GreenAmpt

The Green-Ampt infiltration model is a simple analytical model used in hydrology to describe infiltration into a homogeneous, initially uniform soil profile under ponded conditions or infiltration-limited rainfall. Developed by Henry Green and George Ampt in 1911, it assumes a sharp wetting front advancing into the soil, with a sudden change from θi to θs at depth Zf.

The soil parameters are saturated hydraulic conductivity Ks and the soil-water suction at the wetting front ψf (positive). The depth of the wetting front is Zf = F/Δθ where F is cumulative infiltration and Δθ = θs − θi.

The infiltration rate f = dF/dt is given by f = Ks [1 + ψf / Zf] = Ks [1 + ψf Δθ / F]. This yields a time-infiltration relationship, t = (F / Ks) − (ψf Δθ / Ks) ln(1 + F /(ψf Δθ)). Infiltration continues until the rainfall intensity falls to the infiltration capacity, after which ponding may occur.

Applications: used to estimate infiltration during rainfall or irrigation, design of irrigation systems, rainwater harvesting, and hydrological modeling where a simple infiltration law is needed.

Limitations: assumes homogeneous soil, a sharp wetting front, constant Ks and ψf, and negligible preferential flow; does not capture suction changes with depth or soil layering, and may be less accurate for early-time infiltration or nonponding conditions.

See also: infiltration models, Horton, Philip, Richards equation.