Investigation of the Efficiency of Different Methods for Parameters Estimation of Nash's instantaneous unit Hydrograph in Simulating Flood Hydrograph (Case Study: Manshad Watershed)

In Nash's cascade model that is simple and efficient model for simulating hydrological response in watersheds, catchment behavior is considered same as n linear reservoirs with k storage coefficient. In this context, the accuracy of different methods of estimating the Nash model parameters (n and k) including moment, experimental Nash formula, Han’s graphical, time to peak-time to inflection point and Bhunya et al methods have been investigated in Manshad watershed in Yazd province. The n and k parameters were calculated using three storm events and physiographic parameters were used for experimental Nash formula. The mean value of calibrated parameters, were used for simulation of second subset of storm events as model validation. The results show that using Bhunya and Han’s graphical methods for n and k parameters estimation, lead to better agreement between observed and simulated hydrographs. Percentage of relative error in simulating peak discharge, time to peak and flood volume, in Bhunya method, were estimated equal to 28.24, 29.48 & 8.20% and in Han’s graphical method were estimated equal to 28.29, 29.48 and 8.52% , respectively. Also, the results indicate that time to peak-time to inflection point method and experimental Nash formula give weak results for simulating some flood characteristics. Thus, using these methods in similar catchments is not advised without more researches.