Soils are classified by the Natural Resource Conservation Service into four
Hydrologic Soil Groups based on the soil's runoff potential. The four Hydrologic
Soils Groups are A, B, C and D. Where A's generally have the smallest runoff
potential and Ds the greatest.
Details of this classification can be found in ‘Urban Hydrology for Small
Watersheds’ published by the Engineering Division of the Natural Resource
Conservation Service, United States Department of Agriculture, Technical Release–55.
Group A is sand, loamy sand or sandy loam
types of soils. It has low runoff potential and high infiltration rates
even when thoroughly wetted. They consist chiefly of deep, well to excessively
drained sands or gravels and have a high rate of water transmission.
Group B is silt loam or loam. It has a moderate
infiltration rate when thoroughly wetted and consists chiefly or moderately
deep to deep, moderately well to well drained soils with moderately fine
to moderately coarse textures.
Group C soils are sandy clay loam. They
have low infiltration rates when thoroughly wetted and consist chiefly
of soils with a layer that impedes downward movement of water and soils
with moderately fine to fine structure.
Group D soils are clay loam, silty clay loam,
sandy clay, silty clay or clay. This HSG has the highest runoff potential.
They have very low infiltration rates when thoroughly wetted and consist
chiefly of clay soils with a high swelling potential, soils with a permanent
high water table, soils with a claypan or clay layer at or near the surface
and shallow soils over nearly impervious material.