| Design and operation of smallholder irrigation in South Asia |
|Chapter 10 - Durability of canal linings|
The question of whether to line a canal system, and which categories of canal to line, involves technical, economic, and financial considerations. Canal lining is a major cost item, but the record of performance ranges from excellent to very poor, underlining the need for full analysis of alternative courses and technical options in each particular case.
A principal reason for lining may be to reduce seepage losses. However, as an alternative to lining, seepage may be recoverable by groundwater extraction. Indeed recharge by canal seepage may be highly desirable if the quality of the groundwater and the nature of the aquifer favor well development in the particular area. However, if the groundwater is unsuited to irrigation or if the underlying formation is not favorable to well development, canal seepage may present a drainage problem. To illustrate, a canal was excavated in thinly-bedded sandstones dipping parallel with the ground surface, down slope from the canal. The bedding planes provided seepage paths for water, but insufficient yield for economic well development. The seepage produced waterlogging for a distance of about a kilometer down-slope from the canal and paralleling it for several kilometers. Lining was the only available solution, justified in this case by the loss of production from the water-logged area. Some credit could also be taken for the value of water lost from the canal, although seepage water is seldom entirely lost. It may reappear downstream as a contribution to stream-flow and may subsequently be developed for irrigation (although at some cost). However, it is lost to economic use if it joins a body of groundwater of quality unsuited to irrigation or if it is by unproductive evapotranspiration in areas of waterlogging caused by the seepage. A regional rise in watertable and threat of extensive waterlogging may in fact be the principal reason for lining a canal system. Even where the character of the formation is suited to groundwater development, cultivators in the areas may not generally be in a financial position to install wells, at least not on a sufficient scale to control the rise in watertable, preferring to use canal supply only. Further, once waterlogging has occurred the economic productivity of the area falls and cultivators are even less likely to be willing to undertake well installation. Large scale canal lining may than be the solution.
Another factor which may influence a decision to line is erosion, particularly in the vicinity of structures or bends in canal alignment. In some clay-soils (previously discussed) the impossibility of preserving the section of an unlined channel due to sloughing or lateral erosion of the canal banks may be a reason for lining.
The tertiary canal requires special consideration in the matter of lining. In view of the small flow in relation to the "wetted perimeter" of these small channels seepage in permeable soils can cause disproportionate loss. Lining of at least the main stem of the tertiary may be necessary in such soils, if reliable delivery is to be maintained at the outer perimeter of the tertiary command. In dune sand areas, lining of the whole length of the tertiary is virtually mandatory. Another reason for considering lining of tertiaries is heavy infestation with phreatophyte plants (particularly bull-rushes or "typha") in areas of perennially high watertable. Maintenance of a small unlined water-channel in such circumstances can be very difficult. This raises the question of responsibility for maintenance of tertiary channels, lined or unlined, also for meeting the cost of their construction. Payment by cultivators for the cost of tertiary lining has been successfully practiced to a limited extent, but is very much the exception. Government view, and that of some international financing agencies, is generally that tertiaries are the communal property of the cultivators within the tertiary command, not part of the canal system proper. The cost of construction and subsequent maintenance should therefore lie with the cultivators. Cultivators understandably take the opposite view. Their position is complicated by the fact that lining of the main stem of the tertiary, primarily for the benefit of cultivators on the outer (downstream) perimeter, does not significantly benefit upstream cultivators. So why should the upstream cultivators contribute to its cost? Acceptance of the idea of communal property does not apparently extend to acceptance of communality of costs. Efforts to provide credit to cultivators to meet the cost of lining, or to set up intermediary institutions which borrow for that purpose and endeavor to collect from cultivators, have not made payment of the cost of lining of tertiaries any more palatable to cultivators and have generally been unsuccessful. It has usually become a Government cost, and it can be a very substantial budgetary item. For this reason lining is often limited to the main stem, or a certain proportion of the length of the channel.