Performance evaluation of irrigation outlets with physical model study in Indus basin irrigation system of Pakistan

The efficiency of water distribution at primary, secondary, and tertiary levels in the Indus Basin Irrigation System (IBIS) has historically suffered due to poor design, suboptimal operation, and water scarcity. To address these issues, the system has been designed with ungated irrigation outlets to ensure equitable water allocation at secondary and tertiary levels. This research evaluates the hydraulic performance of three irrigation outlets: adjustable proportional module (APM), adjustable orifice semi-module (AOSM), and open flume (OF) using a physical model study. A distributary channel model with these outlets was constructed at the Centre of Excellence in Water Resources Engineering, where discharge coefficients (Cd) were calibrated and measured under various hydraulic and geometric conditions, including free and submerged flow conditions, and with adjustments to flow depths and outlet settings. The results showed variability in Cd values under free flow and submerged flow conditions with APM and AOSM ranging from 6.07 to 8.20 and 0.56 to 0.74, respectively, and OF between 2.46 and 4.31. Additionally, the behavior of outlet tampering on Cd values was also assessed under three scenarios: tampering with the half wing wall (1st), tampering with the full wing wall (2nd), and lowering the bed level downstream of the outlet (3rd). The increase in Cd values for APM and AOSM was + 10.84% and + 14.49% under 1st scenario, + 17.12% and + 22.36% under 2nd scenario, and + 24.25% and + 26.30% under 3rd scenario, respectively. The results reveal that even minor tampering with outlet structures can lead to significant deviations in performance, highlighting the importance of maintaining stringent control over outlet configurations to ensure equitable and efficient water distribution. There is a critical need for rigorous, site-specific calibration of irrigation outlets to optimize their performance under local conditions and redesigning outlet structures to minimize the impacts of tampering, thereby enhancing the overall sustainability of water use in large-scale irrigation systems. The findings from this study provide essential insights for irrigation engineers and policymakers tasked with upgrading and maintaining irrigation infrastructure. By adopting a more customized approach to outlet design and management, it is possible to significantly improve water use efficiency and achieve more sustainable irrigation practices.