Estimating Flow Inefficiencies In The Kaddumurivu Irrigation System Through Integrated Hydrodynamic Modeling And Real-Time On-Site Measurements

Abstract

Irrigation water scarcity in Sri Lanka is intensified by conveyance losses in unlined tank-cascade canals. This study quantifies flow inefficiencies in a 650 m reach of the Kaddumurivu Irrigation System (KIS) by integrating field measurements with a one-dimensional steady-flow HEC-RAS model. Eight canal stations were surveyed using a Total Station, while discharge and water-surface elevations were measured under steady operating conditions using a current meter and staff gauges. The model was calibrated by adjusting Manning's roughness coefficient until simulated water-surface elevations agreed with observations within ±0.05 m and was subsequently validated using an independent flow dataset. The measured discharge decreased from 1.0052 m3/s upstream to 0.7681 m3/s downstream, giving a volumetric loss of 0.2371 m3/s equivalent to 23.6% of inflow, and a conveyance efficiency of 76.4%. Energy- based analysis gave a total head loss of 0.30 m, of which 0.12 m was attributed to friction and 0.18 m (60%) to non- friction inefficiency. The calibrated Manning's n values were 0.039 for the upper unlined reach, 0.012 for the concrete-lined reach, and 0.020 for the lower unlined reach. Spatial analysis identified the lower unlined reach and short structural-transition segments as critical loss locations. The results suggest that non-friction mechanisms, potentially including seepage, are significant contributors to the observed inefficiency, supporting targeted lining, maintenance, structural inspection, and continued monitoring rather than uniform rehabilitation of the entire canal.