Determination of optimized sediment rating equation and its relationship with physical characteristics of watershed in semiarid regions: A case study of Pol-Doab watershed, Iran

Managers always consider the precise estimation of sediments in watersheds due to various conditions, such assoil and water resources management, construction, infrastructure and economical and social issues. In this condition,an optimized determination of sediment rating equation (typical method until now for sediment yield estimation) isessential to investigate sediment yield in rivers. In this study, the best sediment rating equation was determined forfour hydrometric stations of Pol-Doab watershed in Markazi province using sediment rating curves types (singlelinear,multi-linear, mean loads) together with bias correction factors (FAO, Quasi-Maximum Likelihood Estimator[QMLE], Smearing, Minimum Variance Unbiased Estimator [MVUE] and β). The results showed that the optimizedequation in stations is the mean loads (MVUE), which can used for prediction of sediment yield in annual scale.Moreover, FAO factor is more accurate for the estimation of sediment yield in high variability conditions formonthly, weekly and daily scales. According to the obtained results, accurate representation of variability intensity ofsediment yield is associated with the rating curves types, since the monthly rating curve is more accurate. Also, theresults indicated that the watershed average slope has direct relation with b coefficient of rating equation, and whenusing this parameter, the rate of sediment yield can be determined for month, season and hydrological periods. Basedon the obtained results, with increase in the watershed average slope, the slope of suspended sediment concentration(SSC) equation is also increased.