An eco-hydrological approach for determining environmental flow in anthropogenic rivers of arid and semi-arid regions

The Zayandehroud River, located in central Iran, is a highly regulated watercourse within a semi-arid basin. Intensive upstream interventions, including dam construction, inter-basin transfers, and extensive water withdrawals, have altered the natural flow regime, resulting in the degradation of downstream aquatic ecosystems and increased ecological stress. This study applies four environmental flow (EF) assessment methods, the modified Tennant method, wetted perimeter method, ecological hydraulic radius method, and habitat simulation using HEC-RAS. Flow naturalization is employed to reconstruct pre-regulation flow regimes, thereby enhancing the accuracy of hydrological and hydraulic approaches. To assess method suitability, two new metrics, failure index and damage severity index, are introduced. Findings reveal that the ecological hydraulic radius method, which incorporates species-specific velocity requirements, yielded the most reliable EF estimates compared to habitat simulations. Optimal ecological flows, estimated at ∼37 % of the mean annual natural flow, far exceed current allocations, underscoring the ecological cost of overuse. The modified Tennant method, when applied to naturalized flows, provides a practical baseline for EF provision. Overall, results emphasize the critical role of flow naturalization in improving EF assessments and support ecologically grounded water management in arid and semi-arid regulated rivers. • River flows in arid regions are altered to meet human water demands. • E-flow assessment methods are essential for sustaining river ecosystems. • A structured approach is proposed for e-flow assessment in anthropogenic rivers. • Habitat models serve as benchmarks for evaluating e-flow method suitability. • Failure and damage indexes evaluate the reliability of e-flow methods.