Infiltration

Global subcatchment infiltration data. This data is referenced from individual subcatchments.

Infiltration from pervious areas may be computed by either the Horton (1933, 1940) or Green-Ampt (1911) equations or as a uniform loss. Parameters required by the two methods are quite different. Depression storage is also related to infiltration, especially for pervious areas. Manning's roughness relates more to runoff routing and is grouped with subcatchment infiltration parameters.

Infiltration Method

Horton Infiltration

This dialog is used to define a Global Database record for Horton infiltration parameters.

Horton's model is empirical and is perhaps the best known of the infiltration equations. Many hydrologists have a "feel" for the best values of its three parameters despite the lack of published information.

The model gives infiltration capacity as a function of time as:

Where:

Fp = infiltration rate into soil, in./hr (mm/hr)

Fc = minimum or asymtopic value of Fp, in./hr (mm/hr)

Fo = maximum or initial value of Fp, in./hr (mm/hr)

t = time from beginning of storm, sec

k = decay coefficient, 1/sec

This equation describes the familiar exponential decay of infiltration capacity evident during heavy storms. However, the program uses the integrated form to avoid an unwanted reduction in infiltration capacity during periods of light rainfall.

Continuous Simulation

For continuous simulation, infiltration capacity will be regenerated during dry weather. The recovery of the infiltration rate during dry weather is calculated by the equation:

Where:

kp = decay coefficient for the recovery curve = k × REGEN

tw = projected time at which Fp = Fo, sec

REGEN = coefficient of soil regeneration

The value of for REGEN is typically << 1, suggesting that soil regeneration rate is much slower that initial decay rate. The input of REGEN is located in the Runoff Job Control dialog. The default value is 0.01. Further information may be found in Appendix V of the EPA SWMM User’s Manual for Version 4.

Maximum (Initial) Infiltration Rate, Fo

The maximum or initial infiltration capacity, in./hr [mm/hr]. This parameter depends primarily on soil type, initial moisture content and surface vegetation conditions. For single event simulation the initial moisture content is important. The values listed in the following table can be used as a rough guide.

Minimum (Asymptotic) Infiltration Rate, Fc

The minimum or ultimate value of infiltration capacity, in./hr [mm/hr]. This parameter is essentially the saturated hydraulic conductivity, or "permeability", of soils. The following table lists ranges of this parameter for various soil groups (Musgrave, 1955). 

Values of Horton Equation parameters suggested by Akan (1993):

The minimum (asymptotic) infiltration rate is often close to the saturated hydraulic conductivity of the soil.

Decay rate of Infiltration

Max Infiltration Volume

Max infiltration volume is accumulative. The infiltration rate becomes zero once max volume is attained. Zero in the max volume cell means that no max volume will be attained.

Green-Ampt Infiltration

Data required for using the Green Ampt infiltration method. Although not as well known as the Horton equation, this method has physically based parameters that can be predicted.

The Mein-Larson (1973) formulation of the Green-Ampt equation is a two-stage model; the first step predicts the volume of water which will infiltrate before the surface becomes saturated; from this point, infiltration capacity is predicted by the Green-Ampt equation. The algorithm is described as follows:

IF F < Fs THEN
f = i
IF i > Ks THEN
Fs = (Su * IMD) / (i/Ks - 1)
END
ELSE
f = Fp
Fp = Ks * (1 + Su * IMD / F)
END

Where:

Average Capillary Suction

The average capillary suction, in. (mm) of water is perhaps the most difficult parameter to quantify. The following table summarizes several published values.

This parameter can be derived from soil moisture conductivity data if available.

Initial Moisture Deficit

The fractional difference between soil porosity and actual moisture content, non-dimensional. This parameter is the most sensitive of the three. Values for dry antecedent conditions tend to be higher for sandy soils than clay soils because the water is held weakly in the soil pores of sandy soils. The following table gives typical values for various soil types (Clapp and Hornberger, 1973). 

For single event simulations these values would apply only to very dry antecedent conditions. For moist or very wet antecedent conditions, lower values should be used. Note that since sandy soils drain more quickly than clayey soils, the value for sandy soil will be closer to the above tabulated values than it would be for clayey soil for the same period since the previous event.

Saturated Hydraulic Conductivity

Saturated hydraulic conductivity of soil, in./hr [mm/hr]. This parameter is the same as the corresponding Horton parameter.

Uniform Loss

The uniform loss method allows for simulating infiltration as an initial amount followed by a constant rate.

The initial loss in. (mm) specifies the depth of rainfall that infiltrates before any runoff occurs.

The continuing loss occurs after the initial loss has been satisfied. It may be defined as an absolute value, in./hr (mm/hr) or as a rate proportional to rainfall, 0.0 – 1.0.

SCS Curve Number

The SCS Curve Number Method for simulating infiltration may be used with the following routing methods:

• Runoff (EPA SWMM procedure)
• Laurenson
• Unit Hydrography
  • Nash
  • Synder
  • Synder (Alameda)
  • Clark

A curve number is selected for the pervious area in the subcatchment. The initial abstraction can be quantified as an absolute depth (in. or mm) or as a fraction of the precipitation depth.

Depression Storage, Mannings “n” and Zero Detention % entries in the Infiltration dialog are active fields when using the SCS Loss infiltration method in conjunction with the SWMM Runoff routing method. In this case the SCS parameters are only used to compute infiltration. The typical way to use this combination would be to include typical estimates for the SCS parameters. However, we encourage the Green-Ampt infiltration method be used with SWMM Runoff with as it is the most physically descriptive infiltration option.

For further in information see SCS Hydrology. 

Depression Storage

The volume, in inches [mm], that must be filled prior to the occurrence of runoff. It represents the loss or "initial abstraction" caused by such phenomena as surface ponding, surface wetting, interception and evaporation. Depression storage may be treated as a calibration parameter, particularly to adjust runoff volumes. Separate depression stores are required for pervious and impervious areas.

When a subarea impervious% value > 0 is used the subarea is divided into two components representing the impervious and pervious portions. Horton or Green Ampt losses are only applied to the pervious portion of the subarea. In some runoff procedures including the EPA runoff, kinematic wave method, and some unit hydrograph methods currently excluding the time/area method also include additional depression storage loss. Impervious depression storage loss is the only loss applied to the impervious portion.

Separate Horton or Green Ampt losses can be applied to both impervious and pervious portions of a sub-catchment by defining two subareas each with impervious% set to 0 (zero). Then use the pervious parameters within the infiltration dialog to define each impervious and pervious areas respectively.

Impervious Depression Storage

Impervious area depression storage, in. [mm]. Water stored as depression storage on impervious areas is depleted by evaporation. A relationship for depression storage versus catchment slope has been developed as follows (Kidd, 1978):

Dp = 0.0303 * S ^ -0.49 (Correlation coefficient 0.85)

Where:

Dp = depression storage, inch.

S = catchment slope, percent.

Pervious Depression Storage

Pervious area depression storage, in. [mm]. Water stored as depression storage is subject to both infiltration and evaporation. This parameter is best represented as an interception loss, based on the type of surface vegetation. For grassed urban surfaces, a value of 0.10 in. (2.5 mm) is typical.

Manning's Roughness

The Manning's roughness for the subcatchment pervious and impervious areas. Values of Manning's roughness coefficient are not as well known for overland flow as for channel flow because of the considerable variability in ground cover, transitions between laminar and turbulent flow, very small depths, etc. Some estimates of Manning's roughness are given in the following tables. The user is advised to refer appropriate texts for values applicable to their project.

The following table was compiled by Crawford and Linsley by calibration using the Stanford Watershed Model.

The next table was compiled by Engman (1986) by kinematic wave and storage analysis of measured rainfall-runoff data.

Zero Detention

Percentage of the subcatchment impervious area with zero detention (immediate runoff), 0.0 - 100.0 percent. This parameter assigns a percentage of the impervious area a zero depression storage in order to promote immediate runoff.