A runoff coefficient is a factor used in some methods to estimate how much rainfall becomes surface runoff from an area. Values depend on land cover, soil, slope, and the selected method. It simplifies complex conditions and should be chosen according to applicable guidance. It is an input to analysis, not a measured property of every parcel.
Why it matters for owners
Coefficient assumptions can influence flow estimates and design scope. State the method and source in technical deliverables.
