Time to peak, time from the beginning of the rising limb to the occurrence of the peak discharge, is one of important time parameters and used to peak flow and flood frequency analysis. Because of its key role in flood control projects, watershed management and soil conservation, artificial recharge and designing of water structures and urban culverts, it's important to have exact calculation of tp by using of observed data and index hydrograph. In areas where observed data is not available, empirical formulas are used to estimation of time to peak. Due to the nature of these relationships to be regional and their usability in certain areas and conditions, it's necessary to investigate their accuracy and efficiency in other places. In this study, performance of 15 empirical equations to estimate the time to peak were analyzed by observed data from four watersheds Amameh, Kan, Sira and Javestan. Results of relative error show that accuracy of studies empirical equations is sensitive to the regional conditions. So that in Jovestan and Amameh watersheds, the both equations of Schneider and Gray estimated time to peak with high accuracy and error percentage was less than 25%, while in Kan and Sira watersheds, the equations of James et al and Bedient & haber have more precise estimations respectively. Differences between observed and estimated data, reflect this fact that each empirical method developed and calibrated for certain area and their application in other places with differences conditions is depend on investigation of these equation's accuracy.
