Source file: scs_runoff.html
Estimates the direct runoff depth resulting from a given total rainfall depth, based on the watershed's soil/land-cover characteristics, using the SCS (NRCS) Curve Number method.
Direct runoff depth:
Q = (P − 0.2S)² / (P + 0.8S) [valid only when P > 0.2S; otherwise Q = 0]
Potential maximum retention:
S = (25400 / CN) − 254 (metric form, S and P in mm)
Initial abstraction:
Ia = 0.2 × S
Where:
- P — total storm rainfall depth (mm)
- CN — SCS Curve Number (0–100), representing runoff potential based on soil group, land use, and antecedent moisture condition
- S — potential maximum retention after runoff begins (mm)
- Ia — initial abstraction (interception, depression storage, and infiltration before runoff starts) — standard SCS assumption sets Ia = 0.2S
- Q — direct runoff depth (mm)
| Input | Symbol | Notes |
|---|---|---|
| Total rainfall depth | P |
mm |
| Curve Number | CN |
Selected from preset dropdown: • Paved parking lots/roofs: CN = 98 • Residential-urban/clay soils: CN = 80 • Cultivated agricultural land: CN = 70 (default) • Forest/woodlands: CN = 55 |
Direct (non-iterative) calculation:
1. Compute S from CN using the metric formula.
2. Compute Ia = 0.2 × S.
3. If P > Ia, compute Q using the runoff equation; otherwise Q = 0 (all rainfall is absorbed by initial abstraction — no runoff generated).
S, mm)Ia, mm)Q, mm)CN value; real watersheds typically require an area-weighted composite CN across multiple soil/land-cover combinations (from NRCS hydrologic soil group tables), which this tool does not compute automatically.Ia = 0.2S assumption; some current NRCS guidance recommends Ia = 0.05S for certain conditions, which would change results — not configurable here.