Understanding the SCS Runoff Method: A Guide for Stormwater Designers

Hello stormwater designers, and welcome back to Clear Creek Solutions’ Hydrology Education Series. In today’s article, we’ll be diving into the SCS Runoff Method—a key tool in the hydrology toolbox for estimating stormwater runoff.

We’ll break down the essential equations, explore curve numbers, and walk through sample problems so you can apply this method confidently in your own designs.

What Is the SCS Runoff Method?

While the Rational Method is one of the most commonly used approaches for estimating peak runoff, it’s not the only one. The SCS (Soil Conservation Service) Runoff Method, also known as the NRCS Method, provides an alternative, especially useful for calculating runoff volume and sizing detention or retention facilities.

This method takes into account soil type, land use, and rainfall to estimate runoff from a given site.

Key Concepts and Variables

The SCS Runoff Method includes the following core components:

• P: Rainfall (in inches)

• Q: Runoff depth (in inches)

• S: Maximum potential retention after runoff begins (in inches)

• Iₐ: Initial abstraction (inches)—this includes water lost to infiltration, evaporation, and surface storage before runoff begins.

These variables allow us to model how rainfall interacts with the land surface, particularly how much water becomes runoff.

Core Equations of the SCS Runoff Method

Here are the primary equations you'll use:

1. Runoff Equation:

   Q=(P−Ia)2(P−Ia+S)Q = \frac{(P - Iₐ)^2}{(P - Iₐ + S)}Q=(P−Ia​+S)(P−Ia​)2​

2. Maximum Retention (S):

   S=1000CN−10S = \frac{1000}{CN} - 10S=CN1000​−10

   Where CN is the Curve Number.

3. Initial Abstraction (Iₐ):

   Ia=0.2×SIₐ = 0.2 \times SIa​=0.2×S

These equations are derived empirically and used to model single-event hydrology—meaning they apply to individual storm events, not continuous simulations.

Understanding Curve Numbers (CN)

Curve Numbers are values assigned based on land use, ground cover, hydrologic condition, and soil group (A, B, C, or D). Higher curve numbers indicate more runoff potential.

Example Curve Numbers:

Land Use & CoverSoil Group ASoil Group BSoil Group CSoil Group DPasture, Poor Condition (<50% cover) Wood-Grass Combination, Good Condition

As you can see, poorer conditions or less permeable soils (like Group D) result in higher curve numbers, indicating greater runoff.

Sample Problem 1: Calculating Runoff Depth

Problem:
A basin has a curve number of 75 and receives 6 inches of precipitation. Calculate the runoff depth (Q).

Step-by-Step:

1. Calculate S:

   S=100075−10=3.33 inchesS = \frac{1000}{75} - 10 = 3.33 \text{ inches}S=751000​−10=3.33 inches

2. Calculate Iₐ:

   Ia=0.2×3.33=0.666Iₐ = 0.2 \times 3.33 = 0.666Ia​=0.2×3.33=0.666

3. Use the runoff equation:

   Q=(6−0.666)26−0.666+3.33=(5.334)29.664≈2.95 inchesQ = \frac{(6 - 0.666)^2}{6 - 0.666 + 3.33} = \frac{(5.334)^2}{9.664} \approx 2.95 \text{ inches}Q=6−0.666+3.33(6−0.666)2​=9.664(5.334)2​≈2.95 inches

Sample Problem 2: Finding Precipitation from Known Runoff

Problem:
A basin with poor meadow grass in Soil Group B produces 1.5 inches of runoff. Find the required precipitation.

Step-by-Step:

1. Determine CN:
   From the CN chart, poor meadow grass on Soil Group B = 67

2. Calculate S:

   S=100067−10=4.93S = \frac{1000}{67} - 10 = 4.93S=671000​−10=4.93

3. Calculate Iₐ:

   Ia=0.2×4.93=0.986Iₐ = 0.2 \times 4.93 = 0.986Ia​=0.2×4.93=0.986

4. Use the runoff equation:

   1.5=(P−0.986)2P−0.986+4.931.5 = \frac{(P - 0.986)^2}{P - 0.986 + 4.93}1.5=P−0.986+4.93(P−0.986)2​

   Solving this equation (quadratically), we find:

   P≈4.56 inchesP \approx 4.56 \text{ inches}P≈4.56 inches

Conclusion

The SCS Runoff Method is a powerful tool for stormwater modeling, particularly when you're estimating runoff volume for design of detention and retention systems. By understanding how to apply curve numbers and equations correctly, you can produce reliable runoff estimates based on real-world conditions.

If you’re curious about other hydrology methods, including continuous simulation (like those used in advanced stormwater models), download our Ultimate Hydrology Guide—available for free in the video description.

Don’t forget to like and subscribe to our channel for more helpful tutorials on hydrology and stormwater design. See you in the next video!