Abstract
Environmental flow assessment (EFA) is crucial for maintaining river ecosystem health and determining the minimum water requirements for aquatic life and riparian vegetation. This study evaluates the environmental flow of the SiminehRood and ZarinehRood Rivers in the Lake Urmia Basin, Iran, using three hydrological methods: Tennant, Tessman, and Flow Duration Curve (FDC) analysis. Long-term daily discharge data from hydrometric stations were collected and analyzed. The Tennant method, recommending 30% of mean annual runoff (MAR) for low-flow seasons and 10% for high-flow seasons, provided environmental flow values inconsistent with the rivers' natural regimes. The Tessman method, utilizing a combination of mean monthly flow (MMF) and MAR, revealed exceedance probabilities below 50% during certain months, indicating inadequate environmental flow provision. The FDC method assessed low-flow ranges by calculating exceedance probabilities. For the SiminehRood River, low-flow ranges were 1.06-0.1 m3/s (Qizil Ghonbad), 1.45-0.36 m3/s (Dashband), and 2.4-0.26 m3/s (Miandoab Bridge). For the ZarinehRood River, low-flow ranges were 1.41-0.1 m3/s (Anian Bridge), 10.7-2 m3/s (Sarighamish), and 3.43-0.9 m3/s (Nezam Abad). The study highlights the limitations of solely relying on hydrological methods and the need to incorporate ecological components and field research to determine acceptable living conditions for aquatic species. Recommendations include exercising caution when using the Tennant method, modifying it to suit local conditions.