Determination of environmental flow requirement of Sojasrood River using water quality and Decision Tree methods and comparison with hydrological methods

In this study, the environmental flow of the Sojasrood River (Yengikand station) at Zanjan province was calculated using the water quality )the so-called Q relationship) and M5 tree decision methods and then the results were compared with the four hydrological methods (Smakhtin, desktop reserve model, FDC shifting and single low flow indices (7Q10 and 7Q2). Evaluation of the hydrological methods showed that the FDC shifting method has the proper accuracy than other methods in determining the environmental flow of the Sojasrood River. Based on the FDC shifting method, the environmental flow (Environmental Management Class C) at Yengikand station was equal to 0.99 (m3/s), respectively, which is equal to 29/4% of MAR. The maximum value of environmental flow was obtained by the water quality method (1.48 m3/s, equal to 43.9% of MAR) and the minimum value of environmental flow was obtained by 7Q10 and 7Q2 method (0.26 m3/s, equal to 7.7% of MAR) while, the environmental flow was determined 1.11 m3/s (equal to 32.8% of MAR) based on the tree decision method. In sum, the tree decision method (M5) extracted the equations of flow duration curve and could properly determine the environmental flow and is able to provide the values of characteristic discharges of a river.