The rational method is an old, simple, widely used (and often criticized) method employed in the
determination of peak discharges from a given watershed. The method is based on the idea that the peak
rate of surface outflow from a given watershed will be proportional to the watershed area and the
average rainfall intensity over a period of time just sufficient for all parts of the watershed to contribute
to the outflow (Reference 38). The constant of proportionality is then supposed to reflect all those
characteristics of the watershed. In its simplest form, the rational formula is written as
Q = CiA EQ