Runoff Coefficient
The rational method uses a runoff coefficient to separate the rainfall depth into losses and runoff. It defines a simple percentage for the amount of rainfall that runs off. The rational equation is given as:
Q = CiA
where: C = runoff coefficient (a value between 0.0 and 1.0)
i = rainfall intensity (inches/hour)
A
= area (acres)
If C represents the percentage of rainfall that becomes runoff then (1-C) would be the losses. In the case of runoff coefficients all losses, including initial abstractions and infiltration are accounted for with this single value.
Some examples of different C values for different kinds of surface condition are given in the following table.
| Surface condition | Runoff Coefficient |
| Business districts | 0.70 - 0.95 |
| Residential | 0.50 - 0.75 |
| Industrial | 0.50 - 0.90 |
| Parks and grass | 0.05 - 0.30 |
| Streets and driveways | 0.75 - 0.95 |
| Agricultural and unimproved | 0.10 - 0.30 |
If a watershed contains multiple surface conditions, a composite runoff coefficient should be calculated using an area weighted average.
