Due to assumptions regarding homogeneity of rainfall and equilibrium conditions at the time of peak flow, the rational method should not be used on areas larger than about 1 square mile without subdividing the overall watershed into sub-basins including the effect of routing through any drainage channels.
WMS includes two different methods for determining runoff from larger watersheds, subdivided into smaller sub-basins, including the ability to account for routing and lag through drainage channels and detention basins.
Related Topics:
Rational Method Overview
Rational Equation