The Green-Ampt infiltration model is derived from Darcy's law and assumes unsaturated flow through a homogeneous soil profile. It models the movement of a sharp wetting front through the soil. Key variables in the model include the soil porosity, residual water content, effective porosity, suction head, and hydraulic conductivity. The document provides Green-Ampt's equations for cumulative infiltration and infiltration rate, defines the model's variables, and gives an example problem of applying the model to compute infiltration over time into a silty clay soil.

2. GREEN-AMPT
INFILTRATION MODEL
Prepared by: Jeric Ryan B. Galan

3. Infiltration Methods
• Green – Ampt
–Infiltration model developed from
an approximate theory to an exact
solution
–is derived from the application of
Darcy's law to unsaturated flow in
a homogeneous soil profile.

4. Green-Ampt Assumptions
Wetted Zone
Wetting Front
Ground Surface
Dry Soil
L


n
i


z

 = increase in moisture
content as wetting front
passes

 = Suction head at “sharp”
wetting front
Conductivity, K
L = Wetted depth
K = Conductivity in wetted
zone
Ponded Water 0
h
0
h = Depth of water ponding
on surface (small)

6. Green-Ampt’s equation for cumulative
infiltration:
Green-Ampt’s equation for infiltration rate:

7. In the Green Amp model, the potential
infiltration rate is computed as

8. Green-Ampt Soil Water Variables
Wetted Zone
Wetting Front
Ground Surface
Dry Soil
L


n
i


z
r
 e

n = 𝛳𝑟+ 𝛳𝑒
i
 = initial moisture content of
dry soil before infiltration
happens

 = increase in moisture
content as wetting front
passes
 = moisture content (volume
of water/total volume of soil)
r
 = residual water content of
very dry soil
e
 = effective porosity
n = porosity
n = 𝛳𝑖+ 𝛳

9. Additional:
While solving, there are some other technical
terms that need to be taken care of:

10. Green-Ampt Parameters
(Data from Table 4.3.1)
Texture Porosity n
Residual
Porosity ϴr
Effective
Porosity ϴe
Suction
Head ψ (cm)
Conductivity
K (cm/hr)
Sand 0.437 0.020 0.417 4.95 11.78
Loamy Sand 0.437 0.036 0.401 6.13 2.99
Sandy Loam 0.453 0.041 0.412 11.01 1.09
Loam 0.463 0.029 0.434 8.89 0.34
Silt Loam 0.501 0.015 0.486 16.68 0.65
Sandy Clay
Loam 0.398 0.068 0.330 21.85 0.15
Clay Loam 0.464 0.155 0.309 20.88 0.10
Silty Clay Loam 0.471 0.039 0.432 27.30 0.10
Sandy Clay 0.430 0.109 0.321 23.90 0.06
Silty Clay 0.470 0.047 0.423 29.22 0.05
Clay 0.475 0.090 0.385 31.63 0.03

11. Example: Use Green-Ampt method to
compute infiltration rate and cumulative
infiltration at every 0.25-hr from the
beginning of infiltration for a silty clay soil
with porosity, ε = 0.479; Suction head, Ψ =
29.22 cm; K = 0.05 cm/hr. Assume the initial
effective saturation is 30 percent and it have
continuous ponding. The effective porosity
for the silty clay soil is 0.423.

12. Example: Use Green-Ampt Method to
compute the infiltration rate f and
cumulative infiltration F after one hour
of infiltration into a silt loam soil that
initially had an effective saturation of 30
%. Assume water is ponded to a small
but negligible depth on the surface.