Systematic Determination of the Form ofthe Dimensionless Unit Hydrographs

The unit hydrograph ordinate value, as given by the mathematical expression, for the unit hydrographs, derived by 0. G. Edson (and also by J. E. Nash and Doose) has been found to check fairly well with those of 2-hour unit hydrographs derived for four different watersheds lying in diverse, hydrometeorological zones in India. In most of the cases examined, the closeness of the fit, is remarkably good.

In this paper the expression for the dimensionless unit hydrograph (which involves incomplete gamma function) has been transformed into a straight line, which provides a simple method for deriving the discharge ordinates of the unit hydrographs. Graphs have also been prepared to facilitate direct reading of the ordinates, for any shape of dimensionless unit hydrograph. This method is, therefore, recommended to be used for deriving discharge ordinates of a synthetic unit hydro-graph, when only two parameters, viz., “peak” and “time to peak” of the unit hydrograph are known or estimated.