Double Counting Impervious Areas

In SCS Curve Number hydrology, double counting occurs when the same impervious surface is represented twice in a subbasin: once inside the curve number selected by the user and again as a separate percent impervious value. The model has no way of knowing that the rooftops, roads, and sidewalks have already been baked into the CN, so it adds an additional impervious calculation on top of it. The result is consistently overestimated peak flows, oversized stormwater infrastructure, and overly conservative floodplain mapping.

This article explains how to avoid double counting impervious areas during hydrologic analysis, ensuring that impervious surfaces are represented only once.

Why Double Counting Matters

Engineers rely on hydrologic models to size storm sewers, culverts, detention basins, channels, and floodplain limits. Each of those design decisions is sensitive to peak discharge. If imperviousness is counted twice, the modeled peak grows for reasons that have nothing to do with the watershed; the impact compounds as the design moves downstream.

Common consequences of double counting include:

• Inflated peak discharges for the design storm, leading to oversized inlets, pipes, and culverts.
• Larger detention requirements than the project necessitates, increasing land use requirements and construction costs.
• Overly conservative floodplain limits that can affect insurance ratings, lot layouts, and permitting.
• Calibration drift where the user attempts to match observed flows by tweaking unrelated parameters such as time of concentration or initial abstraction, masking the underlying input error.

Curve Number and Impervious Surface

The SCS curve number (CN) is a dimensionless index that estimates direct runoff from a rainfall depth based on land use, soil type, and antecedent moisture conditions. Standard CN tables, such as those in the NRCS TR-55 manual and NEH-630 Chapter 9 already include the effect of typical impervious cover for many urban and developed land use categories.

The impervious surface percentage defines the fraction of a subbasin where rainfall does not infiltrate, such as roofs, paved roads, parking lots, and sidewalks. When entered separately, it tells the model to compute runoff from that fraction of the subbasin using a fully impervious response and to combine it with the runoff from the pervious portion.

Both inputs describe imperviousness, but they describe it in two different ways. The trouble starts when both are used to describe the same impervious cover at the same time.

Where This Issue Can Occur

When the user models runoff with the SCS Curve Number method, both the curve number and the impervious surface percentage are entered together in the Subbasin Data dialog box. The dialog box accepts both inputs without warning, which is why it is up to the user to make sure they are consistent. Refer to this article in our knowledge base to learn more about the Subbasin Data command.

How to Avoid Double Counting

The fix is to make sure imperviousness is represented in exactly one place in each subbasin. There are two reliable ways to do this. Both produce technically correct results. The right choice depends on how the curve number was derived.

Method 1 — Use a Pre-Weighted Curve Number

If the selected CN value already includes the effect of impervious cover for the land use (for example, a value taken from a standard table for “1/4-acre urban residential” or “commercial and business areas”), then the Impervious surface percentage in the Subbasin Data dialog box should be set to 0%. The CN already accounts for the imperviousness; entering it again would double count it.

The standard NRCS TR-55 table shown below is a good example of pre-weighted values. Each developed land use has an “average percent impervious area” included in the listed CN.

Method 2 — Separate Pervious CN with Explicit Impervious Percentage

If the CN represents only the pervious portion of the subbasin (for example, a value for open space, meadow, woods, or row crops), then enter the actual impervious percentage in the Impervious surface entry field. The software then computes runoff from the pervious area using the entered CN, computes runoff from the impervious area using a fully impervious response and combines the two contributions to give total subbasin runoff.

This method is useful when:

• The user has measured impervious cover from aerial mapping, GIS data, or CivilGEO’s built-in imperviousness coverage and wants the model to reflect the actual existing site conditions.
• The user is evaluating future build-out where the impervious fraction will change but the underlying pervious land use will not.
• The user is comparing existing vs proposed scenarios and wants imperviousness to be the single variable that changes between runs.

Quick Decision Guide

Use the table below as a quick check before running the model. The goal is to make sure the CN value and the impervious surface input are consistent and do not represent the same impervious area twice.

Double Counting Example Scenario — New Residential Subdivision

A user is sizing a detention basin for a proposed residential subdivision on Hydrologic Soil Group C. The subdivision has roughly 30% impervious cover from streets, driveways, rooftops, and sidewalks. The subbasin is set up with the SCS Curve Number loss method.

Incorrect Setup — Double Counted

The user picks the CN for “1/4-acre urban residential” from a standard table, which already accounts for typical urban imperviousness, and enters an additional 30% impervious value. The same imperviousness is now represented twice. The computed peak discharge is artificially inflated, and the detention basin is sized for a flow that does not exist on the actual site.

Correct Method 1 — Pre-Weighted CN

The user picks the CN for “1/4-acre urban residential” and sets the impervious value to 0%. Imperviousness is represented once inside the CN, and the computed peak reflects the developed condition consistent with the CN table.

Correct Method 2 — Pervious CN with Explicit Imperviousness

The user picks a pervious-only CN (for example, “open space, good condition” on HSG C) and enters 30% in impervious value. The software computes runoff from the 70% pervious portion using the CN and from the 30% impervious portion as fully impervious, then combines the two. Imperviousness is again represented once.

Conclusion

Double counting impervious areas is one of the most common sources of overestimated runoff in SCS Curve Number modeling. Before computing runoff for a subbasin, the user should confirm one of two things: either the curve number already represents the developed land use and the impervious surface field is set to 0%, or the curve number represents only the pervious land cover, and the impervious surface field carries the actual percent imperviousness.

When impervious cover is represented correctly, the model produces peak discharges that are consistent with the source data, detention sizing that reflects the real site, and floodplain limits that support agency review.