Calculate Weighted Curve Number
Professional Hydrology & Stormwater Analysis Tool
Enter the area and Curve Number (CN) for each land use type in your drainage basin. The weighted CN is essential for estimating direct runoff from rainfall events.
| Land Use Description | Area (acres/sq ft) | Curve Number (0-100) |
|---|---|---|
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CN must be 0-100
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CN must be 0-100
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CN must be 0-100
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CN must be 0-100
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Chart: Contribution of each land use area to the total weighted CN value.
What is the Weighted Curve Number?
To calculate weighted curve number is a fundamental step in modern hydrology and civil engineering. The Runoff Curve Number (CN) is an empirical parameter used in hydrology for predicting direct runoff or infiltration from rainfall excess. The method was developed by the USDA Natural Resources Conservation Service (NRCS), formerly known as the Soil Conservation Service (SCS).
However, real-world drainage basins rarely consist of a single type of land use or soil. A watershed might contain a mix of dense forests, residential lawns, paved roads, and agricultural fields. Each of these surfaces absorbs and sheds water differently. The Weighted Curve Number is a composite index that represents the average runoff potential of the entire heterogeneous area.
Engineers, hydrologists, and urban planners use this calculation to design stormwater management systems, detention ponds, and culverts that can handle peak flow events effectively.
Formula and Mathematical Explanation
The calculation is a standard area-weighted average. The goal is to determine a single CN value that mathematically represents the aggregate hydrologic response of the catchment area.
Where:
- CNweighted: The final composite curve number for the basin.
- Areai: The surface area of a specific land use section (sub-area).
- CNi: The curve number assigned to that specific sub-area.
Variables Table
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| CN | Curve Number | Dimensionless | 30 (permeable) to 98 (impervious) |
| Area | Surface Area | Acres, sq ft, ha, m² | > 0 |
| Product | Weighted Contribution | Area-CN Units | Dependent on Area |
Practical Examples
Example 1: Small Residential Development
Consider a 10-acre development site proposed for a suburban area. The engineer breaks down the land use based on the site plan:
- 5 acres of residential lots (1/4 acre size) with a CN of 75.
- 3 acres of open green space (good condition) with a CN of 39.
- 2 acres of impervious streets and driveways with a CN of 98.
Step 1: Calculate Products
(5 × 75) = 375
(3 × 39) = 117
(2 × 98) = 196
Step 2: Sum totals
Total Area = 10 acres
Total Product = 688
Result: 688 / 10 = 68.8 CN. This value is then used to calculate expected runoff volume.
Example 2: Commercial Park Retrofit
A 50,000 sq ft commercial zone is being analyzed. It consists of 40,000 sq ft of roof and pavement (CN 98) and 10,000 sq ft of gravel borders (CN 76).
The weighted calculation would be:
((40,000 × 98) + (10,000 × 76)) / 50,000 = 93.6 CN.
This high CN indicates very high runoff potential, requiring significant stormwater detention.
How to Use This Weighted Curve Number Calculator
- Identify Sub-Areas: Divide your total catchment area into distinct sections based on land use (e.g., woods, grass, pavement).
- Assign CN Values: Look up the standard SCS Curve Number for each section based on its soil group (A, B, C, or D) and cover type.
- Enter Data: Input the description, area size, and CN for each section into the calculator rows above.
- Review Results: The tool automatically updates the Weighted CN. Use the "Copy Results" button to paste the data into your drainage report.
- Interpret the Graph: The bar chart visualizes which sub-areas are contributing most to the overall "imperviousness" or runoff factor of the site.
Key Factors That Affect Curve Number Results
When you calculate weighted curve number, several physical properties of the land influence the final figure. Understanding these helps in designing better site layouts to reduce runoff.
- Hydrologic Soil Group (HSG): Soils are classified into groups A (sand/low runoff), B, C, and D (clay/high runoff). Clay soils result in much higher CN values than sandy soils for the same vegetation.
- Land Cover Type: Vegetation intercepts rain. Woods and thick grass have lower CNs compared to fallow land or pavement.
- Cover Condition: "Good" condition (dense grass) promotes infiltration better than "Poor" condition (sparse grass), resulting in a lower CN.
- Impervious Surfaces: Concrete, asphalt, and roofs prevent water from entering the soil, typically assigned a CN of 98.
- Antecedent Moisture Condition (AMC): While the standard CN assumes average moisture (AMC II), wet soil (AMC III) will yield higher runoff, effectively increasing the CN.
- Urbanization: Compaction of soil during construction often shifts soil characteristics, effectively moving a generic soil group B closer to a group C or D behavior.
Frequently Asked Questions (FAQ)
Theoretically, CN ranges from 0 to 100. Practically, it ranges from about 30 (permeable soils with thick forest) to 98 (impervious surfaces like concrete). 100 would imply zero abstraction, behaving like a glass surface.
No. You must be consistent. If you use acres for one row, you must use acres for all rows. The final Weighted CN is dimensionless, so the area unit cancels out mathematically.
Generally, yes. A higher CN indicates lower infiltration and higher direct runoff. This leads to higher peak flows during storm events, increasing the risk of flooding if not managed.
Significantly. A residential lawn on sandy soil (Group A) might have a CN of 39, while the same lawn on clay soil (Group D) might have a CN of 80.
TR-55 is a comprehensive technical release by the USDA for urban hydrology. This calculator performs the specific weighted average computation found within the TR-55 methodology.
Water bodies are already saturated. Rain falling on a lake becomes immediate runoff (or rise in pool elevation), so it is treated as impervious (CN 98-100) in many models.
Yes. Even for a single residential lot, you can calculate the weighted CN by separating the roof/driveway area from the lawn area.
Yes, as long as the watershed is small enough that the lumped parameter assumption holds. For very large complex basins, distributed modeling might be required.
Related Tools and Internal Resources
Enhance your hydrology analysis with our suite of engineering calculators:
- Time of Concentration Calculator – Estimate how long it takes for water to travel through a watershed.
- Rational Method C Coefficient – Calculate the runoff coefficient for peak flow estimation.
- Stormwater Detention Volume – Determine the required storage for detention ponds.
- Manning's Equation Calculator – Compute flow velocity in open channels.
- Soil Infiltration Rate Estimator – Assess soil absorption capacity.
- Rainfall Intensity Duration Frequency – Lookup rainfall data for your region.