Integrating Hydrological and Hydraulic Approaches for Adaptive Environmental Flow Management: A Multi-Method Approach for Adaptive River Management in Semi-Arid Regions

In this research, different hydrological and hydraulic methods were employed to estimate the environmental flow demands of the Sofi Chay River, Iran. In total, 50 years (1969–2018) of flow data exhibited high variability with a mean annual flow of 9.37 m3/s and standard deviation of 42.15 m3/s. Hydrological techniques included Tennant, Flow Duration Curve, and Range of Variability Approach; recommended minimum flows ranged from 0.53 to 2.66 m3/s, respectively, or in other words, 10–50% of mean annual flow. In contrast, hydraulic techniques such as Wetted Perimeter, R2CROSS, and Hydraulic Habitat Simulation suggested higher flows of 1.60–5.38 m3/s, or 30–101% of mean annual flow. The Hydraulic Habitat Simulation Method provided a maximum Weighted Usable Area for target species at the flow of 5.38 m3/s. Sediment analysis showed that there was a power relationship between discharge and SSC, where SSC = 14.23 × Q1.68 and R2 = 0.99. Integration of methods yielded a proposed environmental flow regime of base flows of 1.5–2.5 m3/s during the dry season and 3.0–5.0 m3/s during the wet season, with small floods contributing 15.0–20.0 m3/s and large floods > 35.0 m3/s to maintain channel morphology and ecosystem functions. After realizing the need to incorporate all the approaches in the environmental flow assessment, the hydraulic methods consistently recommended higher flows than the hydrologic methods. An adaptive management framework has been put forward for implementing and refining these recommendations to ensure long-term ecosystem health, coupled with meeting human water needs within the Sofi Chay River basin.