The Rational Method
Q = A * C * I * k
where:
Q = discharge in cubic metres per second
A = area in square kilometres
C = coefficient of runoff
I = intensity of rainfall in millimetres per second
k = constant of proportionality
Steps in Determining the Peak Flow Discharge of a Drainage Basin with the Rational Method*

1.  Define the perimeter of the watershed and measure the watershed area.
2.  Identify the appropriate rainfall intensity for the location and rainstorm desired.
3.  Subdivide the watershed according to cover types, soils and slopes.  Asssign a coefficient of runoff to each, and measure each subarea.
4.  Determine the percentage of the watershed represented by each subarea and multiply this figure by the coefficient of runoff of each.  This will give you a coefficient adjusted according to the size of subarea.
5.  Sum the adjusted coefficients to determine a coefficient of runoff for the watershed as a whole.
6.  Multiply the watershed area times the coefficient of runoff times the rainfall intensity value to obtain peak discharge in cubic metres per second.

Tables of Runoff Coefficients

Back to Methodology

*  Source:  adapted from William M. Marsh's Landscape Planning:  Environmental Applications, 1983, p. 136.