Implementation - Runoff Discharge
The Rational Method is used to calculate the runoff discharge.
Q=kCiA
where:
Q = runoff
C = runoff coefficient
i = rainfall_intensity
A = contribution drainage area
k = unit convertion coefficient
k = 1.008 for Q in cfs, i in in/hr, A in acre, BUT IT IS NORMALLY NEGLECTED AND TAKEN AS 1.0.
k = 1/43200 for Q in cfs, i in in/hr, A in ft^2, BUT IT IS NORMALLY TAKEN AS 1/43560 to match for acre.
k = 1/360 for Q in cms, i in mm/hr, A in hecta.
k = 1/3600000 for Q in cms, i in mm/hr, A in m^2.
For a drainage areas with differing runoff coefficients, it is necessary to subdivide the area into several subareas and sum all the contribution runoff to get the total runoff.