This study evaluates optimal water allocation within the Subak Bali irrigation system in the Petanu River Basin, focusing on return flow dynamics known locally as “natak tiyis. ” The objective was to enhance water availability for downstream irrigation while ensuring system sustainability under increasing water scarcity. A synthetic rainfall-runoff model was employed to derive dependable discharge (Q80), and net field water requirements were calculated for irrigation area behind each weir, considering both the existing cropping pattern and a shift in planting start periods. Eight allocation scenarios were simulated using DSS-RIBASIM (Decision Support System - River Basin Simulation Model), varying return flow percentages incrementally. Optimal performance was assessed using a multi-criteria framework based on reliable water supply (Q80), water availability gain, irrigation equity, and system sustainability. Results indicate that the eighth scenario, with a 31. 7% return flow, achieved the highest performance, increasing water availability by \: 0. 18m^3/s and expanding irrigable areas downstream. This improvement supports agricultural resilience without requiring structural modifications, demonstrating that optimizing traditional water reuse mechanisms can serve as a sustainable, nature-based solution in UNESCO-recognized cultural landscapes. The findings underscore the value of integrating indigenous knowledge with hydrological modelling for climate-resilient water governance.
Yekti et al. (Tue,) studied this question.