Green-Ampt Equation

Green and Ampt developed an infiltration model based on Darcy’s law. Generally, the governing equation is:
= K(change in potential)/distance

where

• f is the infiltration rate
• K is the hydraulic conductivity of the wetted soil part of the profile
• Lf is the depth of the wetting front
• tau is the pressure head for wetting at the wetting front
• d is the depth of ponding of water on the soil surface

By assuming the ponded depth is small enough to be neglected, the equation can be transformed to:

Where

• F(t) is the cumulative infiltration rate at time t
• tau is the change in water content across the wetting front

Typical Green Ampt parameters are:

Green Ampt Infiltration Parameters (Rawls et all., 1983)
Soil texture	Total porosity	Effective porosity	Pressure head (cm)	K (cm/hr)
Sand	0.437	0.417	4.95	11.78
Loamy sand	0.437	0.401	6.13	2.99
Sandy loam	0.453	0.412	11.01	1.09
Loam	0.463	0.434	8.89	0.34
Silt loam	0.501	0.486	16.68	0.65
Sandy clay loam	0.398	0.330	21.85	0.15
Clay loam	0.464	0.309	20.88	0.10
Silty clay loam	0.471	0.432	27.30	0.10
Sandy clay	0.430	0.321	23.90	0.03
Silty clay	0.479	0.423	29.22	0.05
Clay	0.475	0.385	31.63	0.03