Comparison of different methods for parameter estimation of nash's instantaneous unit hydrograph in jafarAbad watershed

Nash instantaneous unit hydrograph is frequently used for simulating hydrological response in natural watersheds. The accuracy of estimating the parameters n and k in Nash model directly affect simulation results. This study focuses on the evaluating different methods of estimating the Nash model parameters (n and k) in Jafar-Abad watershed (109 km2) located in Golestan province. Using Different methods such as moment، trial-and-error، Han’s graphical method، time to peak – time to inflection point and Bhunya et al methods، the n and k parameters was calculated using 33 stormevents، and physiographic parameters was used for experimental Nash formula. Then the mean value of n and k، were used for simulation of 4 other storm events as model validation. The efficiency of Nash model results was evaluated by statistical comparison with the observed hydrographs. The results show that using moment and trial-and-error methods in n and k parameters estimation، the Nash model predict direct runoff hydrographs with good accuracy، 76% and 70 % respectively according to the Nash-Sutcliffe criterion. Also the results indicate that other methods give weak results comparing with moment and trial-and-error methods. Thus regionalization of Nash’s model parameters (n and k) by mentioned methods، in similar catchments introduced for development and application of this hydrological model.