The SCS-SA Method Explained: Curve Numbers for South African Catchments

The SCS-SA method is South Africa’s localised version of the United States Soil Conservation Service (SCS) runoff procedure — adapted for South African soils, rainfall, and catchment conditions through research at the University of Natal (now UKZN) by Schulze, Schmidt and colleagues. Where the Rational Method gives you a single peak flow, SCS-SA produces a full flood hydrograph: peak, volume, and shape. That makes it the standard choice whenever storage, attenuation, or floodplain routing is part of the question.

The core idea: rainfall in, runoff out, via a curve number

The method converts a design storm into runoff in two steps.

Step 1 — how much of the rain runs off. The SCS runoff equation estimates the runoff depth Q from the storm rainfall P, given the catchment’s potential maximum retention S:

Q = (P − Iₐ)² / (P − Iₐ + S), for P > Iₐ

where Iₐ is the initial abstraction — the rain lost to interception, surface storage, and early infiltration before runoff starts (commonly taken as a fraction of S). The retention S comes from the curve number (CN):

S = (25400 / CN) − 254 (S in mm)

CN runs from around 30 (deep, permeable, well-vegetated soils that absorb almost everything) to 98 (paving, which sheds almost everything). It is the method’s counterpart to the Rational Method’s C — a single calibrated number that encodes soils, land cover, and land management.

Step 2 — how fast it comes off. The runoff depth is distributed in time using the catchment lag and a dimensionless unit hydrograph, producing the design hydrograph and its peak.

What makes it “SCS-SA” rather than just SCS

The South African adaptation is not cosmetic. Its key localisations are:

• Soils. South African soil forms and series were classified into the SCS hydrological soil groups (A through D, with intermediate classes), so CN selection can be grounded in actual SA soil mapping rather than US descriptions.
• Design rainfall and storm distributions. The method uses South African design rainfall depths with regionally appropriate synthetic storm distributions, rather than the US Type curves.
• Catchment response and antecedent moisture. SA research introduced improved estimates of catchment lag and — importantly — adjustment of the curve number for typical antecedent soil moisture ahead of large storms in each region, rather than assuming a fixed “average” condition. Joint consideration of how wet catchments actually are when big storms arrive materially improves design flood estimates.

When SCS-SA is the right tool

SCS-SA is intended for small to medium catchments — indicatively up to about 30 km² — and earns its keep when:

• You need volume and shape, not just a peak — sizing detention basins, checking attenuation requirements in a stormwater management plan, or routing a flood through a dam or wetland.
• Land-use change is the question. Because CN responds directly to land cover, SCS-SA is well suited to before/after development comparisons in catchment studies.
• A reviewer expects method comparison. For floodline determinations and other regulated estimates, standard South African practice is to compute design floods by several independent methods — SCS-SA alongside the Rational Method, empirical methods, and statistical analysis where records allow — and to justify the adopted value. A hydrograph method in that mix is expected.

Its main sensitivities are the mirror of its strengths: the result leans heavily on CN selection (soils mapping and land-cover interpretation), on the storm distribution chosen, and on the antecedent moisture assumption. Two of those three are judgement calls that should be documented, not defaulted.

SCS-SA vs Rational, in one table

	Rational Method	SCS-SA
Output	Peak flow only	Full hydrograph
Catchment size	Small (≲ 15 km²)	Small–medium (≲ 30 km²)
Key parameter	Runoff coefficient C	Curve number CN
Land-use change analysis	Crude	Strong
Storage/attenuation design	Not suitable	Designed for it
Effort	Minutes	Hours (soils + land cover mapping)

In practice the two are complements, not competitors: quick peak checks and pipe sizing lean on the Rational Method; anything involving volume, storage, or regulatory scrutiny brings in SCS-SA. Our hydrological studies routinely run both — agreement builds confidence, and disagreement tells you where to look harder.