Résumé
Recent studies in Pakistan's Punjab have emphasized an increasing inequity in the distribution of canal supllies and frequent occurence of fluctuations in irrigation canals. The paper quantifies these phenomena for the Chishtian sub-division in south-east Punjab, suggesting that the large gap between crop water requirements and canal water supplies contributes considerably to the present predicament. Regulation at the main canal is left to a large extent to local operators reacting to local variables, unaware of future pertubations and targets and generally not taking into account the effect of their operations on downstream portions of the irrigation system. The number of operations per day increases from head to tail of the sub-division, as fluctuations are amplified in size and multiply in number. With the help of hydraulic model, the operation in Chishtian sub-division were analyzed for their impact on the hydraulic state of the canal and water supply to secondary channels, determining the potential for improvements. By implementing an alternative rotation that passes on all pertubations in the system to one secondary canal at the time for at least 7 days, disturbances in the canal are minimized. Two scenarios were simulated to test alternative ways of water distribution. In the first scenario, the present localized control was maintained, minimizing the number of operations and reducing the inflow to distributaries to the design or indent discharges. This was shown to stabilize the hydraulic state of the canal as well as supplies to secondary canals. The second scenario was based on a "feed-forward control", where decisions on operations are centralized involving the system manager. Based on this hydraulic knowledge of the system and on information about future pertubations at the head of the system and future targets, operations can be drasticallly reduced from sometimes 10 to 1 operation a day, which is shown to have a positive impact on the hydraulic state of the canal. While the latter scenario may be relatively far away from present realities, it provides useful elements for the collaboration between the Punjab Irrigation & Power department (PID) and IIMI, in the context of which simulated scenarios will be field tested.
Cet article présente les problèmes d'iniquité de distribution et les fréquentes fluctuations observés sur les canaux d'irrigation du Punjab (Pakistan). Il détaille les règles de gestions actuellement mises en oeuvre pour les gestionnaires et propose des modifications de ces règles, testées sur un modèle de simulation hydraulique des canaux d'irrigation. Le cas du canal de Fordwah, Chistian Sub-Division est étudié par les auteurs. Les simulations montrent que les règles modifées proposées permettent de réduire l'amplitude et les temps de fluctuation du canal.