How do you size the storm water network pipe based on the rain water collection?

based on rain water collection we calculate the discharge by using equation Q = ciA 

where Q = discharge , c = discharge coffienet , i = intensity and A = catchment Area 

after calculatng discharge  we use hydrograph 

hydrograph is a graph showing the rate of flow (discharge) versus time past a specific point in a river, or other channel or conduit carrying flow. The rate of flow is typically expressed in cubic meters or cubic feet per second (cms or cfs).