|Design and Operation of Smallholder Irrigation in South Asia (WB, 1995, 134 p.)|
|Chapter 8 - Hydraulics of canal regulation and types of control structures|
The subject of canal regulation, particularly as applied to third world situations, has generated a considerable volume of literature over the last decade, and considerable contention. Publications include recent monographs by the American Societies of Civil Engineers and of Agricultural Engineers, proceedings of the International Commission on Irrigation and Drainage, internal papers of international development agencies including the World Bank publications of academic and research institutions associated with agricultural development, and papers by professionals in developing countries (many of which are available through the Overseas Development Institute and the International Irrigation Management Institute).
Views on the subject expressed by competent professionals and agencies differ substantially. Differences stem principally from the perennial question of how the individual smallholder will react to situations of varying supply and demand. On the one hand is the view that only a simple preordained system of rotational supply will survive, as it certainly has in some situations. The view is tempered, however, by the experience that in other situations such a system is so obviously contrary to cultivator needs that the cultivators themselves reject it. On the other hand, there is the view that with modem technology water deliveries can be, and should be, much more closely matched to cultivator needs, a view which some experienced practitioners treat with considerable reservation, if not cynicism.
The subject of supply of irrigation from a partially regulated or unregulated source, in an area with capricious monsoonal climate, with highly independent small cultivators, is in fact a difficult one. Much as international lending institutions prefer standard solutions to commonly encountered problems, irrigation in developing countries requires a situation-specific approach.
A feature of most South Asian irrigation schemes is that the supply of water in some seasons, particularly in some years, is less than what the cultivators demand. Distribution in such circumstances has to be on the basis of available supply, rather than on demand, i.e. it is to some extent a system of rationing.
The various supply/demand situations have been discussed in the last chapter, with comments on options available for canal management. Systems without storage (run-of" river) or with fixed high-flow season and low-flow season diversions call for no further comment. They are essentially supply-driven, with upstream control. On the other hand, systems with upstream storage (even where providing partial regulation only) offer the possibility of tailoring irrigation releases much more closely to supply and demand. The extent to which this possible flexibility is to be exercised determines the degree of sophistication of the hydraulic controls required and the intensity of management.
For purposes of illustration, a case is considered in which the storage reservoir will have net capacity of about one-third of the mean annual inflow. Rainfall in the irrigation area averages some 1300 mm annually, mainly recurring in the single monsoon season. Soils include both light well-drained uplands and heavy poorly drained low-lands, in close association. Most of the area is in holdings of less than one hectare, primarily owner cultivated. The area is underlain by hard-rock with limited groundwater potential (dug wells only). Basic irrigated crops expected to be grown in the area when the project comes into service are paddy and maize in the monsoon season, paddy being either monocropped or double-cropped, wheat and oil-seeds in the dry season, and sugarcane as an annual crop. However, there is also keen interest in specialty crops including potatoes, tomatoes, and other vegetables, as well as bananas and citrus. Market projections indicate increasing desirability of diversification into these areas.
Design studies are to include a range of options for regulation of water distribution including possible provision of a high degree of flexibility extending either down to the individual farm, or alternatively to the 30 to 40 ha tertiary command.