Much more challenging than that of groundwater is the improvement of performance of canal systems, which will remain the dominant water transfer technique in developing countries in the future. The standards used for design of irrigation canals in many countries have not kept up with the development of new technologies. Most traditional delivery systems have no or little flexibility built into them. Their control strategies and control structures are inadequate for good water management practices. Water deliveries at farm level are not compatible with the key requirements of modern agriculture. Thoughts on how to assess the performance of canal projects are evolving.
The engineering concept of efficiency at different levels of a canal system is important for planning, designing and even operating a canal irrigation system. However, water resources experts have criticised this concept which could be misleading. These experts argue that seepage losses contribute to the recharge of aquifers and losses from a project can be recovered downstream if the water quality has not deteriorated beyond certain limits. These arguments are valid when considering the development of water resources at basin level or even at broader levels when aquifers extend beyond basin boundaries. Advocates of water conservation measures, such as canal lining, then respond that water quality may decline in the recycling process, lands adjacent to canals may be affected by waterlogging and salinization, and an important volume of water can be lost from wasteland. Furthermore, farmers have to support the cost of groundwater pumping to which opponents can argue that farmers use groundwater more efficiently than canal water which is too often unreliable. The application of water conservation measures in irrigation can have negative or positive impact on third parties. The theoretical debate is endless.
Efficiency alone is not a sufficient indicator to define the performance of an irrigation system. Indicators of water productivity expressed in terms of production or value of production per unit of water are very important in increasingly water-scarce situation. A canal irrigation system may have high conveyance efficiency with a minimum of seepage and operational losses. However, if water delivery is too rigid or unreliable, there will be considerable waste further down at the farm level. A water productivity indicator provides a global indication of the effectiveness of water conservation measures and of the quality of service provided to the users, as well as the farm use of water and other inputs.