Statistical approaches for estimating the environmental flows in a river basin: case study from the Euphrates River catchment, Eastern Anatolian part of Turkey

Environmental flows are the water that is left in a river, or released into it to maintain valued features of the ecosystem. It refers to the water considered sufficient for protecting the structure and function of an ecosystem and its dependent species. In the recent advancement, there has been a rapid proliferation of methods for assessing the environmental flows, ranging from relatively simple, low confidence, desktop approaches, to resource-intensive, high-confidence approaches. Each of these has some merits and limitations. The statistical approaches have a very important role in most researches up to date. In the present study, the statistical approaches were carried out to estimate optimum environmental flow rate in the Euphrates River, which is one of the most important rivers running through the Erzincan plane in the Eastern Anatolian part of Turkey, with discharge values varied between 43.8 and 1,320 m³/s. The Euphrates water is mostly used for covering irrigation water demands, electrical energy production and water sports activities. In the present study, different statistical approaches including hydraulic rating and hydrologic methods were carried out to investigate the available water quantity needed for producing hydroelectrical energy from Kemah Dam and to estimate the optimum environmental flow that should be discharged from the Kemah Dam site and the minimum water depth required to maintain the ecosystem in this basin. The optimum environmental flow rate was estimated using hydraulic rating methods such as (a) wetted perimeter “Wet Environment” (b) river analyses systems (HEC-RAS), and hydrologic methods such as the base flow and the flow duration curve analysis. It is concluded from this study that the optimum environmental flow value at the Kemah hydroelectric power plant is calculated as 18.2 m³/s. This flow values correspond to Q95 on the flow duration curve, and approximately matched the values calculated by the wetted perimeter method (18.0 m³/s). This value represents the minimum environmental flow that should maintain and keep on the ecosystem in Kemah Dam site (for all aquatic organisms at depth of at least 25–30 cm). From the HEC-RAS approaches, the minimum flow rate (18.0 m³/s) corresponds to water depth of 0.25 m and water flow velocity of 0.90 m/s.