Comparative assessment of environmental flow using hydrological methods of low flow indexes, Smakhtin, Tennant and flow duration curve

There are different methods in hydrological approach for estimating the environmental flow and a comparative assessment is necessary. The low flow indexes (7Q2 and 7Q10), Smakhtin, Tennent and flow duration curve were used to estimate the environmental flow of Zohreh River in the southwest of Iran. The Smakhtin, 7Q2, 7Q10 and Tennant methods resulted in the estimation of constant values of 27.2, 12.7, 5.9 and (8 and 24) cms, so that, on average 52.8, 26.9 and 12.3, 36.7 percent of the monthly flow is allocated to the environmental flow. The monthly environmental flow pattern for these methods does not fit well with the monthly flow pattern, and thus it can be concluded that the Smakhtin, 7Q2, 7Q10 and Tennant methods cannot be used in the initial form. The application of the flow duration curve leads to an environmental flow assessment in the range of 6.8–38 cms in different months, whose time pattern completely matched with the monthly flow pattern. In this method, on average, 30.8% (range 18–48%) of the monthly flow allocated to the environmental flow, which is reasonable and acceptable amounts. Investigating the results of this study shows that the time pattern of the results should be analyzed in comparison with the observational flow pattern to estimate the environmental flow with a hydrological approach. The results also suggest that the methods that provide a constant amount of environmental flow in different months of the year should be interpreted cautiously along with other methods.