Soil Infiltration Rate Calculator
Calculate cumulative infiltration and infiltration rate using Philip's Two-Term Method or the Green-Ampt Method for various soil types.
Soil water absorption capacity (typical range: 5–100 mm/hr⁰·⁵)
Steady-state transmission rate (typical range: 1–50 mm/hr)
Elapsed time since infiltration began (must be > 0)
Results will appear here.
Formulas
Philip's Two-Term Method:
- Cumulative Infiltration: F(t) = S·t0.5 + A·t
- Infiltration Rate: f(t) = ½·S·t−0.5 + A
Where S = sorptivity (mm/hr⁰·⁵), A = transmissivity constant (mm/hr), t = time (hr).
Green-Ampt Method:
- Cumulative Infiltration (implicit): F(t) = Ks·t + ψ·IMD·ln(1 + F(t)/(ψ·IMD))
- Infiltration Rate: f(t) = Ks·(1 + ψ·IMD / F(t))
- Initial Moisture Deficit: IMD = θe − θi
Where Ks = saturated hydraulic conductivity (mm/hr), ψ = wetting front suction head (mm), θe = effective porosity, θi = initial moisture content. The implicit equation is solved via Newton-Raphson iteration.
Assumptions & References
- Philip's method assumes homogeneous soil, uniform initial moisture, and ponded surface conditions from t = 0.
- Green-Ampt method assumes a sharp wetting front, saturated conditions above the front, and uniform initial moisture deficit below.
- Green-Ampt soil parameters (Ks, ψ, θe) are mean values from Rawls, Brakensiek & Miller (1983) as tabulated in Chow, Maidment & Mays (1988).
- The Green-Ampt implicit equation is solved iteratively using successive substitution (converges to within 10⁻⁹ mm).
- Both methods are valid for rainfall intensity exceeding the infiltration rate (ponding assumed).
- References: Philip, J.R. (1957). The theory of infiltration. Soil Science, 83(5), 345–357. | Green, W.H. & Ampt, G.A. (1911). Studies on soil physics. Journal of Agricultural Science, 4(1), 1–24. | Rawls, W.J., Brakensiek, D.L. & Miller, N. (1983). Green-Ampt infiltration parameters from soils data. ASCE Journal of Hydraulic Engineering, 109(1), 62–70.