Curve numbers provide a practical way to estimate how much rainfall becomes runoff from a watershed or subbasin. In urban areas, the calculation deserves extra attention because rooftops, streets, parking lots, sidewalks, compacted soils, and storm drainage systems can quickly change how rainfall moves across a site.
Quick summary
A lower curve number generally means more rainfall can infiltrate into the ground. A higher curve number generally means more rainfall becomes surface runoff. Urban impervious areas usually push the curve number higher, especially when runoff is directly connected to a storm sewer, gutter, channel, or other drainage system.
What is a Curve Number?
A curve number, commonly abbreviated as CN, is an empirical value used in hydrology to estimate direct runoff from rainfall. It ties together three major site characteristics: land use or land cover, the underlying hydrologic soil group, and the moisture or runoff condition assumed for the storm event. In most stormwater applications, CN values are treated as practical runoff indicators: the higher the CN, the greater the expected runoff potential.
The curve number method was developed by the USDA Soil Conservation Service, now the Natural Resources Conservation Service (NRCS), and became widely used because it gave engineers a practical way to estimate runoff from storm events using information that is usually available for a project: soils, cover, and drainage conditions. TR-55, first issued by the Soil Conservation Service in 1975 and revised in 1986, helped make the method especially familiar for small and urbanizing watersheds.
In simple terms, the curve number is not a randomly selected modeling input. It is looked up or computed from the combination of what is on the ground surface and what is underneath it. A paved parking lot on poorly draining soil will produce a much higher CN than a forested area on sandy soil. That is why the same rainfall depth can produce very different runoff results from two nearby areas.
Why Urban Areas Need Special Treatment
Urbanization changes the natural water cycle by replacing absorbent ground with surfaces that do not allow much infiltration. Roads, roofs, parking lots, driveways, sidewalks, and compacted soils reduce infiltration and deliver runoff more rapidly to streams or drainage infrastructure. EPA summarizes this as a common hydrologic effect of urbanization: decreased infiltration, increased surface runoff, and faster delivery of runoff through storm drainage systems.
This is why curve numbers are so important in stormwater modeling. A site may look simple in plan view, but hydrologically it can behave very differently depending on whether runoff is allowed to spread across grassed areas or is routed directly into a storm drain. Recent urban hydrology research summarized by the USGS notes that roughly 90 percent of rainfall on impervious surfaces and drainage infrastructure can become runoff, which helps explain why developed watersheds can experience increased flood risk, water quality impacts, and reduced groundwater recharge.
Important modeling point
Not all impervious areas behave the same. A roof downspout connected directly to a storm sewer is hydrologically different from a roof downspout that discharges across a lawn, where some infiltration and flow spreading can occur.
Data Used to Determine Curve Numbers
To determine a curve number, the software needs two main pieces of information: the land use or land cover and the hydrologic soil group. These two datasets are combined spatially, and the appropriate CN value is selected from standard curve number relationships for each land use and soil group combination.
For example, a grassy open space on a well-drained soil will usually have a lower CN than that same grassy area on a poorly drained soil. If the area is paved, the CN will be higher still because most of the rainfall becomes runoff. This is the central idea behind CN lookup tables: each land cover and soil group combination represents a different runoff response.
| Input | What It Represents | Why It Matters |
|---|---|---|
| Land use or land cover | Describes what is on the ground surface, such as open space, forest, pasture, rooftops, roads, parking lots, residential development, commercial development, wetlands, or agricultural land. | Different surfaces produce different runoff responses. Pavement and rooftops generate more runoff than vegetated or wooded areas. |
| Hydrologic soil group | Describes how readily water can move through the soil. Soils are commonly grouped as A, B, C, D, or dual groups such as A/D, B/D, and C/D. | A soil that drains quickly generally produces less runoff than a soil that drains slowly. |
| Drainage connection | Describes whether impervious runoff goes directly to the drainage system or is routed across a pervious area first. | Connected impervious area generally produces more runoff than unconnected impervious area. |
Understanding Hydrologic Soil Groups
The NRCS classifies soils into hydrologic soil groups based on how easily water can move through the soil when it is wet. These groups have a direct impact on curve numbers because they describe the infiltration side of the runoff equation.
A helpful way to think about the progression is this: Group A behaves more like sand, where water can soak in quickly. Group D behaves more like clay or a shallow restrictive soil, where water is much more likely to remain near the surface and become runoff. Groups B and C fall between those two conditions.
| Soil Group | General Runoff Response | Plain-English Description |
|---|---|---|
| A | High infiltration / low runoff potential | Sandy or gravelly soils that transmit water freely. These soils tend to absorb rainfall quickly. |
| B | Moderately low runoff potential | Soils with moderate infiltration, often including sandy loams or silt loams. They generally drain better than Group C soils. |
| C | Moderately high runoff potential | Soils where water movement is somewhat restricted. These often contain more clay and less sand. |
| D | High runoff potential | Soils with very slow water movement, high clay content, shallow restrictive layers, or a high water table. These soils tend to produce the most runoff. |
Dual Hydrologic Soil Groups: A/D, B/D, and C/D
Some soils are assigned dual hydrologic soil groups, such as A/D, B/D, or C/D. These are usually wet soils that behave like Group D soils in their natural, undrained condition because of a shallow seasonal water table. If those soils are adequately drained, they may behave like the first letter in the dual classification.
For example, an A/D soil can behave like Group A if it is adequately drained, but like Group D if it remains undrained. The first letter represents the drained condition, and the second letter represents the undrained condition. For this reason, it is important to confirm whether the project condition being modeled reflects drained or undrained soil behavior.
Connected and Unconnected Impervious Areas
In urban watersheds, it is not enough to know how much impervious area exists. It also matters how that impervious area is connected to the drainage system. TR-55 specifically identifies the percentage of impervious area and the means of conveying runoff from impervious areas to the drainage system as important factors in computing urban curve numbers.
Connected Impervious Areas
An impervious area is considered connected when runoff flows directly into a drainage system. This can include runoff flowing into a curb and gutter, storm sewer inlet, channel, pipe, ditch, or another defined drainage path. TR-55 also treats impervious runoff as connected when it travels as concentrated shallow flow across a pervious area and then enters the drainage system.
Urban CN tables were developed for typical land use relationships using assumed percentages of impervious area. They also assume that pervious urban areas behave like pasture in good hydrologic condition and that impervious areas have a CN of 98 when directly connected to the drainage system. If those assumptions do not match the project site, a composite CN should be computed instead of relying only on the default table value.
Unconnected Impervious Areas
An impervious area is considered unconnected when its runoff is spread over a pervious surface as sheet flow before entering the drainage system. This allows part of the runoff to slow down, infiltrate, or be filtered by vegetation before it reaches a gutter, inlet, or channel.
TR-55 provides a separate procedure for an unconnected impervious area when the total impervious area is less than 30 percent. In that situation, the composite CN accounts for both the total impervious area and the ratio of unconnected impervious area to total impervious area. When the total impervious area is 30 percent or greater, TR-55 recommends using the connected impervious area relationship because the remaining pervious area is not expected to significantly reduce runoff.
The chart below shows the unconnected impervious area composite CN procedure used when the total impervious area is less than 30 percent.
Why This Distinction Matters
The difference between connected and unconnected impervious areas can affect the computed CN. TR-55 gives a simple example: a half-acre residential lot with 20 percent total impervious area and a pervious CN of 61 produces a composite CN of 68 if all impervious area is connected, but a lower composite CN of 66 when 75 percent of the impervious area is unconnected.
Rooftop Disconnection
Rooftop disconnection is a common example of an unconnected impervious area. Instead of directing a roof leader or downspout directly into a storm sewer, the discharge is routed onto a vegetated area. This gives runoff a chance to spread out, slow down, and infiltrate before reaching the drainage system.
When rooftop disconnection is allowed by the reviewing agency, it can reduce the amount of directly connected impervious area used in the composite CN calculation. However, this credit should not be applied automatically. Local plumbing codes, stormwater ordinances, separation distances, soil limitations, and grading requirements should always be checked before applying a rooftop disconnection credit.
Local requirements reminder
Rooftop disconnection requirements vary by jurisdiction. Before applying a disconnection credit, confirm that the proposed discharge location, flow path length, slope, soil conditions, and setback requirements comply with the governing stormwater and plumbing requirements.
Common Rooftop Disconnection Checks
The following checks are commonly seen in local stormwater guidance. They should be treated as examples only; always follow the requirements of the governing jurisdiction.
- The contributing roof area to each disconnected discharge point is limited, often to 500 square feet or less.
- The receiving pervious area is not a hydrologic soil group D, or an equivalent very poorly draining soil.
- The pervious flow path has a gentle slope, commonly 5 percent or less.
- The flow path is continuous, vegetated, and free of intervening impervious surfaces.
- The flow path remains separated from ground-level impervious areas, such as driveways and walkways, where required by local guidance.
- Each discharge point has adequate energy dissipation or spreading, so the flow remains sheet flow instead of causing erosion or concentrated flow.
Example Partial Rooftop Disconnection Credit
Some local stormwater ordinances use a partial-credit relationship based on the length of the pervious flow path. The example below is representative of one commonly used approach and should be checked against local requirements before use.
| Length of Pervious Flow Path (feet) | Roof Area Treated as Disconnected (% of contributing roof area) |
|---|---|
| 0 – 14 | 0 |
| 15 – 29 | 20 |
| 30 – 44 | 40 |
| 45 – 59 | 60 |
| 60 – 74 | 80 |
| 75 or more | 100 |
As an example, consider a 1,000 square foot roof served by two roof leaders, with each leader draining 500 square feet to a lawn area. If the receiving area has a hydrologic soil group B, a slope of 3 percent, and a 65 foot pervious flow path before reaching the street, the table above would treat 80 percent of the contributing roof area as disconnected under that example standard. The remaining connected portion would still be treated as directly connected impervious area.
Automated Curve Number Computation in CivilGEO Software
CivilGEO software can automate much of the curve number computation by combining land use or land cover data with hydrologic soil type data. The software evaluates each selected subbasin, overlays the land cover and soil group datasets, and computes the curve number based on the resulting land use and soil group combinations.
For United States projects, this workflow commonly uses online land cover data, such as NLCD, together with NRCS soil survey data. For projects outside the United States, available land cover and soil datasets may vary by country or region. Users can also bring in their own GIS polygon layers when more detailed local land use or soil data is available.
For site-specific projects, user-supplied GIS polygon data is often the best approach because it can represent precise boundaries for rooftops, parking lots, roadways, landscaped areas, open space, and other important site features. Pixel-based land cover datasets can work well for regional studies, but they may not capture enough detail for smaller urban sites.
Software workflow note
For detailed or small urban projects, consider using project-specific GIS polygon layers for land use and soil data when available. This helps the computed CN reflect the actual site layout rather than relying only on generalized regional datasets.
In CivilGEO software, the Compute CN command can be used to automatically compute the curve number based on the underlying land use and hydrologic soil type data. To learn more about the Compute CN command, refer to this article in our knowledge base.
Intermediate Results and QA Review
A useful quality-check step is to review the intermediate land use and soil group combinations used by the software. When intermediate results are generated, each unique land use and soil group combination can be represented as a separate GIS polygon. This helps the modeler verify which land cover types and soil groups are driving the final composite curve number for each subbasin.
This is especially helpful for urban projects where small areas can have a large hydrologic effect. For example, a parking lot on Group C soil, a lawn on Group B soil, and a rooftop routed directly to a storm sewer may all exist in the same subbasin. Reviewing the intermediate results makes it easier to confirm that the final CN reflects the intended spatial data and assumptions.
Quality-check tip
Before finalizing the model, review whether the computed CN values make sense for the project area. If a subbasin has a surprisingly high or low CN, check the land use boundaries, hydrologic soil groups, and impervious area connection assumptions.
Practical Modeling Guidance
Curve numbers are simple enough to use efficiently, but they still require engineering judgment. The following checks can help avoid common mistakes:
- Confirm that land use categories match the project condition being modeled, such as existing conditions, proposed conditions, or post-development conditions.
- Use site-specific GIS land use polygons when a project requires more detail than an online land cover dataset can provide.
- Verify hydrologic soil groups, especially where soils are disturbed, compacted, filled, drained, or mapped as dual hydrologic soil groups.
- Do not treat all impervious areas the same. Determine whether impervious runoff is connected or unconnected to the drainage system.
- Apply rooftop disconnection credits only when the site meets the governing jurisdiction’s criteria and local code requirements.
- Review intermediate results when available to confirm that the spatial overlay and computed CN values make sense.
Conclusion
The curve number method remains popular because it is practical, familiar, and able to translate land cover, soil behavior, and drainage assumptions into a single runoff parameter. In urban watersheds, the most important modeling step is often not just identifying how much impervious area exists, but understanding how that impervious area drains. Connected impervious areas typically produce more runoff, while properly unconnected areas may allow some infiltration and runoff reduction. By combining accurate land use data, reliable hydrologic soil group data, and careful review of connected versus unconnected impervious areas, the computed CN values can provide a more defensible basis for stormwater modeling and design.