Calculator Inputs
Use the grid below for large, medium, and mobile screens. Submit to calculate runoff depth, runoff volume, and peak flow.
Example Data Table
These sample scenarios help compare the effects of rainfall depth, watershed area, land response, and storm intensity on runoff behavior.
| Scenario | Rainfall | Area | CN | C | Intensity | Runoff Depth | Peak Flow |
|---|---|---|---|---|---|---|---|
| Urban Catchment | 90 mm | 10 ha | 88 | 0.80 | 55 mm/hr | 54.9 mm | 1.22 m³/s |
| Mixed Watershed | 85 mm | 12.5 ha | 78 | 0.60 | 42 mm/hr | 31.6 mm | 0.88 m³/s |
| Rural Basin | 70 mm | 18 ha | 68 | 0.35 | 28 mm/hr | 14.3 mm | 0.49 m³/s |
Formula Used
1) SCS Curve Number Retention
S = (25400 / CN) − 254 for metric depth units.
S is the potential maximum retention after runoff begins. Larger curve numbers reduce storage and usually increase runoff.
2) Initial Abstraction
Ia = λ × S
Initial abstraction covers interception, depression storage, and early infiltration before direct runoff begins.
3) Direct Runoff Depth
Q = (P − Ia)² / (P − Ia + S) for P > Ia
If rainfall depth P does not exceed Ia, direct runoff depth Q is zero.
4) Runoff Volume
Volume = Q × Area
Runoff depth is converted to a volume using total watershed area.
5) Rational Peak Discharge
Peak Flow = C × i × A
The calculator converts intensity and area into consistent SI units internally, then reports the result in the selected unit system.
6) Timing Indicators
Lag Time = 0.6 × Tc
Time to Peak ≈ D/2 + Lag Time, where D is storm duration and Tc is time of concentration.
How to Use This Calculator
- Choose metric or imperial units first so every field is interpreted correctly.
- Enter rainfall depth and watershed area for the event you want to evaluate.
- Set the SCS curve number based on land cover, soil group, and treatment condition.
- Leave λ at 0.20 unless your method requires a different abstraction ratio.
- Enter Rational Method coefficient C and rainfall intensity for peak discharge estimation.
- Add storm duration and time of concentration to interpret timing and hydro-response.
- Press Calculate Runoff to show results above the form.
- Use the CSV and PDF buttons to export a clean summary for reporting or documentation.
FAQs
1) What does a rainfall runoff calculator estimate?
It estimates how much rainfall becomes direct runoff, how much volume leaves the watershed, and how large peak discharge may become during a storm event.
2) Why are both SCS and Rational methods included?
They answer different questions. SCS helps estimate runoff depth and volume. Rational Method focuses on peak discharge during short critical storms for drainage sizing.
3) How do I choose the curve number?
Pick a value based on land use, hydrologic soil group, surface condition, and antecedent moisture assumptions. Higher values usually represent lower infiltration and greater runoff.
4) What does the runoff coefficient C represent?
C represents the fraction of rainfall intensity expected to appear as peak runoff response. Impervious, paved, or densely built areas usually use higher values.
5) Why is runoff depth lower than rainfall depth?
Some rainfall is lost to interception, storage, infiltration, and other abstractions. Only the excess portion becomes direct runoff in this event-based approach.
6) What is time of concentration?
It is the travel time required for runoff from the hydraulically most distant point to reach the outlet. It affects timing and peak response interpretation.
7) When should I use metric versus imperial units?
Use whichever matches your project data. The calculator converts internally, so the final answers stay consistent as long as every input follows the selected unit system.
8) Can this calculator replace a full hydrologic model?
No. It is a practical screening and design-support tool. Detailed routing, channel storage, spatial rainfall variation, and calibration need more advanced modeling workflows.