Hydrologic Soil Groups
Definition
of Hydrologic soil groups from the National Soil Survey Handbook
Chapter 7 National Engineering Handbook - criteria
National Water and Climate Center - Hydraulics and Hydrology
Update: Make sure to check new values using Web Soil Survey after October 1 2014 as the hydro groups are now calculated values!
A grouping of soils having the same runoff potential under similar storm and cover conditions. Hydrologic groups are used in equations that estimate runoff from rainfall. Soil properties that influence runoff potential are those that influence the minimum rate of infiltration for a bare soil after prolonged wetting and when not frozen. The soils of the U.S. are placed into four groups A, B, C, D. Definitions of the classes are as follows:
A. Soils with low runoff potential. Soils having high infiltration rates even when thoroughly wetted and consisting chiefly of deep, well drained to excessively well-drained sands or gravels.
B. Soils having moderate infiltration rates even when thoroughly wetted and consisting chiefly of moderately deep to deep, moderately well drained to well drained soils with moderately fine to moderately coarse textures.
C. Soils having slow infiltration rates even when thoroughly wetted and consisting chiefly of soils with a layer that impedes downward movement of water, or soils with moderately fine to fine textures.
D. Soils with high runoff potential. Soils having very slow infiltration rates even when thoroughly wetted and consisting 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.
Official hydrologic soil group listing can be found (some states do not have this information posted) in Section II of the electronic Field Office Technical Guide for the State you are interested in: http://efotg.sc.egov.usda.gov/efotg_locator.aspx
