In Iran, limitation of water resources is natural due to arid and semi-arid climate, therefore, studies to determine the environmental water requirements have been considered as a necessity in recent years. Environmental flow is one of the most important environmental parameters that should be carefully considered in agricultural water development planning studies. Water resource development projects, especially dam construction and inter-basin transfer of water, are occasionally inevitable due to lack of water and poor spatial and temporal distribution of precipitation. Environmental water demand assessment methods were developed to stabilize environmental flows, the protection of ecosystems or save a particular species (endangered). Methodology approaches in general is divided into two groups: prescription and interactive. Prescriptive approach usually addresses a certain goal and suggests the value of the individual or the individual flow regime. Interactive approach focuses on the relationship between changes in the river (or runoff). Prescriptive approaches include four groups: hydrological index, hydraulic ratio, habitat-simulation, comprehensive approaches. Assessment methods of flow with interactive approach are more complex than prescriptive methods and often include habitat simulation and holistic approaches. The most widely used environmental assessment method is hydrological index (prescriptive). In Iran, due to the lack of complete information regarding the characteristics of aquatic ecosystems and patterns precipitation in any area or the state of underground aquifers, and considering that people in many regions of the country (especially away from the center) are dependent on the river for their livelihoods, the assessment should also include socio-economic considerations. So, technical and economic feasibility should be considered for projects that require inter-basin transfer as water rights for future.
