HEC-HMS vs PondPack Difference in Peak Flow

[litebox]

I am recreating a model I had in HEC-HMS in PondPack (SCS Unit Hydrograph, CN), the runoff volumes are identical between the two, however the peak flows vary up to 4.3%, is this normal? Or should I be getting closer results between both models. I tried using the same Time-Interval/Output Increment on both models but I keep getting these differences.

Any help will be appreciated.

 

[psmart]

Could be a difference in the rainfall distributions or perhaps the Unit Hydrograph. Any such differences will be most pronounced at short Tc values. What rainfall distribution and Tc are you using?

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[litebox]

I am using a Tc of 56.64 minutes and the Rainfall distribution is provided by the local code. I am using the 100 Year Storm with 3 hour duration, this data was provided by the city in 5 minute increments for the 180 min duration. I have entered this data in HEC-HMS and PondPack identically.

 

[psmart]

Strange. 5 minute increments should be fine for a 56 minute Tc. I would check the other runoff options in each program. Perhaps the UH.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[Drew08]

HEC-HMS takes the Lag Time as input. Not the Time of concentration. I don't know about PondPack. This could be the issue if PondPack takes Time of concentration and you entered the values identically.

 

[rlondeen]

If I remember correctly, HEC-HMS uses a different delta d (hydrograph time step) than pond pack for computing the hydrograph. The "time step" that the user has the ability to change only changes the interpolated output values taken from the actual hydrograph.