SCS Curve Number Runoff Calculator

SCS/NRCS event runoff with explicit abstraction fraction and mm storage25400/CN−254, no assumedCN from land class. Caller independently establishes units, regime, calibration and applicability; this does not establish design-code compliance, authenticity, operational safety, treatment efficacy or a validated forecast.

Description

The SCS curve-number runoff model relates precipitation to potential retention and initial abstraction.

The SCS curve-number runoff model relates precipitation to potential retention and initial abstraction.^[1]

The adopted curve number and abstraction fraction require independent hydrologic justification. Use this tool to calculate SCS Curve Number Runoff from the supplied measurements and assumptions.

When to use SCS Curve Number Runoff Calculator

Use this tool to calculate SCS Curve Number Runoff from the supplied measurements and assumptions. Establish the SI units or caller unit basis, load signs, measurement interval, model conditions and independently chosen coefficients or references.

How to use SCS Curve Number Runoff Calculator and understand its results

Establish the SI units or caller unit basis, load signs, measurement interval, model conditions and independently chosen coefficients or references.
Enter rainfallDepthMm, callerCurveNumber, callerInitialAbstractionFraction, using the field meanings and rules below, then run the tool.
Read scsStorageMm, scsRunoffMm. Interpret the fields using their units, signs and evidence conventions below.
If input validation fails, correct the reported field and run again. Before applying the result, check reference scope, model assumptions and any missing evidence or placeholder values.

Input field	Meaning	Value type	Requirement
rainfallDepthMm	Rainfall depth mm	number	Required
callerCurveNumber	Caller curve number	number	Required
callerInitialAbstractionFraction	Caller initial abstraction fraction	number	Required

Enter nonnegative precipitation in mm, curve number in (0,100] and initial-abstraction fraction in [0,1]. Set S=25400/CN−254 and excess=P−fraction×S. Return zero when excess≤0; otherwise return P if S=0, or excess²/(excess+S). No default abstraction fraction, curve-number lookup or calibration is supplied.

Qmm=0 if P≤λS, otherwise (P−λS)²/(P−λS+S); S=25400/CN−254

Use the exact units, boundary rules, ordering and missing-evidence conventions above.

Output field	Meaning	Value type
scsStorageMm	Scs Storage Mm; follow the units, sign, ordering and evidence convention stated above	number
scsRunoffMm	Scs Runoff Mm; follow the units, sign, ordering and evidence convention stated above	number

json

{
  "input": {
    "rainfallDepthMm": 100,
    "callerCurveNumber": 80,
    "callerInitialAbstractionFraction": 0.2
  },
  "output": {
    "scsStorageMm": 63.5,
    "scsRunoffMm": 50.53905835543766
  }
}

Limitations and assumptions

The adopted curve number and abstraction fraction require independent hydrologic justification. Interpretation depends on the units, sampling scope and exact model described above.

All listed fields require explicit values. Unexpected fields, incorrect types, nonfinite numbers and sparse evidence arrays fail validation; the specific bounds and alignment requirements are stated above. Extremely large inputs or outputs can also be rejected. An input error is distinct from a successful result reporting false, no matches or missing evidence.

Alternative or Complementary methods

Use Muskingum Flood Routing Calculator for this related task: Muskingum routing uses a storage-time parameter and weighting to advance an outflow sequence.

Use Snyder Unit Hydrograph Calculator when its separate inputs and assumptions fit the question: Snyder-style hydrograph relations use basin dimensions and calibrated coefficients.

References

SCS Curve Number Runoff Calculator — tool.io

Similar or alternative tools

Muskingum Flood Routing Calculator
Muskingum routing uses a storage-time parameter and weighting to advance an outflow sequence.
Snyder Unit Hydrograph Calculator
Snyder-style hydrograph relations use basin dimensions and calibrated coefficients.