Stage-discharge relationship in natural rivers especially in large rivers is one of the most important issues in flood control projects. In the current paper, an efficient method presented to estimate the rating curves in some large rivers using the concept of isovel contours with the corresponding hydro-geometric parameters of the cross sections which was first introduced by Maghrebi (2006). The most important advantage of the proposed method in estimation of the rating curve is its inherent simplicity since it does not need any calibration. The proposed relationship is obtained by minimization of the differences between the calculated and analytical discharges at each identical stage. For verification, the results of the proposed method are compared with the observed stage-discharge data taken from the Cuntan and Luoshan stations in Yangtze River in China, the Colorado River in Argentina and the Nazli-chay River in Iran. The results show the high accuracy of the proposed method in such a manner that the mean values of MAPE and NRMSE of all the selected rivers do not exceed 6.3% and 0.06, respectively.

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