Transmuting Synthetic Unit Hydrographs into Gamma Distribution

A simple method for transmuting popular synthetic unit hydrographs (SUHs), such as those of Snyder, the Soil Conservation Service (SCS), and Gray, into the Gamma distribution has been proposed, the calculations for which can be performed on a calculator. It gives a smooth shape to an SUH, the area under which is guaranteed to be unity. Previous methods of SUH are shown to be the special forms of the Gamma distribution. The SUHs obtained using these methods are observed to be in serious error. The peak rate coefficient Cp in Snyderʹs method and the peak rate factor Df in the SCS method have imparted physical and conceptual meaning. The possible unified conceptual interpretation of the popular SUHs in line with the conceptual models of instantaneous unit hydrograph (IUH) has been presented. The method can do away with the calculations for W50, W75 and the exceptionally long tail of UH as is usually obtained using Snyderʹs method for fast responding catchments. It opens up the possibility for a variable dimensionless UH in the SCS method and gives a unified basis for the three popular methods in particular and the other methods in general. Application of the method has been illustrated through problems.