of existing schemes or construction of new schemes, whether there are, in fact, many perennial streams suitable for development which are not already preempted by existing schemes, and what economic criteria should be used in determining the viability of new schemes (undeveloped sites are often undeveloped for the reason that they are problem sites and thus costly). In terms of the criteria usually employed in evaluation of irrigation projects many schemes on small unregulated nonperennial streams would be judged non-viable. However, in the development of such small schemes there can be important socio-economic factors which lie outside the compass of conventional economic evaluation of larger irrigation projects (such as, in the case of hill projects, the alternate cost of transport of food into remote areas without road access, or the social cost of migration from such areas). In any case, it would be impractical to analyze in detail each scheme as small as two or three hundred hectares, and simplified economic criteria have to be devised. The nature of these criteria strongly influences the scope for participation of international financing agencies in village-level irrigation development (Martin 1987, Sundar 1990).
In addition to the economic criteria, the irrigation system design approach needs to be tailored to the particular situation of the village scheme. The development in most cases is likely to be improvement or extension to existing schemes, and the concept of farmer ownership must be preserved at all costs. The improvements should be primarily those requested by the farmers, i.e. works aimed at remedying problems perceived by the farmers rather than those conceived by the lending agency as being desirable. This approach is doubly necessary if the farmers are to be asked to pay for or contribute to the work. However, it is reasonable to expect that the lending agency would wish to see some degree of up-grading as the result of its participation. In this regard, there are modern technologies which could indeed be introduced with considerable benefit, provided that farmers were persuaded of their value. Such technologies must, however, be adapted to the particular circumstances of such schemes, notably maximum use of local materials, minimum transportation (particularly in remote hill projects), and minimum cost to the farmers concerned. These requirements will often rule out designs which are scaled-down versions of structures conventionally used in larger projects.
In cases where the proposed improvements to an existing scheme go beyond the construction of a more permanent intake weir and include reconstruction and possible partial lining of the conveyance canal, an issue may be whether the size of the service area should be reexamined, particularly in the context of increased canal capacity. Any such reexamination poses two questions. First whether the crops currently grown are indeed appropriate to the site, and second whether the cultivators would agree to any change (these are "farmer-owned" schemes). For example, in Nepalese hill projects, paddy, the favored crop, is being grown in soils with infiltration rates many times higher than the rate normally considered to be the appropriate upper limit. Hence, should the improved scheme still cater to such service, or should a change in cropping pattern be suggested and if so to what alternative crops. If the cultivators being served by the scheme strongly prefer to stay with rice, as is likely to be the case, should a change from wet-land practice to up-land be suggested (much of the rice at present being grown in such areas is in fact grown largely under up-land conditions). The problem is that any reduction in supply of water to existing irrigators, in the interests of supply to others, is likely to be resisted by the present irrigators, whose rights may be of long standing. Furthermore, any increase in consumptive diversion to one scheme may reduce the flow available to some downstream scheme. It is evident that the approach to irrigation system design in a small hill scheme may be very different from the approach to the design of a major scheme. A case by case approach is necessary, with as much attention to sociological as to technical factors.
With regard to technical design, the possibility of introducing newer materials and methods into the traditional construction of small schemes is worth consideration and is attractive to international lending agencies assisting in this area. Possible improvements to intakes (a perennial problem to farmers) include greater use of wire-mesh stone-filled crib work, and particularly the use of cribs fabricated from light steel rod rather than wire to better withstand abrasion by rocks during passage of floods in a mountain stream. Tethered boulders, retained by steel rods with upstream anchorage have been used elsewhere in weir construction in torrential streams, and would appear attractive for hill schemes in some situations. Improvements to channel linings by incorporating modern geotextile fabrics behind traditional masonry lining also offers possibilities, and in some locations the use of light-weight portable prefabricated channel lining units constructed of G.R.C. (fiber-glass reinforced mortar).
The design of improvements to an existing small scheme or the design of a new one is not necessarily simple. All of the physical factors entering into the design of a larger project are present in the smaller scheme, but with diseconomies of scale. Indeed, more engineering judgement may be required in the design of improvements to a small scheme than in the more conventional text book design of a larger one. In addition, much of the topographic and hydrologic data normally available in the case of the larger project may not be available in the case of the small schemes. Added to the problem is the difficulty of access to the small scheme (commonly several days on foot in hill schemes) for staff carrying out investigation, design and technical supervision of construction.
Important issues are the extent to which work on small schemes should be carried out by the beneficiary farmers versus by contractor, responsibility for supervision of construction and disbursement of funds, and the respective roles of the national agencies which may be concerned (Irrigation, Agriculture, Rural Development, etc.)
Small farmer-constructed diversion schemes are undoubtedly an important component of agricultural development in some areas, and one which deserves the support of international development institutions. However, any direct involvement (other than simply financial support to concerned national agencies) must face a number of issues which have proved troublesome in the past and are likely to limit the scope for channeling financial assistance in this direction.