ENVIRONMENTAL CHALLENGES AND UNCERTAINTIES OF HYDROLOGIC AND HYDRAULIC APPROACHES FOR ENVIRONMENTAL FLOW ASSESSMENT IN STREAMS OF IRAN

Hydrological and hydraulic approaches are the simplest approaches in estimation of environmental flow in streams. Hydrological methods are used where hydrologic records are available. In hydraulic approach, the relationship between a hydraulic property and discharge is used as an appropriate technique for determining the minimum flow allowable for environmental purposes. Hydrological and hydraulic approaches do not focus on ecology. At the present research these two approaches are evaluated in a mountainous stream. Based on the results, differences among methods are very large. For instance, in Delichai stream in Tehran province, the method of maximum slope estimates minimum environmental flow 33 times greater than corresponding value in Tenant method. These differences demonstrate that hydrological and hydraulic approaches have many ambiguous points and using of these methods would be questionable for experts in professional negotiations. Also setting the environmental policies based on these methods can produce environmental challenges or social problems. These types of methods are not applicable for streams in arid or semi-arid regions. Thus estimation of environmental flows and definition of environmental scenarios must be based on the hydro-ecological approaches.In this paper, questionable cases of hydrological and hydraulic approaches are investigated and analyzed carefully