Enter watershed and storm data
The calculator combines Rational Method flow, SCS runoff depth, and Kirpich travel timing for a practical engineering estimate.
Event hydrograph
The chart uses the computed peak discharge, time to peak, and base time to display a practical triangular event hydrograph.
Submit the form to generate the hydrograph.
Engineering equations behind the calculator
Q = 0.00278 × C × i × A
Q = direct peak discharge in m³/s, C = runoff coefficient, i = rainfall intensity in mm/hr, A = catchment area in hectares.
S = (25400 / CN) − 254
S = storage parameter in mm, CN = curve number.
Ia = λ × S
Ia = initial abstraction in mm, λ = abstraction factor.
Qr = (P − Ia)² / (P − Ia + S), when P > Ia
Qr = runoff depth in mm, P = rainfall depth in mm.
V = (Qr / 1000) × Am²
V = runoff volume in m³, Am² = catchment area in square meters.
Tc = 0.01947 × L0.77 × Slope−0.385
Tc = time of concentration in minutes, L = flow length in meters, Slope = slope in m/m.
Simple workflow
- Enter the catchment area in hectares.
- Provide rainfall intensity and event rainfall depth.
- Choose a runoff coefficient matching the land cover.
- Enter the curve number for watershed abstraction behavior.
- Supply the hydraulic flow length and basin slope.
- Set the storm duration in minutes.
- Keep the abstraction factor at 0.20 unless your method differs.
- Optionally include baseflow and infiltration rate for added context.
- Press the calculate button.
- Review the result cards, table, hydrograph, and download options.
Worked sample
| Catchment Area (ha) | Intensity (mm/hr) | Runoff Coefficient | Rainfall Depth (mm) | Curve Number | Flow Length (m) | Slope (%) | Storm Duration (min) | Peak Discharge (m³/s) | Runoff Depth (mm) | Runoff Volume (m³) | Tc (min) |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 125.00 | 65.00 | 0.58 | 92.00 | 78.00 | 1850.00 | 3.40 | 75.00 | 14.301 | 40.404 | 50505.607 | 23.468 |
Common questions
1) What does this hydrology calculator estimate?
It estimates peak discharge, runoff depth, runoff volume, watershed timing, storage behavior, and a simple event hydrograph from rainfall and basin inputs.
2) Why does it use both the Rational Method and curve number method?
The Rational Method is useful for peak flow. The curve number method estimates event runoff depth and losses. Using both gives broader design insight.
3) What runoff coefficient should I use?
Use a value that reflects land cover, imperviousness, slope, and drainage condition. Paved urban areas are higher. Grass and forested areas are lower.
4) What is the curve number?
The curve number represents watershed runoff potential. It depends on soil type, cover, treatment, and antecedent condition. Higher values produce more runoff.
5) Why is time of concentration important?
It estimates how long water needs to travel from the most distant point to the outlet. It strongly affects hydrograph timing and design response.
6) What does baseflow change in the results?
Baseflow adds background discharge to the direct runoff peak. It helps approximate total flow when the basin already carries water before the storm.
7) Can I use this for final regulatory design?
Use it for screening, concept design, and comparison. Final regulatory work should follow your local criteria, approved storm frequencies, and calibrated models.
8) Why might my runoff depth become zero?
If rainfall depth does not exceed initial abstraction, the SCS runoff equation returns zero direct runoff. That means early losses consume the event.