This paper studies one of the most important problems of those countries that are confronted with the water deficit and competition of rivals for the water allocation. In this research, by discussing on the currently used methods for minimum environmental water definition in Iran, a method that is applicable efficiently and more precisely in the areas with no biological habitat data is introduced. In this regard, the Tenant (Montana) method as a hydrological method and the Wetted perimeter as a hydraulic method are analyzed and compared. According to the obtained results, the Montana method could be an improper selection for the minimum environmental water allocation, while the hydraulic method shows more coincidence with the natural state of the selected perennial river. For defining of the Wetted perimeter method's parameters, the Maximum Curvature and the Slope Algorithm were used. For the case study, the Slope Algorithm gives a minimum water requirement higher than the river discharge that may cause a conflict in the areas with the water deficit crisis.