WilHyd is used to simulate hydrographs from rural watersheds with long recession periods. The program uses the Williams and Hann’s unit hydrographs developed in the original HYMO program and the Modified SCS Curve Number Procedure to calculate the rainfall losses.

TABLE 3-5 WILHYD PARAMETERS

Parameter Name Description Default Value
AA/DWF Printout parameter or if less than 0, to enter a constant Dry Weather Flow or baseflow (m³/s or ft³/s). If AA is positive, the unit hydro-graph will be printed. If AA is 0, neither happens. 0
BB Printout parameter. If BB is positive the rainfall excess ordinates will be printed. If BB is 0, excess ordinates will not be printed. 3
CN SCS Modified Curve Number 80
IA Initial abstraction (mm or in). If IA is negative, the program uses the SCS method where IA = 0.2 × S and S is a function of Curve Num-ber. 5
K Recession constant in the William and Hann unit hydrograph equation (hr) 4
TP Unit hydrograph time to peak (hr). The time step DT should be smaller than TP. 0.2

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