Rational Method

Illustration of approach used in the Rational method.

One of the simplest hydrologic situations to model is the peak runoff from a small area (less than about 80 hectares). The Rational method estimates only the peak flow using rainfall intensity, area, and a land use factor. Note that using this method, there is no timing associated with the peak flow.

The equation is:

Peak Runoff = C*I*A where:

  • C is a unitless, land-use runoff coefficient
  • I is the rainfall intensity, usually in mm hr-1
  • A is the area, usually in hectares.
  • The product would need to be multplied by a conversion factor of 0.00278, for the result to be in the common unit of cubic meters per second
Land use type table for Rational Method Hydrologic Modeling method

The value of C is found by using a table listing land use types.

The Rational method is used for computing peak runoff from small areas such as parking lots, crop lands, or public parks. It is primarily used in small basins where the coverage of impermeable surfaces is important.

This method provides no robust means of accounting for infiltration.