This is the factor used in the Rational Method to allow for a number of influences on the conversion of rainfall to runoff. Such influences include loss and routing effects, and the effects of land-use, catchment slope and other factors.
The interpretation of the C factor is always controversial, as it is meant to account for so many factors in a single number. There are many versions of the rational method in which C values are related to different factors, or are applied in different ways. Earlier versions of the rational method defined C values for different types of surfaces (roof, road, grass, etc.) or land-use types (high-density residential, commercial, parks, etc.). More recent methods tend to divide catchments into impervious and pervious proportions, with some distinguishing roofs or directly-connected and non-directly-connected (supplementary) impervious areas.
Roofs are usually given a C value of 1.0 and impervious ground-level surfaces a value around 0.9. There are various ways of determining C values for pervious surfaces, which can vary from 0.0 to over 1.0, depending on the locality and the particular version of the rational method being applied.
In some methods, such as those in the 1987 Australian Rainfall and Runoff and AS/NZS 3500.3, the user specifies a 10 year average recurrence interval C values, and these are converted to other ARIs by multiplying by the factors:
ARI (years) | Frequency Factor |
1 | 0.80 |
2 | 0.85 |
5 | 0.95 |
10 | 1.00 |
20 | 1.05 |
50 | 1.15 |
100 | 1.20 |
>100 | 1.25
(AS/NZS3500.3 only) |
(Source: p. 307, Australian Rainfall and Runoff, 1987)