Assessment of Hydrologic Alternation of flow due to dam construction in an arid basin (The case of the Gadar River in Urmia Basin)

Gadar River located at the south part of the Urmia Basin is one of the imperative source of water for Urmia Lake and its satellite wetlands. It has been regulated since 2000 by diversion of its flow to the Hasanlu dam for supplying irrigation water. This study was aimed at investigating hydrologic alternation of the river flow during the post-dam period compared to the natural reference condition using the RVA and Eco-deficit methods. Results show that simultaneous with decreased base flow index, occurrence of zero flows has been increased as well as the duration of low flow events. Furthermore, considerable decreases in the frequency and duration of the high flows were detected. Analysis of eco-deficit/surplus curve also indicated that not only the reservoir operation has created no eco-surplus but also it has produced an average eco-deficit of 218 MCM/y. To improve the hydrologic condition of the river through conserving low flow and peak flow components of its natural regime, the acceptable range for the regulated flow was calculated using the RVA method and recommended to be considered by implementing active and restrictive management policies in the reservoir and river, respectively.