SCS-CN is an event loss method estimating direct-runoff depth, also described in NRCS guidance. CN is not the Rational Method's runoff coefficient C. The output is not a discharge time series or peak. Estimating how much rainfall becomes runoff volume differs from estimating when and how rapidly that volume reaches the outlet.
Choosing CN from catchment information
Choose CN using cover, land use, hydrologic soil group, surface condition and the source's moisture convention. Similar-looking sites need not share values. Preserve table assumptions, document area proportions and the combination method, and consider impervious connectivity where relevant. Separate existing and proposed land use rather than change building area while ignoring other assumptions.
Retention and initial abstraction
In millimetres, S = 25400/CN − 254. For P > Ia, runoff depth R = (P − Ia)²/(P − Ia + S); for P ≤ Ia, R = 0. Conventional tables commonly use Ia = 0.2S. A different ratio requires consistency with CN calibration and source assumptions. R denotes depth, not discharge. Keep rainfall, retention and losses in one unit system.
Example runoff-depth calculation
For illustrative CN = 75 and P = 60 mm, S ≈ 84.67 mm, Ia ≈ 16.93 mm and R ≈ 14.52 mm. Uniformly over 1 km², this is approximately 14,520 m³ because 1 mm over 1 km² equals 1,000 m³. Volume is not discharge; arbitrary division by a duration does not generate a peak. The inputs are examples, not catchment recommendations.
Additional steps for a hydrograph
A discharge hydrograph needs rainfall timing, losses, a transformation model and any routing components. Response time, subcatchments and storage affect peaks even for equal rainfall totals. Loss and transformation models remain distinct. Explain which component determines volume and which timing, and compare hydrograph volume with runoff depth to identify unit or connectivity errors.
Sensitivity and method limitations
CN and abstraction changes can strongly affect runoff under some storms. Use soil and surface evidence and observations where available. Event losses do not constitute a continuous water budget or full groundwater model. Consider alternatives outside table assumptions. Reject invalid CN values and correctly handle the zero-retention limit at CN = 100. Report the abstraction ratio and influential uncertainties.
Traceability and reporting scope
For traceable hydrological calculations, record the source, observation period, units and missing-data treatment of the rainfall or streamflow series. Keep the catchment boundary, area calculation, land use, soil information and parameters of the selected method in the same data package. Calculation sheets should show inputs and unit conversions as well as the answer. Another engineer opening the files should be able to understand where each datum came from and the step in which it was used.
It is helpful for the project owner to share the location, study boundary and available mapping at the outset. Missing information does not automatically justify completing the calculation with a single assumption: surveys, alternative data or a narrower scope may be needed. Explain why the selected method suits the catchment and study purpose. The report should present uncertainties and checking scenarios alongside the central assumptions. Hydrological discharge and hydraulic water level are different outputs; where needed, a separate hydraulic model evaluates the effect of that discharge on structures or parcel elevations.
Sources and related articles
Let's discuss your project's data requirements and reporting scope.
Request a project quote