Infiltration Rate (Green–Ampt) Calculator

Example data table

Formula used

The Green–Ampt approach treats infiltration as a sharp wetting front moving downward through soil. It links infiltration to soil hydraulic conductivity, capillary suction, and the moisture deficit ahead of the front.

• Δθ = θs − θi (moisture deficit, dimensionless)
• A = ψf·Δθ (mm)
• F = Ks·t + A·ln(1 + F/A) (mm, solved numerically)
• f = Ks·(1 + A/F) (mm/hr, infiltration capacity)
• Actual infiltration = min(i, f) (mm/hr)

How to use this calculator

1. Enter Ks using the closest available units.
2. Enter ψf (wetting front suction head) and select its units.
3. Provide θs and θi as decimal fractions (0 to 1).
4. Set the time at which you want the infiltration capacity.
5. Optionally enter i to compute the actual rate as min(i,f).
6. Press Calculate. Results appear above this form.
7. Use Download CSV or Download PDF for reporting.

Green–Ampt infiltration in field practice

1) Why the Green–Ampt approach is used

Green–Ampt is widely used in hydrology because it connects infiltration to measurable soil properties. It assumes a distinct wetting front and produces an infiltration capacity that typically declines with time. This makes it practical for stormwater design checks and irrigation scheduling when steady laboratory curves are unavailable.

2) Key inputs and where data comes from

The calculator needs saturated hydraulic conductivity (Ks), wetting front suction head (ψf), and moisture contents (θs and θi). Ks is often estimated from texture-based tables or field tests like double-ring infiltrometers. ψf relates to pore-size distribution and commonly varies with soil type and structure.

3) Typical magnitude ranges you can sanity-check

In practice, Ks can range from a few mm/hr in compacted clays to hundreds of mm/hr in coarse sands. ψf often falls between tens and a few hundred millimeters when expressed as an equivalent suction head. Moisture contents are dimensionless and typically sit between 0.05 and 0.60, depending on porosity and antecedent wetness.

4) Moisture deficit drives early-time infiltration

The moisture deficit Δθ = θs − θi controls how much water the soil can absorb before reaching near-saturation. Dry soils (larger Δθ) usually show higher early infiltration capacity because the wetting front advances faster. Small Δθ values indicate already-wet soil and reduce capillary-driven intake.

5) Time dependence and cumulative infiltration

Green–Ampt computes cumulative infiltration F by solving an implicit equation. As F grows, the term A/F decreases, so the infiltration capacity approaches Ks. For example, with Ks = 10 mm/hr and A = 25 mm, early-time capacity can exceed Ks but will trend downward toward 10 mm/hr.

6) Rainfall-limited versus soil-limited conditions

Real infiltration is capped by the available input rate i from rainfall or irrigation. The calculator reports actual infiltration as min(i, f). If i is 25 mm/hr and f is 19 mm/hr, the soil limits intake and excess water may pond or run off. If i is 8 mm/hr and f is 20 mm/hr, the storm limits intake and there is no ponding from infiltration capacity alone.

7) Interpreting outputs for decisions

Use the infiltration capacity f to compare soil scenarios, and use the actual rate to estimate infiltrated depth over a period. The exported CSV and PDF help document assumptions, unit conversions, and intermediate values such as A and Δθ for reporting.

8) Practical cautions and limits

The model assumes homogeneous soil and a sharp wetting front, so layered soils, crusting, macropores, and preferential flow can deviate. When calibrating, adjust Ks and ψf within realistic bounds and compare predicted infiltration against observed ponding times or field curves. Treat results as engineering estimates unless site measurements support the parameters.

FAQs

1) What does the “actual infiltration rate” mean?

It is the smaller of the rainfall/irrigation intensity i and the infiltration capacity f. This indicates how fast water enters the soil at that moment, assuming uniform surface supply and no other losses.

2) Why is cumulative infiltration F solved numerically?

The Green–Ampt cumulative equation contains F inside a logarithm, so it is implicit. The calculator uses a stable numerical method to find F, then computes the infiltration capacity from that value.

3) How should I choose θs and θi?

θs is often close to porosity and can be taken from lab data or published ranges for the texture. θi should represent antecedent wetness, such as after a dry spell or a recent storm.

4) What happens if ψf is set to zero?

If ψf is zero, wetting-front suction is ignored, so infiltration reduces to a conductivity-only case. The model behaves like F ≈ Ks·t, and f approaches Ks without early-time enhancement.

5) Can I use this for infiltration basins or stormwater systems?

Yes for preliminary estimates of infiltration capacity over time. For final design, confirm parameters with site tests and consider clogging, compaction, and layered soils that can reduce infiltration significantly.

6) Why can infiltration capacity be higher than Ks initially?

Capillary suction draws water into dry pores near the wetting front. That suction contribution appears as A/F, making f larger early on. As F grows, A/F decreases and f tends toward Ks.

7) What units should I trust in the outputs?

The calculator converts inputs to base units and reports results in millimeters and hours for consistency. Check the converted inputs panel to confirm the values match your intended units before exporting.