| Scenario | Total rainfall (mm) | Method | Key parameter | Effective rainfall (mm) | Runoff volume (m³) for 2,500 m² |
|---|---|---|---|---|---|
| Concrete staging area | 40 | Runoff coefficient | C = 0.85 | 34.00 | 85.000 |
| Bare compacted soil | 40 | Loss-based | Losses = 6 mm (total) | 34.00 | 85.000 |
| Mixed site drainage | 55 | SCS Curve Number | CN = 80, λ = 0.20 | ~24.86 | ~62.150 |
These examples are illustrative. Local guidance and calibration may be required for design.
Pe = max(0, P − L)
Where L is the sum of interception, initial abstraction, infiltration, evapotranspiration, and storage losses.
Infiltration = infiltration rate × duration.
Pe = P × C
Use C between 0 and 1 based on surface type and compaction.
Implied loss = P × (1 − C).
S = (25400/CN) − 254
Ia = λ × S
Q = (P − Ia)² / (P − Ia + S)
If P ≤ Ia, then Q = 0.
- Runoff volume (m³) = (Pe/1000) × Area(m²)
- Peak flow (m³/s) ≈ 0.278 × C × I(mm/hr) × A(km²)
Peak flow is a quick estimate and depends on time of concentration and local standards.
- Select a calculation method that matches your available data.
- Choose the depth unit and enter rainfall depth, or enable intensity × duration.
- For loss-based, enter realistic losses and (if needed) duration for infiltration.
- For coefficient, pick a surface preset or enter your own runoff coefficient.
- For curve number, enter CN and adjust λ only if your guidance requires it.
- Optionally add catchment area to compute runoff volume and peak flow.
- Press Calculate. Download CSV or PDF for records.
Effective Rainfall in Site Water Balance
Effective rainfall is the portion of measured precipitation that becomes usable water on a project site after immediate losses. In construction planning it supports dewatering estimates, erosion control, and temporary drainage sizing. Track values by event, day, or month to match your reporting period. Use site logs to calibrate losses after several rainfall events.
Runoff and Loss Factors
Losses typically include interception, surface wetting, infiltration, and small depression storage. The Loss Method applies an initial loss plus a continuing loss rate; it suits compacted subgrade and mixed surfaces. Typical starting values might be 1–5 mm initial loss and 0.5–5 mm/h continuing loss, adjusted to site conditions. The Coefficient Method multiplies rainfall by a runoff coefficient, often 0.10–0.30 for lawns, 0.30–0.60 for gravel, and 0.70–0.95 for paved areas.
Design Storm Selection and Time Step
If you enter intensity and duration, the calculator converts them into total depth for the event. Use a duration that represents the critical period for your catchment and your reporting objective. Short time steps highlight peak runoff, while longer steps are better for storage and pumping volumes.
Using the Calculator Outputs
Effective depth (mm) can be converted to volume using area: volume = depth/1000 × area. If area is provided, the tool also reports runoff volume and an indicative peak flow using a simplified Rational approach. Use the peak value as a screening number, then confirm with your local standard method for final design. Export CSV for logs and PDF for submittals with assumptions.
Quality Checks and Common Pitfalls
Keep units consistent: mm with m² or hectares, and verify intensity is in mm/hour. Avoid coefficients outside a realistic range for your surface, and review Curve Number assumptions against soil group and cover. When results look high, check duration, area, and whether losses were double-counted. When results look low, confirm rainfall depth is not already net of losses.
Q1. What is effective rainfall in construction planning?
It is the portion of rainfall expected to become runoff or usable inflow after immediate losses such as infiltration and surface wetting. It helps estimate pumping, storage, and temporary drainage needs.
Q2. Which method should I choose: Loss, Coefficient, or Curve Number?
Use Loss when you know initial and continuing losses, Coefficient when you have a representative runoff coefficient, and Curve Number when you can justify a CN from land cover and soil group. Pick the method your stakeholders accept.
Q3. Can I use rainfall intensity instead of depth?
Yes. Enable intensity mode, enter intensity and duration, and the tool converts them to total depth. Ensure the intensity units are mm/hour and the duration represents the critical storm period.
Q4. How is runoff volume calculated from effective rainfall?
Runoff volume is effective depth converted to meters and multiplied by catchment area. The calculator reports the result in cubic meters when area is provided, which is useful for tank sizing and pump run-time checks.
Q5. Is the peak flow result suitable for final design?
Treat it as a quick screening estimate only. Peak flow depends on time of concentration and local criteria. For permitting or permanent works, confirm using your accepted hydrology and hydraulic standards.
Q6. How do I validate my inputs on site?
Compare outputs against observed runoff, sump inflows, or pump logs after real events. Adjust losses, coefficients, or CN so results match typical conditions, then document the calibration assumptions in your reports.