Calculator Inputs
Example Data Table
| Scenario | Method | Key Inputs | Typical Output |
|---|---|---|---|
| Small test pit infiltration check | Field test | Area 1.0 m², Volume 10 L, Time 20 min | Depth 10 mm, Rate 30 mm/hr |
| Decreasing capacity during soak | Horton | f0 120 mm/hr, fc 15 mm/hr, k 2.2 1/hr, t 0.75 hr | f(t) ≈ 30 mm/hr, F(t) ≈ 34 mm |
| Soil-based infiltration basin design check | Green-Ampt | Ks 20 mm/hr, ψf 110 mm, Δθ 0.20, t 1.5 hr | F and f computed from model equations |
Formula Used
1) Field Test (Average Rate)
Depth = V / A
Rate = Depth / t
V is water volume, A is test area, t is duration. Outputs are average values over the test.
2) Horton Model
f(t) = fc + (f0 − fc) e^(−k t)
F(t) = fc t + (f0 − fc) (1 − e^(−k t)) / k
f(t) is infiltration capacity; F(t) is cumulative infiltration depth at time t.
3) Green-Ampt Model
f = Ks ( 1 + (ψf Δθ) / F )
F = Ks t + ψf Δθ ln( 1 + F / (ψf Δθ) )
The cumulative depth F is solved numerically, then f is computed from F.
How to Use This Calculator
- Select a method that matches your data source (field test, Horton, or Green-Ampt).
- Choose metric or imperial units, then enter the area for volume estimation.
- Fill only the inputs for the chosen method; leave other fields blank.
- Press Calculate to see results above the form.
- Download CSV or PDF to attach to design notes and reports.
- Validate assumptions with soil logs, compaction state, and site drainage criteria.
1) Why infiltration rate matters
Infiltration rate controls how fast water can enter soil from basins, trenches, soakaways, and permeable surfaces. It influences detention sizing, drawdown time, and the risk of surface ponding near slabs and footings. A realistic rate supports safer drainage and reduces nuisance flooding.
2) Typical ranges by soil texture
Coarse sands often show high rates, while silts and clays are slower and more variable. As a practical reference, many sites observe roughly 25–250 mm/hr in sands, 5–50 mm/hr in loams, and 0.5–10 mm/hr in clayey soils, depending on structure and saturation history.
3) Field-test data that drives decisions
Simple volume–area–time tests translate directly into an average infiltration rate. Improve reliability by using a defined test area, pre-wetting to reduce early-time bias, and recording multiple intervals. If 10 L infiltrates through 1.0 m² in 20 minutes, the depth is 10 mm and the average rate is 30 mm/hr.
4) Horton model for declining capacity
Horton represents a common pattern: high early capacity that decays toward a steady value as pores fill. The parameters f0, fc, and k can be fitted from repeated measurements during a soak test. Designers often use fc or a conservatively reduced f(t) when targeting long-duration storms.
5) Green-Ampt for physics-based checks
Green-Ampt links infiltration to hydraulic conductivity (Ks), wetting front suction (ψf), and moisture deficit (Δθ). Ks can be estimated from field permeability tests, while ψf and Δθ can come from soil type correlations and moisture measurements. The model helps compare scenarios like dry versus near-saturated initial conditions.
6) Converting rates into volumes
For planning, convert cumulative infiltration depth to a volume using the contributing area. For example, 35 mm infiltrated across 25 m² equals 0.035 m × 25 m² = 0.875 m³. This supports drawdown checks, storage sizing, and pump-free drainage verification for infiltration features.
7) Construction realities that reduce infiltration
Compaction, fines migration, sediment loading, and biological growth can reduce performance dramatically. A factor of safety is common: many projects apply a reduction factor such as 2–5× (or more) based on soil sensitivity and maintenance plans. Include pretreatment (sumps, filters) to protect the infiltration surface.
8) Reporting for QA and approvals
Document method, unit system, test geometry, water volume, timing, soil description, groundwater depth, and weather. Exportable summaries help reviewers trace assumptions. When possible, report both a representative rate and a conservative design rate, and state how reductions were applied for long-term operation.
FAQs
1) Which method should I use?
Use the field test when you have measured volume, area, and time. Use Horton when you observe a clear decline over time. Use Green-Ampt when you have soil parameters and want a physics-based comparison.
2) Why is my early-time rate unusually high?
Dry soil, surface cracks, and air-filled pores can cause an initial spike. Pre-wetting and measuring multiple intervals usually reduces this bias, producing a steadier estimate suitable for design and reporting.
3) Should I design using the average rate?
Often no. Designers commonly use a conservative rate that reflects clogging, compaction, and long-term performance. If you have multiple tests, consider using lower-percentile results or applying a reduction factor.
4) What area should I enter?
Enter the infiltration surface area relevant to your estimate: test ring area, trench bottom area, or basin footprint. The calculator uses area to convert cumulative depth into an estimated infiltrated volume.
5) What does Δθ represent in Green-Ampt?
Δθ is the soil moisture deficit, typically θs minus θi. It describes how much additional water the soil can hold per unit volume before reaching near-saturation at the wetting front.
6) Can I compare metric and imperial outputs directly?
Yes, if you keep units consistent within each run. Metric outputs report mm and mm/hr, while imperial outputs report inches and inches per hour. Re-run with the other unit system to compare.
7) Is this a substitute for a geotechnical report?
No. Use it to organize calculations and summaries. Final design should follow site investigation findings, groundwater considerations, and local drainage criteria, with appropriate safety factors and professional review.
Use results thoughtfully, and confirm with local standards always.