Interpreting L-THIA's Results: Hydrologic Soil Groups
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.
