The changes in design discharge using the Rational Method are as follows:
1. The major change in the Rational Method is the coefficient of runoff. In the second edition, it is read from a design chart and varies according to the types of landuse, the rainfall intensities and whether it is urban or rural catchments. But in the second edition, it is fixed according to the landuse- like in the P&DP No. 1 (1975).
2. There is no change in the size of catchment area where the Rational Method can be applied. Both editions specify that the Rational Method should not be used for catchment area greater than 80 ha.
3. The magnitude of changes in the design discharge using the Rational Method is covered in the Case Study (Quek, 2013b).