|Design and Operation of Smallholder Irrigation in South Asia (WB, 1995, 134 p.)|
|Chapter 7 - Canal systems for smallholder irrigation|
|Distribution at the tertiary level|
The importance of involving cultivators in the layout and construction of tertiary channels has been referred to earlier. An issue which frequently arises in areas of irregular field boundaries or irregular topography is the extent to which tertiary alignment should follow boundaries or be routed more directly, crossing properties where necessary to do so. The latter course minimizes length and cost of channel. However, it maximizes interruption to access for cultivation by bisecting fields and for the smallholder may result in disproportionate loss of cultivated area. Furthermore, in many cases compensation is not paid for right-of-way for tertiaries, the channels being regarded as communal property. Following property boundaries, although inconvenient from the layout viewpoint, does come close to equitable contribution of land for tertiary construction. However, routing the tertiary along boundaries is not without problems, particularly where there is considerable difference in elevation between fields lying on either side of the boundary, as is commonly the case in areas already terraced for rainfed cropping.
Determination of size of tertiary command and its division into smaller sub-units have already been discussed. While this procedure is conceptually simple, some site situations can make implementation difficult and may require departure from the idealized design. A case in point is supply to the long narrow strip of command frequently encountered between a primary and a secondary canal in its upper reaches. A tertiary channel serving, for instance, 30 ha in such a location could be several kilometers in length, running generally along the top of the primary canal embankment. In some situations, this can be impractical for a number of site-specific reasons. One expedient is to use a number of "direct" outlets, each taking off from the main canal and serving an area of a few hectares. While solving the problem in one sense, this arrangement requires that the direct outlets be independently operated (rotated), otherwise they would run continuously with the primary canal, taking an excessive amount of water. There is no entirely satisfactory solution to this situation.
A further problem is encountered in irregular topography where a secondary canal runs down a narrow spur, with the area to be irrigated lying on the slopes on either side. Conventional layout would require relatively long tertiaries running parallel to and on either side of the secondary. However, the crest of the spur may not be sufficiently wide to accommodate all three channels if they are of regular trapezoidal section. One option is to omit the tertiaries and use direct outlets on the secondary, each serving an area of a few hectares. However, such outlets would require independent rotational operation, as the capacity (size of delivery stream) would be excessively small if scaled down commensurate with the small size of the area served so that they could be run continuously with the secondary. A second option has both secondary and parallel tertiaries. But in order to accommodate all three on the narrow crest of the spur, a rectangular composite flume section is used (concrete or brick) incorporating all three channels. This avoids the separate operation of outlets required in the first option. Choice between the two is a trade off between construction cost and operational simplicity.
A further example of conflict between technical and operational factors in tertiary system design involves the number of delivery points (turnouts) to a holding. To facilitate adherence to the rotational delivery schedule within the tertiary command, it is usual to observe the convention of a single turnout from the tertiary or field channel to each holding. Indeed this is very strictly enforced in some areas. However, there are site situations which strongly indicate otherwise. A case in point is a sandy soil with very high seepage losses in unlined channels. The tertiary is lined, but farm channels are not. A tertiary parallels the boundary of a holding of several hectares. The single turn-out rule would require a farm channel (unlined) to parallel the lined tertiary for several hundred meters, supplying down-slope branches. However, seepage losses in the farm channel would be high. The alternative would be to provide a second turnout from the lined tertiary at about half-way down the length of the boundary, thereby much reducing seepage losses when supplying the lower end of the holding. However, the existence of two turnouts to the holding, although intended to be operated as alternates, invites the possibility of their being operated together, doubling the rate of diversion to the holding and infringing upon the rotational- schedule. This again is a case of conflict between technical desirability and simplicity of operational control.
Finally, there is the question of possible use of multiple siphon tubes for delivery from tertiary/field channel to the holding. Such plastic siphon tubes (about 5 cm diameter) can be used in groups of five or more, functioning in effect as a portable turn-out or separately supplying individual furrows directly from the tertiary, thereby eliminating the farm channel. This can be attractive in certain situations and is widely practiced in Western irrigation. However, the presence of large numbers of siphon tubes in a smallholder situation would be likely to result in out-of-turn diversions and would write off any possibility of maintaining rotational distribution, unless the cultivators concerned were unusually capable of policing their system.
The above discussion of tertiary systems underlines the fact that their design is not always straightforward. Generalized layout criteria can be formulated, but judgement at the field level has to be exercised in their implementation. Experience in South Asian irrigation is that construction of tertiary/field channel systems is the controlling factor in completion of new projects and often lags years behind "creation of irrigation potential" (the construction of the main canal system). The efforts of international lending institutions to expedite such works by making reimbursement against primary and secondary canal construction conditional on completion of the associated tertiary systems have not entirely solved the problem. Training courses in tertiary distribution for irrigation engineers have undoubtedly assisted, but the subject remains a principal area of concern.