	Applies To
	Product(s):	PondPack, CivilStorm, SewerGEMS
	Version(s):	V8i, CONNECT Edition
	Area:	Calculations
	Original Author:	Mark Pachlhofer, Bentley Technical Support Group

Problem

The total runoff depth and peak flow are different between HEC-HMS and PondPack (or CivilStorm / SewerGEMS) when using the same settings for a SCS Unit hydrograph method catchment.

Solution

You should be able to get almost identical runoff from a single catchment/subbasin when comparing between PondPack/CivilStorm/SewerGEMS and HEC-HMS.

Double check the data entry especially the units (e.g. HEC-HMS uses square miles as the area for catchments/basins while PondPack defaults to acres), since that is often a source of errors.

In HEC-HMS your subbasin should have no canopy method, no surface method, and no baseflow method. Your loss method should be SCS Curve Number and your Transform Method should be SCS Unit Hydrograph. On the Loss tab be careful with setting an impervious area since that can change the effective curve number used in the calculations. If you do use a value greater than zero here make sure you do the same in PondPack / CivilStorm / SewerGEMS. Also, make sure that the "Initial Abstraction" in HEC-HMS is left blank (not zero), so that it is automatically calculated as 0.2 times the "potential retention", as that is what the SCS Unit Hydrograph method implementation in PondPack, CivilStorm and SewerGEMS will always do.

Other differences to note are that HEC-HMS uses “lag time” whereas PondPack uses Time of Concentration. These are not the same thing. Lag time = 0.6 * Tc.

HEC-HMS uses the HEC-1 unit hydrograph simulation method, whereas PondPack (by default) uses a hybrid approach of HEC-1 and TR-20. In order to match up the results in PondPack, go to Tools > Options > Project tab and change the Unit Hydrograph simulation option to “HEC-1 method”.

HEC-HMS results seem to use an increment equal to the increment that the precipitation was entered, so it’s important to use that in PondPack/CivilStorm/SewerGEMS to get identical results. This could be important to note because sometimes the 'true peak' of the runoff hydrograph can occur between timesteps. For example if you use an output increment of 1 minute in PPK, you might get a peak that’s higher if it occurred between the 5 minute increments. This means that PondPack/CivilStorm/SewerGEMS provides more flexibility and accuracy in such cases.

Catchment's total runoff and peak flow are different than HEC-HMS

Problem

Solution

See Also