Hydrologic Group (618.35) - From National Soils Handbbook (430-VI-NSSH, 2001)


(a) Definition

Hydrologic group is a group of soils having similar runoff potential under similar storm
and cover conditions. 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. These properties are depth to a seasonally high water table, intake
rate and permeability after prolonged wetting, and depth to a very slowly permeable
layer. The influence of ground cover is treated independently. 

(b) Classes

The soils in the United States are placed into four groups, A, B, C, and D, and three dual
classes, A/D, B/D, and C/D. In the definitions of the classes, infiltration rate is the rate
at which water enters the soil at the surface and is controlled by the surface
conditions. Transmission rate is the rate at which water moves in the soil and is
controlled by soil properties. Definitions of the classes are as follows: 

A. (Low runoff potential). The soils have a high infiltration rate even when
thoroughly wetted. They chiefly consist of deep, well drained to excessively
drained sands or gravels. They have a high rate of water transmission. 

B. The soils have a moderate infiltration rate when thoroughly wetted. They
chiefly are moderately deep to deep, moderately well drained to well
drained soils that have moderately fine to moderately coarse textures. They
have a moderate rate of water transmission. 

C. The soils have a slow infiltration rate when thoroughly wetted. They
chiefly have a layer that impedes downward movement of water or have
moderately fine to fine texture. They have a slow rate of water
transmission. 

D. (High runoff potential). The soils have a very slow infiltration rate when
thoroughly wetted. They chiefly consist of clay soils that have a high
swelling potential, soils that have a permanent high water table, soils that
have a claypan or clay layer at or near the surface, and shallow soils over
nearly impervious material. They have a very slow rate of water
transmission. 

(1) Dual hydrologic groups, A/D, B/D, and C/D, are given for certain wet soils that can
be adequately drained. The first letter applies to the drained condition, the second to
the undrained. Only soils that are rated D in their natural condition are assigned to dual
classes. Soils may be assigned to dual groups if drainage is feasible and practical. 

(2) Chapter 7 of the NRCS National Engineering Handbook-4, Hydrology, discusses
specific hydrologic groups of soils. 

(c) Significance

Hydrologic groups are used in equations that estimate runoff from rainfall. These
estimates are needed for solving hydrologic problems that arise in planning
watershed-protection and flood-prevention projects and for planning or designing
structures for the use, control, and disposal of water.

(d) Measurements

The original classifications assigned to soils were based on the use of rainfall-runoff data
from small watersheds and infiltrometer plots. From these data, relationships between
soil properties and hydrologic groups were established. 

(e) Estimates

Assignment of soils to hydrologic groups is based on the relationship between soil
properties and hydrologic groups. Wetness characteristics, permeability after prolonged
wetting, and depth to very slowly permeable layers are properties that assist in
estimating hydrologic groups. 

(f) Entries

Enter the soil hydrologic group, such as A, B, C, D, A/D, B/D, or C/D.