Hydrologic Soil Groups: A Complete Guide

Hydrologic Soil Groups (HSGs) classify soils based on their runoff potential and infiltration characteristics. This classification is essential for using the SCS Curve Number method and for designing infiltration-based stormwater systems.

What Are Hydrologic Soil Groups?

The NRCS assigns soils to one of four groups (A, B, C, or D) based on their minimum infiltration rate when thoroughly wetted. This classification indicates how quickly water can enter the soil and, consequently, how much runoff to expect.

The Four Soil Groups

Group A: High Infiltration, Low Runoff

Characteristics:

• Minimum infiltration rate: >0.30 inches/hour
• Deep, well-drained sands and gravels
• High saturated hydraulic conductivity
• Lowest runoff potential

Typical soils:

• Sand
• Loamy sand
• Sandy loam

Examples: Beach sand, outwash plains, well-drained alluvial soils

Group B: Moderate Infiltration, Moderate Runoff

Characteristics:

• Minimum infiltration rate: 0.15 - 0.30 inches/hour
• Moderately deep, moderately well-drained
• Moderate saturated hydraulic conductivity
• Moderate runoff potential

Typical soils:

• Silt loam
• Loam

Examples: Many agricultural soils in the Midwest, well-developed urban lawns

Group C: Low Infiltration, High Runoff

Characteristics:

• Minimum infiltration rate: 0.05 - 0.15 inches/hour
• Layer that impedes downward movement
• Low saturated hydraulic conductivity
• High runoff potential

Typical soils:

• Sandy clay loam
• Clay loam

Examples: Soils with fragipans, clay layers, or moderately fine textures

Group D: Very Low Infiltration, Highest Runoff

Characteristics:

• Minimum infiltration rate: <0.05 inches/hour
• Clay soils with high shrink-swell potential
• Soils with permanent high water table
• Very low saturated hydraulic conductivity
• Highest runoff potential

Typical soils:

• Clay
• Silty clay
• Sandy clay
• Soils with high water table

Examples: Heavy clays, poorly drained bottomlands, soils with shallow bedrock

Summary Table

Determining Soil Group

Method 1: NRCS Web Soil Survey (Preferred)

The most reliable source for HSG data is the NRCS Web Soil Survey:

1. Go to websoilsurvey.sc.egov.usda.gov
2. Navigate to your site location
3. Define your Area of Interest (AOI)
4. Go to “Soil Data Explorer” → “Soil Properties and Qualities”
5. Select “Hydrologic Soil Group”
6. Generate the map and report

Method 2: Soil Texture Method

If HSG data isn’t available, estimate from soil texture:

Method 3: Infiltration Testing

Direct infiltration testing provides site-specific data:

• Double-ring infiltrometer: Standard method for determining saturated infiltration rate
• Falling head permeameter: Laboratory test on soil samples
• Guelph permeameter: Field measurement of hydraulic conductivity

Dual Hydrologic Soil Groups

Some soils are assigned dual groups (A/D, B/D, or C/D) indicating:

• First letter: Drained condition
• Second letter: Undrained condition (high water table)

Understanding Dual Groups

Using Dual Groups

For natural conditions: Use the second letter (D)

For drained conditions: Use the first letter, but only if:

• Permanent drainage is installed
• Water table is lowered sufficiently
• System is maintained

Impact on Curve Numbers

HSG directly affects CN selection. The same land use can have dramatically different CNs:

For a 5-inch rainfall on open space in good condition:

The difference is substantial: D soils produce over 13 times more runoff than A soils for the same conditions!

Soil Group and Infiltration Design

Minimum Infiltration Rates for Design

When to Test

Always conduct infiltration testing when:

• Designing infiltration BMPs
• Soil data is uncertain or unavailable
• Site has been disturbed or filled
• Results significantly impact design
• Local regulations require testing

Urban Area Considerations

Fill and Disturbed Soils

Urban sites often have:

• Imported fill of unknown origin
• Compacted subgrade
• Mixed or layered soils
• Contamination concerns

Compaction Effects

Construction traffic can change soil HSG:

• Group A soil compacted → may behave like Group C
• Compaction can be partially reversed through deep tillage
• Specify soil decompaction in landscaping specs

Urban Fill Classification

When native soil is covered by fill:

• If fill depth >24 inches: Classify based on fill properties
• If fill depth <24 inches: Consider both fill and native soil
• Consult local guidance for specific requirements

Mapping Multiple Soil Types

Real sites often have multiple soil types. Options include:

1. Composite Analysis

Calculate a single composite CN using area-weighted average of CN values (not soil groups).

2. Segmented Analysis

Divide the site into sub-areas by soil type and analyze each separately. This is more accurate but more complex.

3. Conservative Approach

Use the most conservative (highest CN) soil group for the entire site. This ensures adequate capacity but may result in oversizing.

Common Mistakes

1. Using Regional Generalizations

“Clay soils in Texas” doesn’t tell you the HSG. Use site-specific data.

2. Assuming Surface = Subsurface

Surface soil may differ from layers at infiltration depth. Verify conditions at design depth.

3. Ignoring Seasonal Variation

Water tables fluctuate seasonally. Design for worst-case (wet season) conditions.

4. Overlooking Fill History

Old maps may show native soil that’s been covered by fill. Investigate site history.

5. Using Inappropriate Tests

Percolation tests (for septic systems) aren’t the same as infiltration tests. Use appropriate methods.

Summary

Hydrologic Soil Groups are fundamental to drainage design because they:

• Determine Curve Numbers for runoff estimation
• Control feasibility of infiltration practices
• Indicate groundwater conditions
• Affect stormwater system sizing

Always:

• Use NRCS Web Soil Survey as primary source
• Verify with field testing for critical applications
• Consider construction impacts on urban sites
• Account for seasonal water table variation

References

1. Natural Resources Conservation Service. (2007). National Engineering Handbook, Part 630: Hydrology, Chapter 7: Hydrologic Soil Groups. U.S. Department of Agriculture.

2. Natural Resources Conservation Service. (2024). Web Soil Survey. U.S. Department of Agriculture. https://websoilsurvey.sc.egov.usda.gov

3. Rawls, W. J., Brakensiek, D. L., & Saxton, K. E. (1982). Estimation of soil water properties. Transactions of the ASAE, 25(5), 1316-1320.

4. Musgrave, G. W. (1955). How much of the rain enters the soil? Yearbook of Agriculture, 151-159.

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