جمیل بهرامی

In most hydrologic models, time parameters, like time of concentration (tc) and time to peak (tp), are essential inputs and calculated by field methods, empirical equations and hydrograph-hyetograph analysis. Empirical equations are used anywhere without sufficient data. These equations developed for different places and therefore their application in other places may have unsuitable results. Object of this study is to introduce suitable empirical equations for estimation of concentration time and time to peak in Tehran province. In 4 catchments (37.2 to 197 km2 area), 39 rainfall-runoff events data was gathered and by hydrograph-hyetograph analysis of these events, observed values of tc and tp was calculated. 30 and 9 empirical equations were selected to tc and tp estimation respectively. Estimated and observed data were compared by relative error percentage (ER). Results show that most of selected equations, predict tc and tp under real value. For tc estimation in all studied watersheds, Bransby-Williams and Pilgrim-McDermott equations had suitable results. Accuracy of Ventora, FAA, SCS and Black equations affected by slope, in low slope regions, Ventora and FAA formulas as well as in high slope areas SCS and black formulas have better results. 4 parameters method is reasonable in small basins. For tp estimation, in small watersheds, Snyder and gray is suitable as well as in large watersheds, Bedient - Huber has reasonable results.