Composite Catchments
Most catchments are comprised of more than one type of ground cover. For example, a roadside drainage inlet may accept flow from the paved roadway, the curbside grass, and a nearby wooded area. To account for the effects of each of these areas, multiply each corresponding sub-catchment area and rational coefficient, then add the values to obtain the total CA (C·A) for the entire catchment.
Note: Since the rational coefficient is unitless, CA values have units of area. A weighted value for the rational coefficient can be determined by dividing the catchment's total CA by the total catchment area. Rather than tracking area and weighted rational coefficients separately, rational loads are often described solely by using the total CA value. Since the C coefficient and area are multiplied together in the rational formula anyway, there is no adverse effect of this simplification on flow determination.