A new habitat methodology for river environmental flow requirements

This paper introduces a new framework for habitat methodology to identify the environmental flow requirements for rivers using bioecological, hydrological and hydraulic criteria. Using the new method, the environmental flow of the Gorganrud River in Iran is calculated for all months of the year using the critical months' ratio as a guide. Although the critical environmental flow is calculated as twice that calculated by the Tennant method, the annual average outflow to the sea is only 1% more. Moreover, there are the same benefits for the two methods, including maintaining flows to the estuary throughout the year and flood risk management around 30% of the time. However, the probability of environmental flow demand in critical months is increased from 67% in the Tennant method to 100% using the new method. There are also positive significant effects on water basin management, such as an emphasis on the maintenance of environmental aspects, specifically in the spring and summer months, in comparison with the Tennant method. The Tennant method is therefore not recommended for use in defining environmental flow in the Gorganrud basin or in the north of Iran.