| Product(s): | SewerGEMS, CivilStorm, PondPack |
| Version(s): | 08.11.XX.XX and higher |
| Area: | Calculations |
Is the Initial Abstraction taken into account regarding the Storm Rainfall Depth versus the Runoff Depth?
Initial Abstraction is a parameter that accounts for all losses prior to runoff and consists mainly of interception, infiltration, evaporation, and surface depression storage. In theory all Rainfall minus Initial Abstraction will generate the runoff from a specified Catchment.
The formula for Initial Abstraction is utilized in the SCS method. The formula is:
Ia = 0.2S
Where S = (1000/CN)-10
In PondPack, to visualize the Initial Abstraction leaving the storm rainfall total you can use the following steps.
You can compare the storm rainfall with the water runoff by viewing the Runoff Hydrograph - Unit Hydrograph Summary in the reports section of PondPack. In this Unit Hydrograph Summary report you will see Depth of the Storm Event (in.)
This is the initial rainfall value. Now to find the Runoff volume (in), you can utilize the following equation.
Q = ((P-0.2S)^2) / (P+0.8S)
Where Q= Total Runoff volume (in)
P = Total Precipitation (in)
S = Potential maximum retention (in) after runoff.
As you can see, the Initial Abstraction is taken into account in this Runoff formula. So the calculations take the Initial Depth of the Storm Event and calculate the Cumulative Runoff Depth which is also in the Unit Hydrograph summary report.
If you are required to use a different initial abstraction than what the software assumes (for example a ratio of 0.05 instead of 0.2), you can compensate by adjusting the SCS CN value to achieve the desired end result. Meaning, use the formulas above to back calculate the adjusted CN that achieves the desired end result CN based on your assumed initial abstraction ratio.
Catchment's total runoff and peak flow are different than HEC-HMS