Infiltration Model
Rainfall infiltration from the permeable area of a subcatchment to the upper zone of unsaturated soil can be described using five different models:
- Classic Horton Infiltration
- Maximum Infiltration - Maximum infiltration rate in the Horton curve (in/h or mm/h)
- Dry soils with little or no vegetation
- Sandy soils: 5 in/h
- Floor soils: 3 in/h
- Clay soils: 1 inch/h
- Dry soils with dense vegetation
- Sandy soils: 10 in/h
- Floor soils: 6 in/h
- Clay soils: 2 in/h
- Moist soils
- Soils that have drained but not dried out (i.e. field capacity)
divide the values of A and B by 3.
- Soils close to saturation
choose the value close to min. infiltration rate
- Partly dry soils
divide the values of A and B by 1.5 - 2.5
- Minimum Leakage - Minimum seepage rate on the Horton curve (in/h or mm/h).
Equivalent to saturated hydraulic Conductivity.
| Soil Texture Class |
Hydraulic Conductivity
(in/hr) |
Suction
(in) |
Porosity
(fraction) |
Field Capacity
(fraction) |
Wilting Point
(fraction) |
| Sand |
4.74 |
1.93 |
0.437 |
0.062 |
0.024 |
| Loamy Sand |
1.18 |
2.40 |
0.437 |
0.105 |
0.047 |
| Sandy Loam |
0.43 |
4.33 |
0.453 |
0.190 |
0.085 |
| Loam |
0.13 |
3.50 |
0.463 |
0.232 |
0.116 |
| Silt Loam |
0.26 |
6.69 |
0.501 |
0.284 |
0.135 |
| Sandy Clay Loam |
0.06 |
8.66 |
0.398 |
0.244 |
0.136 |
| Clay Loam |
0.04 |
8.27 |
0.464 |
0.310 |
0.187 |
| Silty Clay Loam |
0.04 |
10.63 |
0.471 |
0.342 |
0.210 |
| Sandy Clay |
0.02 |
9.45 |
0.430 |
0.321 |
0.221 |
| Silty Clay |
0.02 |
11.42 |
0.479 |
0.371 |
0.251 |
| Clay |
0.01 |
12.60 |
0.475 |
0.378 |
0.265 |
Source: Rawls, W.J. et al., (1983). J Hyd. Engr., 109:1316
- Decay Constant - Infiltration rate decay constant for the Horton curve (1/hour). Typical values range from 2 to 7.
- Drying time - Time in days for a fully saturated soil to dry completely.
Typical values range from 2 to 14 days
- Infiltration Volume - Maximum possible infiltration volume (inches or mm, 0 if not applicable).
It can be estimated as the difference between a soil's porosity and its wilting point times the depth of the infiltration zone.
- Modified Horton Infiltration
- Green-Ampt Infiltration
- Suction - Mean capillary suction value of the soil along the wetting front (inches or mm)
- Conductivity - Soil saturated hydraulic Conductivity (in/h or mm/h)
- Initial deficit - Fraction of soil volume that is initially dry (i.e. difference between soil porosity and initial moisture content)
The initial deficit for a completely drained soil is the difference between the soil porosity and its field capacity. Typical values for all these parameters can be found in the Soil Characteristics Table.
- Modified Green-Ampt Infiltration
- SCS Curve Number Infiltration
- Curve Number - This is the SCS curve number that is tabulated in the publication
SCS Urban Hydrology for Small Watersheds, 2nd Ed., (TR-55), June 1986
See the Table of Curve Numbers for a list of values by group of
solo and the accompanying Soil Group table for the definitions of the various groups.
Adjustments will be necessary when a subcatchment contains permeable and impermeable fractions
separated and a curve number is selected from a table where the two land uses are grouped.
- Drying Time - The number of days that a fully saturated soil takes to dry.
Typical values range from 2 to 14 days