The Green and Ampt equation is used to determine the rate of infiltration.  Two coefficients (A and B) are used in the equation.  Values for the A and B coefficients may be derived mathematically from the saturated vertical hydraulic conductivity and the soil water characteristic.  The coefficients A and B are entered along with depth to the water table in the Infiltration tab of the Soil input screen. Again, the last set of values should correspond to a water table depth of 1000 cm.

 

The Reference Report (Skaggs, 1980) discusses four methods to determine the Green-Ampt parameters.  The recommended method is #4: calculation of Sav using prediction equations for K and measured soil characteristic data.  The equations for A and B are:

 

A = Ks M Sav

 

B = Ks

 

where Ks is the vertical saturated hydraulic conductivity, M is the fillable porosity (water content at saturation in cm3/cm3 minus the water content at the desired water table depth) and Sav is the suction at the wetting front in cm.

 

If the user only has a lateral saturated conductivity, KL, (the K determined with the auger hole method, for example), then a choice of one KL is a starting estimate for the vertical saturated conductivity, Ks.  Typical Sav values by soil textural class are given in Table 8.  Effects of tillage and management practices on Sav, Ks and the soil water retention function were analyzed by Rawls et al. (1983).

 

Table 1.  Sav and bubbling pressure by soil textural class.

 

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Soil Texture        Bubbling Pressure*        Sav**

 (cm)        (cm)

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Sand        7.3        5.0

Loamy Sand        8.7        6.1

Sandy Loam        14.7        11.0

Loam        11.2        8.9

Silt Loam        20.8        16.7

Sandy Clay Loam        28.1        21.9

Clay Loam        25.9        20.9

Silty Clay Loam        32.6        27.3

Sandy Clay        29.2        23.9

Silty Clay        34.2        29.2

Clay        37.3        31.6

 

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* Soil water characteristics from Rawls, Brakensiek and Saxton (1982).

** Brakensiek and Rawls (1982).