This dataset represents the soil hydrologic group from SSURGO and STATSGO soil descriptions for soil map units in the state of northern California that lie within the North Pacific Landscape Conservation Cooperative.
Definition
The complete definition and official criteria for hydrologic
soil groups are available online at (Title
210, National Engineering Handbook, Part 630, Chapter 7, “Hydrologic
Soil Groups”).
“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 seasonal high water table, saturated hydraulic
conductivity after prolonged wetting, and depth to a layer with a very
slow water transmission
rate. Changes in soil properties caused by land management or
climate changes also cause the hydrologic soil group
to change. The influence of ground cover is treated
independently.
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.
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.
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.
Estimates.— Assignment of soils to hydrologic groups is based on the relationship between soil
properties and hydrologic groups. Wetness characteristics, water transmission after prolonged wetting, and depth to
very slowly permeable layers are properties used in estimating hydrologic groups.