Introduction and definitions

It is often stated that all problems with the small irrigator would vanish if he were simply given a regular, dependable, timely supply of water. Unfortunately the monsoon, principal source of water in the region under discussion, is neither entirely dependable nor always timely. Irrigation is conditioned by supply as well as by demand, and conflict between the two is to some extent unavoidable.

Major irrigation systems can be designed and operated with a formal supply system extending down to the individual farm, "formal" implying construction, operation and maintenance by government agency (e.g. departmentally). Alternatively, the formal system may stop some distance away from the individual farm, delivering to an area in which the farmers manage the distribution of water themselves. This may be as little as 30 or 40 ha, as much as 100 ha or more. However, due to the inefficiency of water distribution within that area, encountered in many such systems, the trend over the last two decades has been to reduce its size and to extend departmental activities to the design and, in some cases, the construction of the distribution system within it. Maintenance and operation of that system remain the responsibility of the group of farmers which it serves. For purposes of this discussion, the area served by the outlet from the formal supply system is referred to as the outlet command, and the distribution system within it is referred to as the tertiary system.

A number of factors have led to efforts in recent years to increase the role of cultivator groups in management of the lower end of the system. One is the major cost of staffing an organization capable of operating a formal system extending down to the individual farm, and the difficulty of recovering such cost from cultivators. Another is the conviction that the cultivators themselves are better capable of managing the system at that level. Finally, much faith is placed on the thesis that the tertiary system will be better maintained if the cultivators have full control of it. Group operation of the lower end of the system is not without its problems (discussed later) but in a smallholder situation, particularly in the absence of land consolidation, it is believed to be the practical solution. The remainder of this discussion is in that context.

There are two types of variable which must be considered in the design and operation of an irrigation system. The first is imposed by natural conditions, including variations in availability of water to the canal system, the occurrence and amount of rainfall on the project area, and variations in suitability of lands within the area for cultivation of particular types of crop. The second type of variable is inherent in the freedom of choice of the individual farmer, or group of farmers, regarding crops to be grown and hence the pattern of irrigation requirements. Such choices are often determined by changing market conditions for various crops. The site variables must be accommodated; the cultivator-related variables may be subject to some degree of regulation by project authority (Frederick 1993, Hoffman 1990).

The design of an irrigation system sets out to optimize the use of the water resource, taking into account the variable nature of the supply and demand as discussed above. Two basic issues are the degree of sophistication acceptable in the design and operation of the system, and the degree of freedom of choice in cropping pattern which is to be left to the cultivator.

A system capable of carrying optimization to the ultimate degree would be very sophisticated indeed, and probably inappropriate to the region under discussion. Limits in the practical level of technology in design and operation are imposed by several factors. These include cost in relation to productivity (cost effectiveness), financial resources available, the ability of the agency concerned to-construct, maintain, and operate the facilities including funding of operation and maintenance, and finally the prospective attitude of cultivators to the type of operation proposed. The latter can be a key factor. If an operation, although entirely logical from the system viewpoint, meets with cultivator disfavor it is likely to be subverted by interference with control structures, or construction of unofficial checks or outlets ("destructive self-help" as one commentator puts it). Vulnerability of structures to misoperation and susceptibility to theft of components may be a very real constraint on the degree of sophistication and on the practical limits of efficienct water use which can be aimed at in a particular project situation.

The question of freedom of choice of crop and consequently of timing of irrigation supply, versus project-wide standardization, is a fundamental one in the design of an irrigation system. Where soils and topography are uniform throughout a project area, and groundwater can be utilized by the individual cultivator who has special water needs, standardization of canal supply based upon the requirements of the most commonly grown crops is generally practiced in South Asia. On the other hand, where soils and topographic conditions vary to an extent compelling radically different classes of crop to be grown in different areas of a project, canal delivery tailored to these specific areas is desirable. However, where the variations in soil and topographic conditions commonly occur within small areas, as can be the case in some topographic situations, tailoring delivery to such areas becomes difficult.

Aside from situations in which site conditions impose variations in class of crop, there is the more general case of the cultivator who wishes to depart from the project norm and to grow specialty crops requiring special water scheduling, but is in an area where groundwater is not available. This brings up two questions: should the canal delivery system recognize such needs and should such diversification be encouraged? and what are the practical limits to meeting such demands?

Terms used in referring to the successive orders of the canal system differ regionally, causing some confusion in the literature. The following alternatives are in common use:

Irrigation Sub-System Conveyance System	Alternative Terms Primary canal and branches Secondary canal and branches	Main canal and branches Distributary canal and branches
	Sub-secondary	Minor (or tertiary)
Distribution system commonly serving	Tertiary (or watercourse)	Watercourse (or field channel)
30 to 40 ha	Quaternary (or field channel)
Within the individual holding	Farm Channel	Farm Channel

The capacity or area served by a particular order of conveyance canal varies widely with the size of the scheme. The area served by a tertiary (or watercourse), however, is more specifically defined, as discussed later. The terms employed herein are primary, secondary, sub-secondary, tertiary or watercourse, and field channel.