The condition of river, wetland and estuarine ecosystems is largely determined by the prevailing flow regime. The flow regime can be described by the magnitude, frequency, timing, duration and rate of change of both intra-annual and inter-annual events. The required flow regime, necessary to maintain the ecosystem in a particular condition, is referred to as the environmental flow requirement. To appropriately take environmental flow requirements into consideration in water resources planning and management, it is required that models used in water resources planning can accommodate the specific characteristics of the environmental flow requirements. This paper presents a new functionality in the generic water resources planning package ‘RIBASIM’, which allows for realistic allocation of particularly the high flow pulses and small and large floods that are part of an environmental flow requirement. Tests of this functionality with a model application based on the characteristics of the Mahabad basin in Iran show that the use of this functionality can help to find environmental flow release options that lead to smaller losses for other water users, which would not have been found easily if the traditional approach was used.