Evaluation some of the Empirical Relations to Estimate the Concentration Time and Identify the Most Important Physiographic Factors Affecting in Gheshlagh Dam Basin

Estimated time of concentration is one of the most important topics in the study of watershed physiographic and hydrology and relatively accurate estimates of that effects on other hydrologic parameters, including peak flood discharge. Despite numerous empirical formulas and widespread application in Iran, up to now a study hasn’t been done about the best relationship and important physiographic parameters and the effectiveness of theirs increasing or decreasing of concentration time, in Gheshlagh Dam basin. So in this study, in addition to study the sixteen relationship of concentration time that have extensive application in our country, a method was presented for the path coefficient analysis of important physiographic parameters of concentration time. In this regard, first Using real-time concentration of trace amounts of dissolved salts, the accuracy of concentration time was assessed by RMSE and MRE statistics methods and the best empirical equations was selected. Then using the Path Analysis method, direct and indirect impact of ten important parameters of basin on concentration time was determined. The results showed that in Gheshlagh Dam basin between investigated relationships, California and Bransly Williams formulas are the best equations for estimate time of concentration (mean error relative to 41.1 and 43.5 percent). Also Chaw and Janson kroos have mean and acceptable result (mean error relative fewer than 57%). The path analysis results also showed that the stream length parameter have the most positive effect (1.10) the area of basin have the most negative effect (-0.257) on time of concentration.