Soil properties are fundamental to hydrologic analysis, affecting infiltration, runoff generation, and groundwater recharge. The tables below provide reference data for USDA soil texture classes, Hydrologic Soil Groups (HSG), Green-Ampt infiltration parameters, and soil hydraulic properties.
Soil Texture
Classification based on the relative proportions of sand, silt, and clay particles
Hydrologic Soil Group
Classification by runoff potential and minimum infiltration rate when saturated
Hydraulic Conductivity
Rate at which water moves through saturated soil (Ksat)
USDA soil texture classes are defined by the percentages of sand, silt, and clay. Each texture class has characteristic hydraulic properties.
| Texture Class | Sand (%) | Silt (%) | Clay (%) | Typical HSG |
|---|---|---|---|---|
Sand Contains 85% or more sand; percentage of silt plus 1.5 times percentage of clay is 15 or less | 85-100 | 0-15 | 0-10 | A |
Loamy Sand Contains 70-90% sand; percentage of silt plus 1.5 times percentage of clay is 15 or more | 70-90 | 0-30 | 0-15 | A |
Sandy Loam Contains 50-70% sand and less than 20% clay, or less than 50% sand and less than 7% clay | 43-85 | 0-50 | 0-20 | A |
Loam Contains 23-52% sand, 28-50% silt, and 7-27% clay | 23-52 | 28-50 | 7-27 | B |
Silt Loam Contains 50% or more silt and 12-27% clay, or 50-80% silt and less than 12% clay | 0-50 | 50-88 | 0-27 | B |
Silt Contains 80% or more silt and less than 12% clay | 0-20 | 80-100 | 0-12 | B |
Sandy Clay Loam Contains 45% or more sand and 20-35% clay | 45-80 | 0-28 | 20-35 | C |
Clay Loam Contains 20-45% sand, 15-53% silt, and 27-40% clay | 20-45 | 15-53 | 27-40 | C |
Silty Clay Loam Contains less than 20% sand and 27-40% clay | 0-20 | 40-73 | 27-40 | C |
Sandy Clay Contains 45% or more sand and 35% or more clay | 45-65 | 0-20 | 35-55 | D |
Silty Clay Contains less than 20% sand and 40% or more clay | 0-20 | 40-60 | 40-60 | D |
Clay Contains 40% or more clay, less than 45% sand, and less than 40% silt | 0-45 | 0-40 | 40-100 | D |
USDA Soil Texture Triangle
The USDA texture triangle classifies soil based on the relative proportions of sand (0.05-2.0 mm), silt (0.002-0.05 mm), and clay (<0.002 mm). Percentages must sum to 100%. Laboratory analysis (sieve and hydrometer tests) determines these percentages.
Design Guidance
Determining Soil Properties
- Soil Surveys: NRCS Web Soil Survey provides HSG and other properties for mapped soil units
- Lab Testing: Grain size analysis determines texture; permeability tests measure Ksat
- Field Testing: Infiltrometer tests, bore logs, and visual classification
Using HSG for Design
- HSG is the primary input for SCS curve number selection
- Consider the HSG of the least permeable layer within the root zone
- For dual-classified soils, use the drained group if drainage is provided
- Urban soils may have altered properties due to compaction
Infiltration Analysis
- Green-Ampt parameters vary significantly with soil texture and moisture
- Use typical values for preliminary design; site-specific testing for final design
- Initial moisture content significantly affects early infiltration rates
- Compacted soils may have 10-100x lower infiltration than undisturbed soils
Available Water Capacity
Available water = Field Capacity - Wilting Point. This represents water available for plant uptake and affects evapotranspiration calculations. Sandy soils have low AWC (0.05-0.10 in/in), while loams have higher AWC (0.15-0.20 in/in).
Primary Sources
- USDA Soil Survey Manual (2017). Chapter 3: Examination and Description of Soil Profiles.
- USDA SCS TR-55 (1986). Urban Hydrology for Small Watersheds.
- USDA NRCS NEH Part 630 (2004). Chapter 7: Hydrologic Soil Groups.
- Rawls, W.J., et al. (1982). Estimation of soil water properties. Transactions of the ASAE.
- Rawls, W.J., et al. (1983). Green-Ampt infiltration parameters from soil data. Journal of Hydraulic Engineering.
Related Resources
External Links
- NRCS Web Soil Survey - Official soil maps and data
- USDA Soil Survey Manual - Complete reference for soil classification