NasHyd is used to simulate runoff flows with Nash instantaneous unit hydrograph. This hydro-graph is made of cascade of “N” linear reservoirs. The command is mainly used for rural areas but can also be used for very large urban watersheds and to simulate the effects of infiltration/inflow in sanitary sewers. Rainfall losses can be computed by SCS Modified CN Procedure or Proportional Loss Coefficient.


Parameter Name Description Default Value
CN SCS Modified Curve Number or Proportional Loss Coefficient (if negative value between 0 and -1 entered). 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 Number. 5
N Number of linear reservoirs used for the derivations of Nash Unit Hydrograph. 3
TP Unit Hydrograph time to peak (hr). It is approximately equal to (N-1)/N × TC where TC is the Time of Concentration. 0.2