Unit hydrographs to model quick and slow runoff components of streamflow

This article derives the unit pulse response functions (unit hydrographs (UHs)) for the quick and slow runoff components of streamflow (quick runoff: direct or surface runoff; slow runoff: the sum of rapid subsurface, delayed subsurface, and groundwater runoffs) using the model of three serial tanks with a parallel tank. The interdependence between the tanks is described by exponential functions of the modelʹs parameters. The parameters are identified using the Simplex method. In comparison to traditional unit hydrograph (TUH) methods, the proposed UH method has the following advantages: (1) it can be used to separate quick and slow streamflow components; (2) it is appropriate for forecasting streamflow hydrographs produced by various storms such as single-peak storms and multi-peak storms without resorting to changing modelʹs parameters; and (3) it does not require either an a priori calculation of rainfall excess or baseflow separation for deriving the UH. The TUH models such as the Sherman model and the Nash model with three cascade reservoirs are particular cases of the proposed model. The applicability and validity of the proposed model are verified using observed rainfall–streamflow data from an actual basin. The resulting computed streamflows show a good correspondence with observed ones.