Equitable Water Allocation in Water-Scarce Regions: Insights from the Kankai Irrigation System, Eastern Nepal

Climate and land-use changes have significantly reduced dry-season flows in spring-fed rivers, coinciding with peak irrigation and domestic demand, thereby shrinking the effective command area for spring paddy cultivation in Nepal. This study investigates the relationship between irrigation water requirements (IWR), river discharge, and equitable water allocation in the Kankai Irrigation System in eastern Nepal. Persistent shortages at tail reaches, exacerbated by traditional rotational distribution without quantified allocation, have led to inequitable water sharing. Field surveys, stakeholder consultations, and spatial analyses revealed a 27% increase in net command area (7000 to 8950 Hectare), necessitating improved allocation strategies. Head-discharge relationships and Parshall flume calibrations (C = 0.488–2.3; n = 1.21–3.06) were developed to standardize flow measurement. Using a cropping intensity of 215%, evapotranspiration and IWR were estimated, with peak demand reaching 26 m3/s in March, far exceeding canal (10.15 m3/s), and river capacities (6.5 m3/s). An integrated demand-based water allocation and scheduling enabled equitable flow distribution across secondary canals. As a result, the irrigable spring paddy area doubled (4000 to 8500 Hectare), monsoon paddy expanded by 1502 Hectare, and paddy yield increased by 27,351 Metric-Tons/year. This approach enhanced farmers’ participation, reduced conflicts, and offers a replicable, data-driven model for equitable water management.