The Infiltration Editor is used to specify the method and its
parameters that model the rate at which rainfall infiltrates into the upper
soil zone of a subcatchment's pervious area. The infiltration parameters depend on
which infiltration model is selected for the subcatchment: Horton and Modified Horton,
Green-Ampt and Modified Green-Ampt, or Curve Number. The infiltration model is normally
the default one set by project's Simulation Options or its Default Properties but can
be overridden in the Editor.
Parameter
Description
Max. Infil. Rate
Maximum infiltration rate on the Horton curve (in/hr or mm/hr) (see table below)
Min. Infil. Rate
Minimum infiltration rate on the Horton curve (in/hr or mm/hr). Equivalent to the saturated
hydraulic conductivity. See the Soil Characteristics Table
for typical values.
Decay Const.
Infiltration rate decay constant for the Horton curve (1/hours). Typical values range
between 2 and 7.
Drying Time
Time in days for a fully saturated soil to dry completely. Typical values range from 2 to 14 days.
Max. Infil. Vol.
Maximum infiltration volume possible (inches or mm, 0 if not applicable). Can be estimated as the difference between a soil's porosity and its wilting point times the depth of the infiltration zone.
Representative Values for Max. Infiltration Rate
DRY soils (with little or no vegetation):
Sandy soils: 5 in/hr
Loam soils: 3 in/hr
Clay soils: 1 in/hr
DRY soils (with dense vegetation):
Multiply values given in A. by 2
MOIST soils
Soils which have drained but not dried out (i.e., field capacity):
divide values from A and B by 3.
Soils close to saturation:
choose value close to min. infiltration rate.
Soils which have partially dried out:
divide values from A and B by 1.5 - 2.5.
Parameter
Description
Suction Head
Average value of soil capillary suction along the wetting front (inches or mm)
Conductivity
Soil saturated hydraulic conductivity (in/hr or mm/hr)
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's porosity
nd its field capacity. Typical values for all of these parameters can be found in the
Soil Characteristics Table.
Parameter
Description
Curve Number
This is the SCS curve number which is tabulated in the publication SCS Urban Hydrology
for Small Watersheds, 2nd Ed., (TR-55), June 1986. Consult the
Curve Number Table
for a listing of values by soil group, and the accompaning
Soil Group Table
for the definitions of the various groups. Adjustments will be needed when a subcatchment contains separate pervious and impervious fractions and a Curve Number is selected from a table where the two land uses are lumped together.
Conductivity
This property has been deprecated and is no longer used.
Drying Time
The number of days it takes a fully saturated soil to dry. Typical values range between 2 and 14 days.
Parameter
Description
Infil. Pattern
Name of optional monthly Time Pattern adjustments applied to the pervious area's hydraulic conductivity. Leave blank if not applicable.
for typical values.
for a listing of values by soil group, and the accompaning
Soil Group Table
for the definitions of the various groups. Adjustments will be needed when a subcatchment contains separate pervious and impervious fractions and a Curve Number is selected from a table where the two land uses are lumped together.