Rational Method vs. SCS/NRCS Method: Which Should You Use?

Choosing between the Rational Method and the SCS/NRCS Curve Number Method is one of the most fundamental decisions in stormwater design. Both are widely accepted, but each excels in different situations. This guide helps you make the right choice for your project.

Quick Decision Matrix

Understanding the Fundamental Difference

Rational Method: Peak Flow Focus

The Rational Method calculates the peak discharge that occurs when a storm lasts exactly as long as the time of concentration:

Where:

• Q = Peak flow (cfs)
• C = Runoff coefficient
• i = Rainfall intensity (in/hr)
• A = Drainage area (acres)

What it tells you: The maximum flow rate you need to convey.

What it doesn’t tell you: Total runoff volume, hydrograph shape, or timing.

SCS/NRCS Method: Volume and Peak

The SCS method first calculates runoff depth (volume per unit area), then develops a hydrograph:

Where:

• Qr = Runoff depth (inches)
• P = Precipitation depth (inches)
• Ia = Initial abstraction (typically 0.2S)
• S = Potential maximum retention

The peak flow comes from the SCS dimensionless unit hydrograph:

What it tells you: Total runoff volume AND peak flow rate.

What it enables: Detention pond sizing, water quality volume calculations, hydrograph analysis.

When to Use the Rational Method

Ideal Applications

1. Storm sewer pipe sizing - Direct calculation of required capacity
2. Inlet sizing - Determining how much flow an inlet must capture
3. Small urban drainage - Parking lots, residential subdivisions
4. Preliminary estimates - Quick checks before detailed analysis
5. Local code requirement - Many jurisdictions specify Rational for minor drainage

Advantages

• Simplicity: One equation, no hydrograph routing
• Minimal data: Need C, i, and A only
• Fast: Manual calculations possible
• Conservative: Often gives slightly higher peaks than SCS
• Universal acceptance: Every reviewer knows it

Limitations

• No volume: Cannot size detention without modifications
• Small areas only: Accuracy decreases above 200 acres
• Single peak: Cannot analyze multiple sub-areas with different timing
• No routing: Cannot account for storage or attenuation

When to Use the SCS/NRCS Method

Ideal Applications

1. Detention pond sizing - Requires volume and peak
2. Retention/infiltration design - Needs total volume
3. BMP sizing - Water quality volume calculations
4. Larger watersheds - Up to 2,000 acres
5. Pre vs. post-development - Volume comparisons
6. Regulatory compliance - Volume-based regulations

Advantages

• Volume calculation: Critical for storage design
• Hydrograph output: Enables routing analysis
• Larger scale: Works for watersheds up to 2,000 acres
• Pre/post comparison: Same method for both conditions
• Timing analysis: Can combine sub-watershed flows

Limitations

• More data required: Need CN values, storm distribution
• More complex: Requires understanding of hydrograph methods
• Assumptions: 24-hour storm, specific distributions
• Small area issues: Less accurate below 1 acre

Side-by-Side Comparison: Same Site, Both Methods

Example Site

• Drainage area: 10 acres
• Commercial development
• Post-developed condition
• 10-year design storm
• Location: Central U.S.

Rational Method Calculation

Inputs:

• C = 0.85 (composite for commercial)
• Tc = 12 minutes
• i = 5.5 in/hr (from IDF curve for Tc)

Calculation:

Result: Design pipes for 46.75 cfs

SCS Method Calculation

Inputs:

• CN = 90 (commercial)
• P = 4.5 inches (10-year, 24-hour)
• S = (1000/CN) - 10 = 1.11 inches
• Ia = 0.2 × S = 0.22 inches

Runoff Depth:

Volume:

Peak Flow (using Tc = 0.2 hours):

Results:

• Peak flow: 51.4 cfs
• Total volume: 123,420 cf (2.83 ac-ft)

Comparing the Results

Both methods give similar peak flows for this example, but only SCS provides the volume needed for detention design.

Hybrid Approaches

When to Combine Methods

Many projects use both methods strategically:

1. Rational for conveyance: Size storm sewer pipes using Rational Method
2. SCS for detention: Size detention facilities using SCS Method
3. Consistent parameters: Use comparable assumptions for both

Modified Rational Method

Some jurisdictions allow a “Modified Rational Method” for preliminary detention sizing:

Where:

• V = Required storage volume
• Q = Peak flow from Rational Method
• Td = Storm duration
• Cs = Storage coefficient

Regulatory Considerations

Local Requirements

Check your jurisdiction’s requirements carefully:

Common Jurisdictional Rules

• Texas: TxDOT allows Rational up to 200 acres
• California: Caltrans specifies Rational for small urban areas
• EPA: SWMM defaults to SCS methods for continuous simulation
• ASCE: Recommends Rational for areas ≤ 200 acres

Data Requirements Comparison

Rational Method Needs

SCS Method Needs

The SCS method requires more input data but provides more comprehensive output.

Common Mistakes to Avoid

Mistake 1: Using Rational Method for Detention

Problem: Rational gives peak flow only—no volume for pond sizing.

Solution: Use SCS method or route a hydrograph through your pond.

Mistake 2: Applying SCS to Tiny Areas

Problem: SCS unit hydrograph assumptions break down for very small watersheds.

Solution: For areas under 1-2 acres, the Rational Method may be more appropriate.

Mistake 3: Inconsistent Assumptions

Problem: Using different storms, return periods, or area calculations for the two methods.

Solution: Document assumptions clearly and maintain consistency.

Mistake 4: Ignoring Local Standards

Problem: Using the “wrong” method for your jurisdiction.

Solution: Obtain local drainage criteria before starting design.

Summary: Method Selection Flowchart

1. Is detention/retention required?

   • Yes → Use SCS Method
   • No → Continue

2. Is drainage area > 200 acres?

   • Yes → Use SCS Method
   • No → Continue

3. Do you need runoff volume?

   • Yes → Use SCS Method
   • No → Continue

4. Is this for pipe/inlet sizing?

   • Yes → Use Rational Method
   • No → Evaluate based on local requirements

5. What does your jurisdiction require?

   • Follow local standards first

Calculators for Both Methods

• Rational Method Calculator
• Time of Concentration Calculator
• SCS Curve Number Calculator
• SCS Triangular Hydrograph Calculator

References

1. Natural Resources Conservation Service. (2004). National Engineering Handbook, Part 630: Hydrology. U.S. Department of Agriculture.

2. Federal Highway Administration. (2013). Urban drainage design manual (3rd ed., Hydraulic Engineering Circular No. 22). U.S. Department of Transportation.

3. American Society of Civil Engineers. (2017). Design and construction of urban stormwater management systems (ASCE Manual of Practice No. 77). ASCE Press.

4. Chow, V. T., Maidment, D. R., & Mays, L. W. (1988). Applied hydrology. McGraw-Hill.

5. McCuen, R. H. (2016). Hydrologic analysis and design (4th ed.). Pearson.

6. Texas Department of Transportation. (2019). Hydraulic design manual. TxDOT.