There are several methods for estimating environmental flow (EF), each with its own advantages and disadvantages. This study evaluates the accuracy of two models, PHABSIM and HEC-RAS, in estimating the EF of the Azad River in Sanandaj County, Kurdistan Province. The evaluation examines three scenarios: drought, years with average rainfall, and high rainfall. For the PHABSIM model, the data utilized included water depth, water velocity, channel index, and discharge rate at cross-sections, measured at one-meter intervals from the river. In contrast, the HEC-RAS model used data on the appropriate water depth necessary for the survival of the index species. The results of the PHABSIM model for EF in September, under drought conditions and during years with average and high rainfall, showed an increasing trend. The estimated flows were 0.6 m³/s for drought conditions, 1.5 m³/s for average rainfall, and 2 m³/s for high rainfall. Similarly, the HEC-RAS model exhibited an increasing trend in the same month, with estimates of 0.2 m³/s for drought, 0.52 m³/s for average rainfall, and 1.03 m³/s for high rainfall. When comparing the results of the two models, it was noted that under conditions of greater water depth, where water flows into the river's side surfaces, the HEC-RAS model predicts a higher EF than the PHABSIM model.
