Determination of optimal unit hydrographs and green-ampt parameters for watersheds

Determination of unit hydrographs using observed rainfall-runoff events can be accomplished by minimizing the deviations between the derived and the observed direct runoff hydrograph ordinates, and accurate estimation of loss rate parameters. Linear programming is an optimization tool that can be used to determine the optimal composite unit hydrograph for a watershed from observed multiple rainfall-runoff data, but requires infiltration losses prior to using the model. Nonlinear programming models overcome this problem and can be used to simultaneously determine the unit hydrograph and loss rate parameters. In fact, several known rainfall-runoff events can be used simultaneously to develop a composite unit hydrograph based upon all those events. Such a model has been developed and applied to multiple storms for watersheds near Buckeye, Arizona; Fort Worth, Texas; and San Antonio, Texas. The nonlinear optimization procedures used for this investigation are the general algebraic modeling system (GAMS) and the generalized reduced gradient (GRG) in a spreadsheet application. Results of the two procedures are comparable.