Importance of Travel Time Duration between Isochrones in Estimation of Flood Resulting from Clark Instantaneous Unit Hydrograph

The unit hydrograph is one of the most important and usual methods to estimate flood discharge from observed data. Development of unit hydrographs using precipitation and runoff analysis is impossible in watersheds with lack of basic data. Therefore, empirical methods or models based on watershed characteristics can be used to estimate hydrographs. The proper application of instantaneous unit hydrograph theory is veryefficient for watershed with lack of rainfall data unless the necessary parameters are determined with high accuracy. In this research, the effects of changes in travel time between isochrones in determining time-area curves and leading to Clark instantaneous unit hydrographs were evaluated with the help of geographical information system for forest Kasilian watershed. The comparison of estimated and observed hydrographs byusing qualitative and quantitative criteria for travel times between 0.25 to 3h showed that the 3h-travel time of isochrones has had the highest accuracy in determination of time- area curve in estimation of flood resulting from Clark instantaneous unit hydrograph. The results emphasized on the necessity of proper selcetion and calibration of time intervals in unit hydrograph development.