This study aimed to provide with a simple and practical method for river environmental flow management with the lack of sufficient data on ecological needs. The main target was the modification of the Tennant method to adapt with the seasonal flow distribution and to cope with the national policy for the environmental flow allocations in the water road map on Iran. The Kashkan River at the Pol- Dokhtar Station in Iran was selected. Daily flow data in a long-term 62-year period (1955-2017) were analysed for the natural trend of the flow regime. Environmental flow requirements (EFR) were estimated using six widely applied hydrological methods (i.e. Tennant, Tessmann, VMF, MCY, Smakhtin, and FDC) and three ecohydrological methods (i.e. RVA, DRM, and FDC-Shifting). The Tennant method was modified to adapt with the observed seasonal flow distributions and in line with the national policy for the EFRs allocations. For flexibility in decision making by river managers, three environmental management classes (EMC: Excellent, Good and Acceptable) were included. The modified method was tested under the flow conditions of the Kashkan River in Iran. Under the Good EMC, the six-month low- and high-flow requirements were about 20 and 40 percent of mean period flow (MPF), respectively. In the three-month scenarios of dry, low-flow, intermediate, and high-flow periods, it was 20, 27, 33 and 40 percent of MPF, respectively. The proposed structure of the Tennant method is presented in a tabular format, with the inclusions of the two seasonal scenarios (i.e. 6 and 3 months) under three EMCs.