Résumé
The Indus Basin irrigation system irrigates an area of about 16 million ha in Pakistan. Traditionally, the regulation of irrigation canals aims to distribute water according to the official targets, sharing any shortfall in inflow equitably among the canal commands. Since the demand for water has more than doubled over the years, the irrigation agencies are dealing with a permanent water shortage. On the one hand farmers try to increase their share, infringing on the concept of equitability. On the other hand, farmers have augmented irrigation supplies by tapping the groundwater, which is increasingly leading to problems of sodification and soil degradation due to the poor quality groundwater.This study quantifies the impact of the present pressure in the irrigation system on regulation and on water distribution, using a mathematical unsteady state hydraulic model (SIC). The present operational rules were programmed in a regulation module (Gateman) that is linked with SIC, which generates priority order for the secondary canals (distributaries). Based on this order and based on the inflow on a given day, decisions are taken which distributaries should be open, which closed and which distributary will absorb the fluctuations. The module further generates the gate settings that are necessary to achieve the target levels and discharges that have been defined, following the present practices of gate operators. The effect of these gate operations on the water distribution is calculated in SIC. A calibration of the composite model showed an average accuracy of within 5% in volume delivered to secondary canals.This methodology is applied to the Fordwah Branch and 14 off-taking secondary canals. The impact of an alternative set of operational rules on the water distribution is then evaluated, using the combined SIC-Gateman tool. Operational rules relate mainly to the length and timing of rotational programs, the definition of target discharges during the season and the operational preferences for canal commands. It is shown that it is possible to better achieve present system objectives by modifying the operational rules. System constraints, such as the inflow and the physical condition of the canal system impose limitations on this improvement.There is much discussion in Pakistan about modifying irrigation management of the Indus Basin irrigation system to better fit canal water deliveries with present objectives of a productive, sustainable irrigated agriculture. The methodology that has been developed can be used by system managers to assess a priori the impact of proposed management interventions and thus contribute to matching ambitions and reality.
L'étude quantifie l'impact de la pression actuelle du système irrigué sur la régulation et la distribution en eau à l'aide d'un modèle de simulation des écoulements transitoires à surface libre (SIC). Les règles de gestion actuelles ont été programmées dans un module de régulation (Gateman) relié à SIC, qui génère les ordres de priorité pour les canaux secondaires. Sur la base de cet ordre et du débit en tête, on décide quels sont les secondaires qui doivent être ouverts, fermés, et quel est le secondaire qui doit prendre en charge les fluctuations. Le module génère ensuite les ouvertures de vannes nécessaires pour atteindre les objectifs de niveau et de débit qui ont été définis, selon les pratiques actuelles des aiguadiers. L'effet de ces opérations est calculé dans SIC. Le modèle composite calé a montré une précision relative de 5% en volume délivré aux canaux secondaires.Cette méthodologie est appliquée au canal Fordwah Branch au Pakistan, et aux 14 secondaires. L'impact de nouvelles règles opérationnelles sur la distribution en eau est évaluée, avec le modèle combiné SIC-Gateman. On montre qu'il est possible de mieux satisfaire les objectifs du système en modifiant les règles opérationnelles, tout en prenant en compte les limitations physiques du système, comme le débit en tête et la condition physique du canal.