Ecological Regime versus Minimum Environmental Flow:Comparison of Results for a River in a Semi Mediterranean Region

Abstract

Maintaining Environmental Flow (EF) plays a critical role in protecting rivers and their ecosystems. Because of shortage of data and limited financial resources in developing countries, there is a tendency to use simple hydrologic methods against comprehensive EF assessment methods. In this research, two most common hydrologic methods (Tennant and Q95) were compared with a habitat simulation method (PHABSIM) under the condition of data shortage. It was concluded that while the results of habitat simulation method even using imprecise input data were justified, the impacts of implementing EF discharges of the two hydrologic methods on the ecosystem might be irreversible. It was found that the Tennant and Q95 methods led to dramatically low discharges as fixed minimum environmental flows, while habitat simulation method gave an acceptable ecological regime. In the absence of ecological data and after deciding on the target species in a case study in the southern part of the Caspian Sea, a special Delphi technique was employed for preparing the suitability data. For enhancing the PHABSIM hydraulic module results, HEC-RAS was implemented for hydraulic simulation and then with a simple modification on average velocity, the cross-sectional velocity distribution for deriving the Weighted Usable Area (WUA) was generated. It was found that this method along with maintaining high flows in the river preserved Mean Annual Flow (MAF) during wet months and preserved Mean Annual Low Flow (MALF) during dry months which is equal to maintaining Environmental Flow Requirement (EFR) in such a way that provides the river with an ecological regime near its historical one and guarantees river health.