Infiltration:Green and Ampt (GA)

The Green-Ampt (GA) equation, developed for infiltration into uniform soils, has proven very useful in the field of hydrologic engineering. In the GA model water is assumed to enter the soil as a sharp (completely saturated) wetting front. The soil is assumed to homgeneous and infinite, with no restrictions due to the water table. The water can be thought of as entering the soil as a piston, shown in the figure below.

GA Infiltration Model

The GA method is selected by placing the GREEN_AMPT card in the project file. Four soil hydraulic parameters are needed for modeling the infiltration process using the G&A technique:

GA hydraulic conductivity, K_ga (cm/h), half the saturated hydraulic conductivity K_s
GA wetting front suction head, Φ_f (cm),
effective porosity, θ_e,
and initial soil moisture content, θ_i.

These values may be assigned by using the Mapping Table and an index map or with four GRASS ASCII maps. If using the Mapping Table and an index map the MAPPING_TABLE card is placed in the project file. The MOISTURE card is used with no argument. If using the GRASS ASCII maps to assign values to every cell in the watershed the CONDUCTIVITY, CAPILLARY, POROSITY, and MOISTURE cards are used to specify the GRASS ASCII map names as arguments. Assignment of parameters for all infiltration methods is discussed in Section 7.5.