Through studies of many small agricultural watersheds the initial abstraction (I_a) was found to be approximated by the following equation:

I_a = 0.2S

Substituting the above relationship into the basic runoff equation yields:

It is important to note that P must be greater than 0.2S in order to produce runoff. If P is less than or equal to 0.2S then the runoff is equal to zero.

Removing the initial abstraction (I_a) allows for a combination of S and P to produce a unique runoff amount. The maximum potential retention (S) is related to the soil and cover conditions of the watershed through the curve number equation:

What is the initial abstraction?

The initial abstraction (I_a) is all losses before runoff begins. It includes water retained in surface depressions, water intercepted by vegetation, evaporation and infiltration. I_a is highly variable but generally is correlated with soil and cover parameters.

What criteria are important when selecting a curve number?

The major factors that determine the curve number (CN) are the hydrologic soil group (HSG), cover type, treatment, hydrologic condition and antecedent runoff condition (ARC). Another factor considered is whether impervious areas outlet directly to the drainage system (connected) or whether the flow spreads over pervious areas before entering the drainage system (unconnected).

Where can I look up a curve number?

The table of standard curve number for average antecedent runoff conditions can be found here.

Why do we use curve numbers instead of the maximum potential retention (S)?

The maximum potential retention (S) has the limiting values of 0 and ∞ while curve numbers range from 0 to 100. The finite nature and numerical range of curve numbers make this index more intuitive. The higher the curve number the higher the runoff amount.

What is a hydrologic soil group (HSG)?

The SCS (NRCS) divides soils into four classifications or hydrologic soil groups (A, B, C and D) according to their minimum infiltration rate, which is obtained for bare soil after prolonged wetting.

Group A soils have low runoff potential and high infiltration rates even when thoroughly wetted. They consist chiefly of deep, well to excessively drained sand or gravel and have a high rate of water transmission (greater than 0.30 in/hr).

Group B soils have moderate infiltration rates when thoroughly wetted and consist chiefly of moderately deep to deep, moderately well to well drained soils with moderately fine to moderately coarse textures. These soils have a moderate rate of water transmission (0.15-0.30 in/hr).

Group C soils have low infiltration rates when thoroughly wetted and consist chiefly of soils with a layer that impedes downward movement of water and soils with moderately fine to fine texture. These soils have a low rate of water transmission (0.05-0.15 in/hr).

Group D soils have high runoff potential. They have very low infiltration rates when thoroughly wetted and 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 material. These soils have a very low rate of water transmission (0-0.05 in/hr).

How do I determine the hydrologic soil group (HSG) for my project site?

A soil can be assigned to a hydrology soil group (HSG) using its texture classification:

HSG	Soil Texture
A B C D	Sand, loamy sand or sandy loam Silt loam or loam Sandy clay loam Clay loam, silty clay loam, sandy clay, silty clay or clay

A table relating the physical properties of soil, such as hydraulic conductivity, to soil texture can be found here.

Is the hydrologic soil group (HSG) important in urban areas?

Yes. Most urban areas are only partially covered by impervious surfaces, so the soil remains an important factor in runoff estimates. Urbanization has a greater effect on runoff in watersheds with soils having high infiltration rates (sands and gravels) than in watersheds predominantly of silts and clays, which generally have low infiltration rates.

Any disturbance of a soil profile can significantly change its infiltration characteristics. With urbanization, native soil profiles may be mixed or removed or fill material from other areas may be introduced.

What is hydrologic condition?

Hydrologic condition indicates the effects of cover type and treatment on infiltration and runoff and is generally estimated from density of plant and residue cover on sample areas. Hydrologic condition is one of the factors used when selecting a curve number.

Good hydrologic condition indicates that the soil usually has a low runoff potential for that specific hydrologic soil group, cover type and treatment. Poor hydrologic condition indicates that soil factors impair infiltration and increase runoff. Some factors to consider in estimating the effect of cover on infiltration and runoff are:

• Canopy or density of lawns, crops or other vegetative areas
• Amount of year-round cover
• Amount of grass or close-seeded legumes in rotations
• Percent of residue cover
• Degree of surface roughness

What is the antecedent runoff condition (ARC)?

The index of runoff potential before a storm event is the antecedent runoff condition (ARC). This index is an attempt to account for the variation in a curve number at a site from storm to storm. This antecedent runoff condition is mainly a factor of soil moisture. It is important to remember that curve numbers are usually reported for average antecedent runoff conditions since they are intended for use in design applications.

Can a curve number from the standard table be applied to my project?

The curve numbers listed in the standard table are for average antecedent runoff conditions. While they can be applied directly to homogenous land areas, it may be necessary to modify the standard curve number for impervious land cover in urban areas.
For non-homogeneous areas it is necessary to calculated a weighted curve number.

How do I calculate a weighted curve number?

A weighted curve number is a weighted average based upon the area of each land cover type. Each curve number is multiplied by the area of its respective land cover type. These products are summed then divided by the total area.

For example, a pasture in good hydrologic condition over a B soil has a curve number of 61 for a 30 acre area. A pasture in good hydrologic condition over a C soil has a curve number of 74 for a 70 acre area. The weighted curve number is determined as follows:

How do I modify the standard curve number for impervious land in an urban area?

Several factors, such as the percentage of impervious area and the means of conveying runoff from impervious areas to the drainage system, should be considered in computing the curve number for urban areas. For example, do the impervious areas connect directly to the drainage system, or do they outlet onto lawns or other pervious areas where infiltration can occur?

An impervious area is considered connected if runoff from it flows directly into the drainage system. It is also considered connected if runoff from it occurs as concentrated shallow flow that runs over a pervious area and then into the drainage system.

Urban curve numbers presented in standard tables were developed for typical land use relationships based on specific assumed percentages of impervious area. These curve number values were developed on the assumptions that pervious urban areas are equivalent to pasture in good hydrologic condition and impervious areas have a curve number of 98 and are directly connected to the drainage system. If the impervious area is directly connected to the drainage system and the above assumptions hold true then the standard curve numbers should be used. If not a composite curve number must be calculated.

If all of the impervious area is directly connected to the drainage system, but the impervious area percentages or the pervious land use assumptions in the standard table are not applicable then the figure below can be used to compute a composite curve number.

For example, the standard table gives a curve number of 70 for a 1/2-acre lot in HSG B, with an assumed impervious area of 25 percent. However, if the lot has 20 percent impervious area and a pervious area curve number of 61, the composite curve number obtained from the figure below is 68.

Composite curve number with impervious area (TR-55)

For unconnected impervious areas runoff is spread over a pervious area as sheet flow. To determine the curve number when all or part of the impervious area is not directly connected to the drainage system use the figure below if total impervious area is less than 30 percent. If the total impervious area is equal to or greater than 30 percent use the above figure to calculate the composite curve number.

Composite curve number with unconnected impervious areas and
total impervious area less than 30% (TR-55)

When the impervious area is less than 30 percent, obtain the composite curve number by entering the right half of the figure above with the percentage of total impervious area and the ratio of total unconnected impervious area to total impervious area. Then move left to the appropriate pervious curve number and read down to find the composite curve number.

For example, for a 1/2-acre lot with 20 percent total impervious area (75 percent of which is unconnected) and a pervious curve number of 61, the composite curve number from the figure above is 66. If all of the impervious area is connected, the resulting curve number would be 68.

Can I use the curve number method with historical rainfall data?

Curve numbers describe average conditions that are useful for design purposes. If the rainfall event used is a historical storm, the modeling accuracy decreases. The procedures are primarily for establishing safe limits in design, and for comparing the effectiveness of alternative systems of measures within a watershed project. They are not used to recreate specific features of an actual storm.

Use the runoff curve number equation with caution when re-creating specific features of an actual storm. The equation does not contain an expression for time and, therefore, does not account for rainfall duration or intensity.

What are the limitations of the curve number approach for runoff estimation?

The following points are important to consider when utilizing the curve number method:

• The user should understand the assumption reflected in the initial abstraction term (I_a) and should ascertain that the assumption applies to the situation. I_a, which consists of interception, initial infiltration, surface depression storage, evapotranspiration and other factors, was generalized as 0.2S based on data from agricultural watersheds (S is the potential maximum retention after runoff begins). This approximation can be especially important in an urban application because the combination of impervious areas with pervious areas can imply a significant initial loss that may not take place. The opposite effect, a greater initial loss, can occur if the impervious areas have surface depressions that store some runoff.
  
• Runoff from snowmelt or rain on frozen ground cannot be estimated using these procedures.
• The curve number procedure is less accurate when runoff is less than 0.5 inch. As a check, use another procedure to determine runoff.
• The SCS runoff procedures apply only to direct surface runoff: do not overlook large sources of subsurface flow or high ground water levels that contribute to runoff. These conditions are often related to HSG A soils and forest areas that have been assigned relatively low curve numbers in the standard table. Good judgment and experience based on stream gage records are needed to adjust curve numbers as conditions warrant.
• When the weighted curve number is less than 40, use another procedure to determine runoff.

Why is time not considered in the curve number approach?

Time was not incorporated in the method for estimating runoff for two important, practical reasons. First, sufficient reliable data were not available to define curves of infiltration capacity versus time for wide range in soil, land use and cover conditions.

Second, if time had been incorporated in the method, it would have required a determination of the time distribution of rainfall in storms for which runoff was to be estimated. In a majority of cases, rainfall records on watersheds with which we deal do not permit reliable determination of the time distribution of individual storms.

Does StormNET implement NRCS (SCS) methods such as TR-55?

Yes. With StormNET you can quickly implement either TR-55 or TR-20.

What is TR-55?

Technical Release 55 (TR-55) is one of the most commonly used design methods in the United States for the management of storm water runoff in urban settings. Originally published in 1975 by the Soil Conservation Service (SCS), TR-55 presents procedures for the calculation of storm runoff volume, peak rate of discharge, hydrographs and detention pond storage volumes for small watersheds. The Natural Resources Conservation Service (NRCS), formally known as the SCS, published a revised version of TR-55 in 1986. The methods in TR-55 are also described in the hydrology section of the National Engineering Handbook which was also compiled by the SCS (NRCS).

Where can I get TR-55?

The original TR-55 documentation and the MS-DOS version can be found on the NRCS TR-55 webpage. The WinTR-55 program, documentation and tutorials can be downloaded from the NRCS WinTR-55 webpage. Links to all NRCS hydraulics and hydrologic software can be found here.

Where can I find additional information about the runoff curve number method?

The following documents provide more detailed information about the SCS (NRCS) TR-55 method and curve numbers:

Runoff Curve Number Method – Beyond the Handbook
Runoff Curve Number Method – Examination of the Initial Abstraction Ratio
Runoff Curve Number Method – Origins, Applications and Limitations