hyd_grp

Hydrologic soil group of the soil

The U.S. Natural Resource Conservation Service (NRCS) classifies soils into four hydrologic groups based on infiltration characteristics of the soils. NRCS Soil Survey Staff (1996) defines a hydrologic group as a group of soils having similar runoff potential under similar storm and cover conditions. Soil properties that influence runoff potential are those that impact the minimum rate of infiltration for a bare soil after prolonged wetting and when not frozen. These properties are depth to seasonally-high water table, saturated hydraulic conductivity, and depth to a very slowly permeable layer. The definitions for the different classes are:

A. Soils having high infiltration rates even when thoroughly wetted, consisting chiefly of sands or gravel that are deep and well to excessively drained. These soils have a high rate of water transmission and a low runoff potential.

B. Soils having moderate infiltration rates when thoroughly wetted, chiefly moderately deep to deep, moderately well to well drained, with moderately fine to moderately coarse textures. These soils have a moderate rate of water transmission and a moderate runoff potential.

C. Soils having slow infiltration rates when thoroughly wetted, chiefly with a layer that impedes the downward movement of water or of moderately fine to fine texture and a slow infiltration rate. These soils have a slow rate of water transmission and a high runoff potential.

D. Soils having very slow infiltration rates when thoroughly wetted, chiefly clay soils with a high swelling potential; soils with a high permanent water table; soils with a clay pan or clay layer at or near the surface; and shallow soils over nearly impervious materials. These soils have a very slow rate of water transmission and a very high runoff potential.

Criteria*	A	B	C
Final constant infiltration rate [mm/hr]	7.6-11.4	3.8-7.6	1.3-3.8	0-1.3
Mean permeability of surface layer [mm/hr]	> 254.0	84.0-254.0	8.4-84.0	< 8.4
Mean permeability of most restrictive layer below the surface layer to a depth of 1.0 m [mm/hr]	> 254.0	84.0-254.0	8.4-84.0	< 8.4
Shrink-swell potential of most restrictive layer**	Low	Low	Moderate	(Very) High
Depth to bedrock or cemented pan [mm]	> 1016	> 508	> 508	< 508

Criteria*	A/D	B/D	C/D
Mean depth to water table [m]	< 0.61	< 0.61	< 0.61

*These criteria are guidelines only. They are based on the theory that the minimum permeability occurs within the uppermost 50 cm. If the minimum permeability occurs between a depth of 50 to 100 cm, then the Hydrologic Soil Group is increased one group. For example, C to B. If the minimum permeability occurs below a depth of 100 cm, the Hydrologic Soil Group is based on the permeability above 100 cm, using the rules previously given.

**Shrink-swell potential is assigned to a profile using the following guidelines: Low: All soils with sand, loamy sand, sandy loam, loam or silt loam horizons that are at least 50 cm thick from the surface without a clay horizon within 100 cm of the surface. Medium: All soils with clay loam horizons within 50 cm of the surface or soils with clay horizons from 50 to 100 cm beneath the surface. High: All soils with clay horizons within 50 cm of the surface. Lower the shrink-swell potential one class when kaolinite clay is dominant.