• There are several assumptions that form the basis for rational method hydrology:
• Drainage areas are smaller than 300 acres (120 hectares).
• Peak flow occurs when the entire catchment is contributing.
• Rainfall intensity is uniform over a duration of time equal to or greater than the time of concentration.

Rational coefficients are independent of the intensity of the rainfall.

The Rational Method has been tested on urban catchments up to 150 hectares in area with times of concentration up to about 30 minutes and outfall pipe diameters of up to around 1 metre. The catchments tested had reasonably uniform slopes and impervious area distributions. These tests have shown that the method is as accurate as other more sophisticated urban runoff methods for calculating peak runoff. (Department of Environment, National Water Council, Standing Technical Committee, 1981)

The accuracy of the method for conditions outside those described above is unknown, and therefore the method can't be positively recommended in those cases.