The evaluation of river environmental flow by using the ecohydrological methods Case study: Mahabad-chi River

Regards of water scarcity, inappropriate distribution of precipitation, implementation and development of projects in water resource fields such as dam construction and inter-basin water convenience is unavoidable. For prevention of long-term negative environmental impacts of such projects, and for water allocation acts, it is necessary to studyhydrological and ecological demands of river as an "environmental water demand". The "environmental water demand" usually is defined as a set of discharges which are determined by the magnitudes, frequencies, occurrences and given flows, but the employed methods in this research calculate environmental flows in the form of mean annual runoff (MAR). These flows which provide appropriate conditions for conservation of aquatic life and ecosystem processes for sustainability goal, are called "environmental flows". This article aimed to study the "environmental demand" estimation for Mahabad-chi River. Mahabad-chai sub basin is located in southwest of Urmia lake and in terms of scope is 4th sub basin of Urmia lake basin. Its geographic area is among of 44˚ 45΄ to 45˚ 56΄ eastern longitude and 36˚ 22΄ to 37˚ 10΄ northern latitude and is composed of two main branches “BYTAS & KOWTER”. The ecological river demand was estimated and compared by four hydrological methods (Tenant, Tessman, FDC-shifting and DRM), then suitable method was introduced. The ecological demand of Mahabad-chai river is estimated and compared by four hydrological methods, FDC-shifting method biological class C, because of considering ecological specifications of the river, was chosen and the environmental demand of Mahabad-chi River was estimated to 0.35 cms in BYTAS station and 1.17 cms in KOWTER station. The annually average flow was 1.73 and 6.17 cms in BYTAS and KOWTER hydrometric station respectively. Based on the results, the suggested flow of FDC-shifting method at C class is recommended as minimum environmental flow for Mahabad-chai river, because this method needs the less data, doing early and rapid assessment in data and simulating current hydrological conditions by considering desirable ecological conditions, whereas other methods can’t do this action. Moderated class C (relatively modified) consider about %20 MAR to %30 MAR as environmental flow, that in this condition basic function of ecosystem has not changed and more species is survived. Also moderated class C has good match with regime of flow at several months and is acceptable in area in terms of managerial, agricultural uses, drinking and etc.
The suggested method in this research is not the ultimate solution for environmental problems of Mahabad-chai River. Lack of comprehensive required ecological information at river’s ecosystem studies, cause to estimating eco-hydrology by less coefficient of confidence.