If the drainage area consists of more than one subarea, a weighted C value for the area must be computed. The weighted C for a drainage area is computed by dividing the sum of all subarea CAs by the total area, where CA is the subarea C value multiplied by the area of the subarea.

Example: An engineer wants to compute the weighted C value for the composite drainage area shown below. In this example the C values are not adjusted for storm frequencies.