River stage-discharge rating curve are very crucial for flood control and sustainable development of the river basin. The stage and discharge data can be extracted by rating curves. Recently, a new approach based on the concept of isovel contours for estimating the rating curves is introduced by Maghrebi et al. (2016). It uses the geometric properties of the flow section and roughness variations of the boundary. One of the essential parameters in setting up the rating curves is the Manning roughness coefficient. However, the determination of this parameter is accompanied by some uncertainties. In natural rivers, due to heterogeneous of boundary roughness, changing equivalent roughness with the stage will be important. A proper estimation of equivalent roughness in the proposed rating curve can significantly help to reduce the errors of stage-discharge prediction. The total number of investigated equations of equivalent roughness is 30, which are divided into four groups. Each one of these equations is examined in the La Suela and Trent rivers. This study has shown that choosing the right method to determine the equivalent roughness can significantly affect the performance of the model and play a substantial role in the more accurate estimation of the rating curve. The results show that in the La Suela and Trent rivers, roughness variations in banks create significant uncertainty in the estimation of the rating curves.

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