Green-Ampt Model
The SWMM runoff model uses the Mein and Larson (1973) implementation of the Green-Ampt infiltration equation.
A full description of the Green-Ampt method can be found in the SWMM Reference Manual (Hydrology), which can be downloaded from the EPA website.
Define the following three parameters to use this infiltration model:
Average capillary suction
Specify a value in millimetres or inches depending on your units setting. You can derive this parameter from soil moisture conductivity data or use a value from the following table.
| Soil texture | Typical capillary suction (mm) |
|---|---|
|
Sand |
100 |
|
Sandy loam |
200 |
|
Silt loam |
300 |
|
Loam |
200 |
|
Clay loam |
250 |
|
Clay |
175 |
Saturated hydraulic conductivity
Specify a value in millimetres per hour or inches per hour depending on your units setting. This value is equivalent to the limiting infiltration rate in the Horton model. Please see the references at the bottom of this topic for further information on the Horton model.
Initial moisture deficit
Specify a non-dimensional value. The initial moisture deficit is the fraction difference between soil porosity and actual moisture content. The table below contains suggested values. (Note: These fractions should be specified as percentages when defining rainfall initial conditions in the Rainfall Event Editor).
| Soil texture | Typical initial moisture deficit at soil wilting point |
|---|---|
|
Sand |
0.34 |
|
Sandy loam |
0.33 |
|
Silt loam |
0.32 |
|
Loam |
0.31 |
|
Sandy clay loam |
0.26 |
|
Clay loam |
0.24 |
|
Clay |
0.21 |
