Infiltration Methods

SCS Curve Number

The Curve Number (CN) method is an empirical surface runoff method developed by the US Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS); formerly the NRCS was called the Soil Conservation Service (SCS 1985). After the CN method was first developed by the SCS, the USDA changed SCS to Natural Resources Conservation Service (NRCS). The SCS CN method estimates precipitation excess as a function of the cumulative precipitation, soil cover, land use, and antecedent soil moisture. The input requirements for the SCS CN method within HEC-RAS are:

Curve Number: (scalar value for each cell).
Initial abstraction ratio: (scalar value for each cell) [-].
Length of recovery period which is only included when utilizing the recovery method (optional value for each 2D area) [hrs].
Minimum infiltration rate (optional value for each cell) [in/hr or cm/hr].

The curve number values range from approximately 30 (for permeable soils with high infiltration rates) to 100 (for water bodies, impervious surfaces, and soils with near zero infiltration rates). Publications from the Soil Conservation Service (1971, 1986) provide further background and details on use of the CN model. The initial abstraction may be estimated as a function of the potential maximum retention as:

$\begin{array}{l}\displaystyle I_a = rS\end{array}$

where r is the user-defined initial abstraction ratio, typically ranging between 0.05 and 0.2. The potential maximum soil retention S is computed from the runoff curve number CN as:

$\begin{array}{l}\displaystyle S = \frac {1000}{CN} -10\end{array}$

where is in inches.

One thing to keep in mind, the curve number CN method was not originally developed for simulating historic events. The same loss amount will be computed for rainfall of 5 inches regardless of whether it occurred in 1 hour or 1 day. However, the CN method has been adapted to single events.

The curve number is related to soil type, soil infiltration capability, land use, and the depth of the water table. The NRCS has divided soils into four hydrologic soil groups (HSGs) according to the ability to infiltrate. The HSGs are defined as follows:

Group A: Soils with high infiltration rates (low runoff potential) even when thoroughly wetted. These consist chiefly of deep, well-drained sands and gravels. These soils have a final infiltration rates greater than 0.30 in/hr (7.6 mm/hr).
Group B: Soils with moderate infiltration rates when thoroughly wetted. These consist mostly of soils that are moderately deep to deep, moderately well drained to well drained with moderately fine to moderately coarse soil textures. These soils have final infiltration rates of 0.15 – 0.30 in/hr (3.8–7.6 mm/hr).
Group C: Soils with slow infiltration rates when thoroughly wetted. These consist chiefly of soils with a layer that impedes downward movement of water or soils with moderately fine to fine textures. These soils have final infiltration rates of 0.05 – 0.15 in/hr (1.3 – 3.8 mm/hr).
Group D: Soils with very slow infiltration rates (high runoff potential) when thoroughly wetted. These consist chiefly of clay soils with a high swelling potential, soils with a permanent high-water table, soils with a claypan or clay layer at or near the surface, and shallow soils over nearly impervious materials. These soils have final infiltration rates of less than 0.05 in/hr (1.3 mm/hr).

Selection of a hydrologic soil group should be done based on measured infiltration rates, soil survey, or judgment from a qualified soil scientist or geotechnical professional. Table 2-2 presents curve numbers for various hydrologic soil-cover complexes.

Table 2-2. Runoff Curve Numbers for Hydrologic Soil-Cover Complexes

Cover			Hydrologic Soil Group
Land Use	Treatment or Practice	Hydrologic Condition	A	B	C	D
Fallows	Straight row		77	86	91	94
Row crops	Straight row	Poor Good	72 67	81 78	88 85	91 89
	Contoured	Poor Good	70 65	79 75	84 82	88 86
	Contoured and terraced	Poor Good	66 62	74 71	80 78	82 81
Small grain	Straight row	Poor Good	65 63	76 75	84 83	88 87
	Contoured	Poor Good	63 61	74 73	82 81	85 84
	Contoured and terraced	Poor Good	61 59	72 70	79 78	82 81
Close-seeded legumes or rotation meadow	Straight row	Poor Good	66 58	77 72	85 81	89 85
	Contoured	Poor Good	64 55	75 69	83 78	85 83
	Contoured and terraced	Poor Good	63 51	73 67	80 76	83 80
Pasture or range	Contoured	Poor Fair Good	68 49 39	79 69 61	86 79 74	89 84 80
Meadow		Good	30	58	71	78
Woods		Poor Fair Good	45 36 25	66 60 55	77 73 70	83 79 77
Farmsteads			59	74	82	86
Roads (dirt)			72	82	87	89
Road (hard surface)			74	84	90	92

The NRCS has also developed Runoff Curve Numbers of other land cover types, including urban areas. However, some of the CN values shown in the NRCS’ developed urban area tables include the percent impervious area already in the development of the Curve Number. Table 2-3 provides the NRCS’ developed runoff CN values (by HSG) for urban areas.

Table 2-3. Runoff Curve Number for Urban Areas

Cover			Hydrologic Soil Group
Land Use	Cover Description	Hydrologic Condition	A	B	C	D
Open space	(lawns, parks, golf courses, cemeteries, etc.)	Poor (<50% grass) Fair (50% to 75% grass) Good (>75% grass)	68 49 39	79 69 61	86 79 74	89 84 80
Impervious areas	Paved	parking lots, roofs, driveways, etc	98	98	98	98
	Streets and Roads	Paved; curbs and storm sewers Paved; open ditches	98 83	98 89	98 92	98 93
	Streets and Roads	Gravel Dirt	76 72	85 82	89 87	91 89
Western Desert Urban Areas	Natural Desert Artificial Desert	Pervious areas only Impervious weed barrier	63 96	77 96	85 96	88 96
Urban Districts	Commercial/Business Industrial	85 % Impervious 72 % Impervious	89 81	92 88	94 91	95 93
Residential Districts by Average Lot Size	1/8 acre (town house) 1/4 acre 1/3 acre 1/2 acre 1 acre 2 acre	65 % Impervious 38 % Impervious 30 % Impervious 25 % Impervious 20 % Impervious 12 % Impervious	77 61 57 54 51 46	85 75 72 70 68 65	90 83 81 80 79 77	92 87 86 85 84 82
Developing Urban Areas	Newly Graded Areas Pervious areas only No vegetation		77	86	91	94

In practice, it is much more accurate to develop runoff Curve Numbers for the pervious area only and define the impervious area separately. In HEC-RAS, impervious area will be treated as 100% runoff with no infiltration. For example, this definition is especially important for urban areas, where runoff will occur at the very beginning of storms due to impervious areas that are directly connected to the storm runoff system.

In HEC-RAS, the recovery method for the SCS CN consists of setting the cumulative rainfall depth to zero (i.e., ) after a user-specified time in which the infiltration is zero.

The SCS CN model outputs the following variables:

Cumulative excess (per cell) [in or cm]
Cumulative precipitation (per cell) [in or cm]
Infiltration rate (per cell) [in/hr or cm/hr]
Infiltration depth (per cell) [in or cm]
Dry time (per cell) [hrs]

Deficit and Constant

The Deficit and Constant loss method uses a hypothetical single soil layer to account for changes in moisture content. This method allows for continuous simulation when used in combination with a potential evapotranspiration time series. Between precipitation events, the soil layer will lose moisture due to evapotranspiration. Unless a potential evapotranspiration time series is supplied, no soil water extraction will occur. If the moisture deficit is greater than zero, precipitation will infiltrate into the soil layer. Until the moisture deficit has been satisfied, no percolation out of the bottom of the soil layer will occur. After the moisture deficit has been satisfied, the rate of infiltration into the soil layer is defined by the constant rate. The percolation rate out of the bottom of the soil layer is also defined by the constant rate while the soil layer remains saturated. Percolation stops as soon as the soil layer drops below saturation (moisture deficit greater than zero). Moisture deficit increases in response to evapotranspiration. The input parameters for the Deficit-Constant method are:

Maximum deficit (scalar value for each cell) [in or mm]
Initial deficit (scalar value for each cell) [in or mm]
Potential evapotranspiration (scalar time-series for each cell) [in/hr or mm/hr]
Potential infiltration rate (scalar value for each cell) [in/hr or mm/hr]

Table 2-4 provides the SCS soil groups and typical potential infiltration rates for the Deficit-Constant method.

Table 2-4. SCS Soil Groups and filtration rates (SCS, 1986; Skaggs and Khaleel 1982).

SCS Soil Group	Description	Range of Loss Rates (in/hr)
A	Deep sand, deep loess, aggregated silts	0.3 - 0.45
B	Shallow loess, sandy lam	0.15 - 0.30
C	Clay loams, shallow sandy loam, soils low in organic content, and soils usually high in clay	0.05 - 0.15
D	Soils that swell significantly when wet, heavy plastic clays, and certain saline soils	0.00 – 0.05

The Deficit and Constant model outputs the following variables:

Cumulative excess (per cell) [in or mm]
Cumulative precipitation (per cell) [in or mm]
Infiltration rate (per cell) [in/hr or mm/hr]
Infiltration depth (per cell) [in or mm]
Soil moisture deficit (per cell) [in or mm]
Percolation rate (per cell) [in/hr or mm/hr]
Percolation depth (per cell) [in/hr or mm/hr]
Evapotranspiration rate (per cell) [in/hr or mm/hr]

Green-Ampt

Green and Ampt (1911) presented a physics-based approach to computing soil infiltration.

For the first implementation of Green-Ampt in HEC-RAS, a simple approach will be used for recovery. The approach assumes that the upper layer of the soil remains saturated even during a hiatus period and solves a simple water balance equation to compute the cumulative infiltration depth. Although simplified, the approach has several important characteristics. Specifically, it is volume conservative and represents soil infiltration, evapotranspiration, and unsaturated gravity driven flow.

The input data required for the Green-Ampt (GA) method are (Table 2-5):

Wetting front suction (scalar value for each cell) [in or mm]
Saturated hydraulic conductivity (scalar value for each cell) [in/hr or mm/hr]
Initial soil water content (scalar value for each cell) [-]
Saturated soil water content (scalar value for each cell) [-]
Soil Potential Evapotranspiration (time series for each cell) [in/hr or mm/hr]

The initial water content should be set by the user based upon the antecedent moisture condition of the soils. The Soil Field Capacity is the amount of water content remaining after the excess water has been drained by gravity. Technically, the soil field capacity is the water remaining in the soil after being subjected to an atmospheric pressure of -0.33 bar (or -4.8 psi). The wilting point is the amount of water that is so tightly bound to soil particles that is cannot be evaporated or transpired. Technically, the wilting point is the water remaining in the soil after being subjected to an atmospheric pressure of -15 bar (or -217.5 psi). The wilting point must be less than the field capacity.

To utilize the Green-Ampt with Redistribution (GAR) the following additional input parameters are required in HEC-RAS (Table 2-5):

Residual soil water content (scalar value for each cell) [in or cm]
Pore-size distribution index (scalar value for each cell) [in or cm]

Table 2-5. Green-Ampt Parameter Estimates and Ranges based on Soil Texture (from Gowdish and Muñoz-Carpena 2009; Rawls and Brakensiek 1982 and Rawls et al. 1982).

Soil Texture	Residual Water Content	Wilting Point	Field Capacity	Total Porosity	Pore-size Distribution Index	Saturated Hydraulic Conductivity (mm/hr)	Wetting Front Suction(mm)
Sand	0.02	0.033	0.048	0.437	0.694	210 – 235.6	96.2 – 106
Loamy sand	0.035	0.055	0.084	0.437	0.553	59.8 – 61.1	119.6 – 142
Sandy loam	0.041	0.095	0.155	0.453	0.378	21.8 – 25.9	215.3 – 222
Loam	0.027	0.117	0.20	0.463	0.252	13.2	175.0 – 315
Silt loam	0.015	0.133	0.261	0.501	0.234	6.8	329.6 – 404
Sandy clay loam	0.068	0.148	0.187	0.398	0.319	3.0 – 4.3	449 – 538.3
Clay loam	0.075	0.197	0.245	0.464	0.242	2.0 – 2.3	408.9 – 446
Silty clay loam	0.040	0.208	0.30	0.471	0.177	1.5 – 2.0	538.3 – 581
Sandy clay	0.109	0.239	0.232	0.430	0.223	1.2	466.5 – 636
Silty clay	0.056	0.250	0.317	0.479	0.150	0.9 – 1.0	577.7 – 647
Clay	0.09.	0.272	0.296	0.475	0.165	0.6	622.5 – 714

Note on Parameter Estimation

The values presented here are meant as initial estimates. This is the same for all sources of similar data including Engineer Manual 1110-2-1417 Flood-Runoff Analysis and the Introduction to Loss Rate Tutorials. Regardless of the source, these initial estimates must be calibrated and validated.