The rational formula is used to estimate the peak discharge of a storm runoff. 

It is given by the equation:

Q = 0.0028 CIA

where:

• Q is the peak discharge (m^3/s)
• C is the runoff coefficient (dimensionless)
• I is the rainfall intensity (mm/hr)

the runoff coefficient is 0.5, the rainfall intensity is 40 mm/hr, and the drainage area is 4 ha. The peak discharge is then:

Q = (0.0028)(0.5)(40 mm/hr)(4 ha) = 0.224 m^3/s

Therefore, the drainage discharge required to dispose the runoff generated from the catchment is 0.224 m^3/s.