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.
This page contains the following topics:
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.
Representative values of Maximum (Initial) Infiltration Capacity, F0 | ||
---|---|---|
A. DRY soils (with little or no vegetation) | in./hr | mm/hr |
Sandy soils | 5 | 127 |
Loamy soils: | 3 | 76.2 |
Clay soils | 1 | 25.4 |
B. DRY soils | ||
Multiply values given in A by 2 | ||
C. MOIST soils (for single event simulation) | ||
Soils which have drained but not dried out: Divide values from A and B by 3 | ||
Soils close to saturation: Choose value close to saturated hydraulic conductivity | ||
Soils partially dried out: Divide values from A and B by 1.5 – 2.5 |
Values suggested by Akan (1993): | ||
Soil Type | (in/hr) | (mm/hr) |
---|---|---|
Dry sandy soils with little or no vegetation | 5.0 | 127 |
Dry loam soils with little or no vegetation | 3.0 | 76.2 |
Dry clay soils with little or no vegetation | 1.0 | 25.4 |
Dry sandy soils with dense vegetation | 10.0 | 254 |
Dry loam soils with dense vegetation | 6.0 | 152 |
Dry clay soils with dense vegetation | 2.0 | 51 |
Moist sandy soils with little or no vegetation | 1.7 | 43 |
Moist loam soils with little or no vegetation | 1.0 | 25 |
Moist clay soils with little or no vegetation | 0.3 | 7.6 |
Moist sandy soils with dense vegetation | 3.3 | 84 |
Moist loam soils with dense vegetation | 2.0 | 5.1 |
Moist clay soils with dense or no vegetation | 0.7 | 18 |
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).
Minimum (Asymptotic) Infiltration Rate, Fc | ||
Hydrologic Soil Group | (in/hr) | (mm/hr) |
---|---|---|
A | 0.30 - 0.45 | 7.6 - 11.4 |
B | 0.15 - 0.30 | 3.8 - 7.6 |
C | 0.05 - 0.15 | 1.3 - 3.8 |
D | 0.00 - 0.05 | 0.0 - 1.3 |
The Hydrological Soil Group corresponds to the classification given by the Soil Conservation Service. Well drained sandy soils are "A"; poorly drained clayey soils are "D". The texture of the layer of least hydraulic conductivity in the soil profile should be considered. Caution should be used in applying values from the above table to sandy soils (Group A) since reported values are often much higher.
Values of Horton Equation parameters suggested by Akan (1993):
Minimum (Asymptotic) Infiltration Capacity, Fc | ||
---|---|---|
Soil Type | (in/hr) | (mm/hr) |
Clay loam, silty clay loam, sandy clay, silty clay, clay | 0.00 - 0.05 | 0.00 - 1.3 |
Sandy clay loam | 0.05 - 0.15 | 1.3 - 3.8 |
Silt loam, loam | 0.15 - 0.30 | 3.8 – 7.6 |
Sand, loamy sand, sandy loam | 0.30 - 0.45 | 7.6 – 11.4 |
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:
f | = infiltration rate, ft/sec |
---|---|
Fp | = infiltration capacity, ft/sec |
i | = rainfall intensity, ft/sec |
F | = cumulative infiltration volume, this event, ft |
Fs | = cumulative infiltration volume required to cause surface saturation, ft |
* Su | = average capillary suction at the wetting front, ft water |
* IMD | = initial moisture deficit for this event, ft/ft |
* Ks | = saturated hydraulic conductivity of soil, ft/sec |
Parameters preceded by an asterisk (*) are parameters required to be entered by the user. Infiltration is thus related to the volume of water infiltrated and the moisture conditions in the surface soil zone.
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.
Soil Texture | Typical Values for Capillary Suction, in. |
---|---|
Sand | 4 |
Sandy Loam | 8 |
Silt Loam | 12 |
Loam | 8 |
Clay Loam | 10 |
Clay | 7 |
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).
Soil Texture | Typical Initial Moisture Deficit at Soil Wilting Point (in.) |
---|---|
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 |
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.
Ground Cover | Manning's n* |
---|---|
Smooth Asphalt | 0.01 |
Asphalt or concrete paving | 0.014 |
Packed clay | 0.03 |
Light turf | 0.20 |
Dense turf | 0.35 |
Dense shrubbery and forest litter | 0.4 |
The next table was compiled by Engman (1986) by kinematic wave and storage analysis of measured rainfall-runoff data.
Ground Cover | Manning's n* | Range |
---|---|---|
Concrete or asphalt | 0.011 | 0.01 - 0.013 |
Bare sand | 0.01 | 0.01 - 0.016 |
Gravelled surface | 0.02 | 0.012 - 0.033 |
Bare clay-loam (eroded) | 0.02 | 0.012 - 0.033 |
Range (natural) | 0.13 | 0.01 - 0.32 |
Bluegrass sod | 0.45 | 0.39 - 0.63 |
Short grass prairie | 0.15 | 0.10 - 0.20 |
Bermuda grass | 0.41 | 0.30 - 0.48 |
n* is akin to Manning’s n but usually higher.
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.