Free Web Tool

SCS Curve Number Runoff

Composite Curve Number from sub-areas + Hydrologic Soil Group, then runoff depth Q for a given rainfall depth (NEH-4 / TR-55).

Inputs

mm

Sub-areas

Method & CN typical values
  • S = (25400/CN) − 254 (mm); Ia = 0.2·S.
  • Q = (P − Ia)² / (P − Ia + S) if P > Ia, else Q = 0.
  • Composite CNw = Σ(CN·A) / ΣA.
  • AMC III: CNIII = 23·CNII/(10 + 0.13·CNII).
  • AMC I: CNI = 4.2·CNII/(10 − 0.058·CNII).

Results

Total Area
Composite CNII
CN (adjusted for AMC)
Potential retention S
Initial Abstraction Ia
Runoff Depth Q
Runoff Volume V