SCS Curve Number & Rainfall Into Runoff
Estimates runoff
Enter rainfall and a curve number (CN) for your soil and land use to estimate runoff and infiltration, plus the potential retention S and initial abstraction Ia — to size ponds, bunds and waterways.
SCS curve number runoff
Next: size your waterways and farm ponds for ~50.5 mm of runoff per 100 mm storm; raising soil cover and infiltration lowers the effective CN and keeps more of the 49.5 mm in the profile.
CN bundles soil hydrologic group, land use and antecedent moisture. Runoff is zero until rainfall exceeds the initial abstraction Ia = 0.2·S.
Curve number runoff — key facts
- Retention S
- (25400 ÷ CN) − 254 mm
- Initial abstraction Ia
- 0.2 × S
- Runoff Q
- (P − Ia)² ÷ (P − Ia + S)
- Infiltration
- P − Q
- High CN
- Compacted, bare, clay → more runoff
- Low CN
- Sandy, vegetated → more infiltration
- CN range
- ≈ 30–100
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One number decides how much rain runs off
When rain falls, some soaks into the ground and some runs off the surface. The SCS curve number method captures that split with a single index — the curve number — that bundles soil type, land use and how wet the ground already is. A high CN (compacted, bare or clay ground) sheds most of the rain as runoff; a low CN (sandy, well-vegetated soil) lets most of it infiltrate. From the CN the method derives the potential retention S, the initial abstraction Ia, and finally the runoff depth.
This tool computes potential retention S, initial abstraction Ia, runoff and infiltration for your rainfall and curve number. Use it to size farm ponds, contour bunds, percolation tanks and grassed waterways, and to see how better land management lowers runoff and stores more water in the soil. Pair it with the Effective Rainfall, Rainwater Harvesting and Farm Pond tools to plan a complete water-harvesting system.
Split rain into runoff and soak
See exactly how much each event yields.
Capture the curve number
Soil, land use and wetness in one index.
Size your structures
Runoff volume sizes ponds, bunds and dams.
Reward good management
Lower CN means more water in the soil.
Frequently Asked Questions
What is the SCS curve number method?+
The SCS-CN method (from the US Soil Conservation Service, now NRCS) estimates how much of a rainfall event becomes surface runoff and how much infiltrates. It packs soil type, land use and antecedent wetness into a single number — the curve number (CN) from 0 to 100 — making it one of the most widely used runoff tools in agriculture and watershed design.
How is runoff calculated from the curve number?+
First the potential maximum retention S = (25400 ÷ CN) − 254 in mm. The initial abstraction Ia (rain absorbed before runoff starts) is taken as 0.2 × S. Then runoff Q = (P − Ia)² ÷ (P − Ia + S) when rainfall P exceeds Ia, otherwise zero. Infiltration is simply P − Q.
What is the curve number (CN)?+
CN is a dimensionless index from about 30 to 100 that summarises how readily a surface produces runoff. Low CN (sandy, well-vegetated, undisturbed) means most rain soaks in; high CN (compacted, bare, clay, paved) means most rain runs off. Standard tables give CN by hydrologic soil group and land use/cover.
What is potential retention S?+
S is the maximum depth of water the soil and surface could absorb after runoff begins, in mm. It's derived directly from CN: a high CN gives a small S (little can be absorbed, lots runs off) and a low CN gives a large S. S is the heart of the method — everything else follows from it.
What is initial abstraction Ia?+
Ia is the rainfall that is intercepted, ponded in surface depressions or infiltrated before any runoff appears. The standard SCS assumption is Ia = 0.2 × S. Rainfall must exceed Ia before runoff starts, which is why a light shower on dry, absorbent ground produces no runoff at all.
What does CN tell me about my soil and land use?+
A higher CN flags more runoff risk: bare or compacted fields, heavy clay, and built or paved areas all push CN up. Lower CN comes from cover crops, mulch, contour cultivation, deep sandy soils and good structure. Lowering CN through management means more water stored in the soil and less erosive runoff.
How is this used to size farm ponds and bunds?+
Runoff depth × catchment area gives the runoff volume from a design storm — the basis for sizing farm ponds, percolation tanks, contour bunds, check dams and grassed waterways. Get the runoff right and you build structures that capture or safely convey the water without being over- or under-sized.
How does antecedent moisture affect runoff?+
Wet ground runs off far more than dry ground. The method handles this with antecedent moisture conditions (AMC I dry, II average, III wet); CN is adjusted upward for wet conditions. Designing for a wet catchment (AMC III) is common for safety, since the biggest runoff events often follow earlier rain.
What are the limits of the CN method?+
It's an event-based estimate, best for design rather than precise prediction, and it's sensitive to the chosen CN and the Ia assumption. It doesn't model rainfall intensity within the storm or detailed soil physics. Used with sensible curve numbers it remains an excellent, practical tool for farm-scale water harvesting and drainage.