Run-off Coefficient for Perforated Piping network below the Softscape

[Buildtech2]

I am calculating the stormwater holding tank capacity for a large park. I need help in considering the run-off coefficient in this case. The stormwater after percolating the soft scape/soil will enter into a perforated piping network which is provided below the soil layer. Eventually, all this stormwater will be collected in a stormwater holding tank.

I would appreciate it if anybody could guide me in considering the run-off coefficient to be considered in calculating the stormwater holding tank capacity.

 

[gbam]

I would expect that the difference in volume is negligible, assuming this will be engineered to convey to pipe. I would size the tank for the contributing area regardless of how it gets there.