6 min read
Canal lining business cases almost always lead with a conveyance efficiency figure: line the canal, and seepage losses drop from perhaps 40-50% to under 10%. These numbers are not wrong in a laboratory sense, but they are frequently presented as system-wide water savings, which is a different claim entirely, and one the field evidence rarely supports as cleanly.
Seepage from an unlined canal is a loss from the conveyance system's perspective, but it is rarely a loss from the basin's perspective. In many command areas, that seepage recharges the shallow aquifer that downstream farmers, and sometimes entire villages, draw from informally through shallow wells. Line the canal and stop the seepage, and you can measurably improve conveyance efficiency while degrading the water security of users who were never accounted for in the original design documents.
This is not an argument against lining. It is an argument for modelling the basin, not just the canal, before publishing an efficiency figure as if it were a net benefit.
The second issue is behavioural, not hydraulic. When conveyance efficiency improves, farmers at the head of the system typically respond by increasing cropping intensity or shifting to higher water-demand crops, since more reliable water arrives more predictably. This is a rational response to improved service, but it means a meaningful share of the "saved" water is reallocated to increased consumption rather than banked as a system-wide saving. Post-project water balance studies that fail to account for this behavioural shift will overstate the programme's water conservation impact, sometimes significantly.
The efficiency figure in the feasibility study is a design parameter, not a monitoring target. Programmes that commission two to three seasons of post-construction water balance monitoring, including shallow groundwater observation near the command area, get a far more honest picture of what a lining programme actually delivered, and a far better basis for designing the next one.