Soil Hydrology of the United States

Soil Hydrology

The Soil Hydrology Map itself shows the wettest drainage class, but the true power of this map lies in what is revealed by clicking any soil map unit. The NRCS SSURGO dataset contains hundreds of soil attributes, and this map brings together the most important soil hydrology attributes from the SSURGO and formats them into one pop-up window.

Drainage Class

This map uses hue and value to display the wettest drainage class for each soil map unit. But click any soil map unit to reveal the maximum, dominant, and weighted average drainage class values for the soil map unit. The maximum drainage class is the value for the component with the largest value in the map unit, while the dominant drainage class is the component with the largest proportion of the map unit. The weighted average drainage class is the mean of all components in the map unit.

The Soil Survey Geographic Database (SSURGO) provides a wide variety of information on the amount and behavior of water in soils. This hydrologic information is useful in a variety of applications.

SSURGO data is organized into map units. Each map unit is composed of one or more components. The Soil Hydrology Application displays SSURGO map units symbolized by the wettest drainage class in the map unit. In the application, clicking on a map unit opens a pop-up window that displays the hydrologic details of the soils in the map unit including: the soil’s hydric group, drainage class, flood frequency, proportion subject to ponding and slope. Where data are available the pop-up also contains a graph of depth to bedrock and the annual and spring depths to the water table. The details window on the right side of the application provides information about these data and links to the Natural Resources Conservation Service’s webpages explaining the data.

The Soil Hydrology Application provides a tool to rapidly assess potential issues associated with soils early in project planning by allowing a user to identify potential wetlands, areas subject to flooding, or poorly drained soils. Identification of these areas early in project planning can result in better designs and identify areas for site specific investigations.

Values included in the pop-up are:

Map Unit

A unique number that identifies the map unit.

Hydric Classification

Hydric soils form where soils are saturated long enough to form anaerobic conditions during the growing season. Wetlands are identified using hydric soils, vegetation associated with saturated soils, and hydrology. This field reports the presence of hydric soils in all, parts of, or none of the map unit.

For more information on hydric soils and how they are identified see the Natural Resources Conservation Service's Hydric Soils webpage.

Hydrologic Group

Hydrologic soil groups are based on estimates of runoff potential. Soils are assigned to one of four groups according to the rate of water infiltration when the soils are not protected by vegetation, are thoroughly wet, and receive precipitation from long-duration storms.

The soils in the United States are assigned to four groups (A, B, C, and D) and three dual classes (A/D, B/D, and C/D). The groups are defined as follows:

Group A - Soils having a high infiltration rate (low runoff potential) when thoroughly wet. These consist mainly of deep, well drained to excessively drained sands or gravelly sands. These soils have a high rate of water transmission.

Group B - Soils having a moderate infiltration rate when thoroughly wet. These consist chiefly of moderately deep or deep, moderately well drained or well drained soils that have moderately fine texture to moderately coarse texture. These soils have a moderate rate of water transmission.

Group C - Soils having a slow infiltration rate when thoroughly wet. These consist chiefly of soils having a layer that impedes the downward movement of water or soils of moderately fine texture or fine texture. These soils have a slow rate of water transmission.

Group D - Soils having a very slow infiltration rate (high runoff potential) when thoroughly wet. These consist chiefly of clays that have a high shrink-swell potential, soils that have a high water table, soils that have a claypan or clay layer at or near the surface, and soils that are shallow over nearly impervious material. These soils have a very slow rate of water transmission.

If a soil is assigned to a dual hydrologic group (A/D, B/D, or C/D), the first letter is for drained areas and the second is for undrained areas. Only the soils that in their natural condition are in group D are assigned to dual classes.

For more information on soil hydrologic groups see the Natural Resources Conservation Services'Soil Survey Manual Chapter 3.

Drainage Class

Drainage class is the natural, undisturbed drainage condition of the soil. For this field we display both the drainage class of the wettest component of the map unit and the drainage class of the most common (dominant) component of the map unit.

The following class definitions are from the Natural Resources Conservation Services' Soil Survey Manual Chapter 3.

Excessively drained - Water is removed very rapidly. The occurrence of internal free water commonly is very rare or very deep. The soils are commonly coarse-textured and have very high hydraulic conductivity or are very shallow.

Somewhat excessively drained - Water is removed from the soil rapidly. Internal free water occurrence commonly is very rare or very deep. The soils are commonly coarse-textured and have high saturated hydraulic conductivity or are very shallow.

Well drained - Water is removed from the soil readily but not rapidly. Internal free water occurrence commonly is deep or very deep; annual duration is not specified. Water is available to plants throughout most of the growing season in humid regions. Wetness does not inhibit growth of roots for significant periods during most growing seasons. The soils are mainly free of the deep to redoximorphic features that are related to wetness.

Moderately well drained - Water is removed from the soil somewhat slowly during some periods of the year. Internal free water occurrence commonly is moderately deep and transitory through permanent. The soils are wet for only a short time within the rooting depth during the growing season, but long enough that most mesophytic crops are affected. They commonly have a moderately low or lower saturated hydraulic conductivity in a layer within the upper 1 m, periodically receive high rainfall, or both.

Somewhat poorly drained - Water is removed slowly so that the soil is wet at a shallow depth for significant periods during the growing season. The occurrence of internal free water commonly is shallow to moderately deep and transitory to permanent. Wetness markedly restricts the growth of mesophytic crops, unless artificial drainage is provided. The soils commonly have one or more of the following characteristics: low or very low saturated hydraulic conductivity, a high water table, additional water from seepage, or nearly continuous rainfall.

Poorly drained - Water is removed so slowly that the soil is wet at shallow depths periodically during the growing season or remains wet for long periods. The occurrence of internal free water is shallow or very shallow and common or persistent. Free water is commonly at or near the surface long enough during the growing season so that most mesophytic crops cannot be grown, unless the soil is artificially drained. The soil, however, is not continuously wet directly below plow-depth. Free water at shallow depth is usually present. This water table is commonly the result of low or very low saturated hydraulic conductivity of nearly continuous rainfall, or of a combination of these.

Very poorly drained - Water is removed from the soil so slowly that free water remains at or very near the ground surface during much of the growing season. The occurrence of internal free water is very shallow and persistent or permanent. Unless the soil is artificially drained, most mesophytic crops cannot be grown. The soils are commonly level or depressed and frequently ponded. If rainfall is high or nearly continuous, slope gradients may be greater.

Flooding Frequency

SSURGO provides the maximum flooding frequency for each map unit, the flooding frequency of the component with the most frequent flooding, and the flooding frequency of the component with the largest proportion of the map unit.

From Part 618 of the National Soil Survey Handbook produced by NRCS:

Flooding is the temporary covering of the soil surface by flowing water from any source, such as streams overflowing their banks, runoff from adjacent or surrounding slopes, inflow from high tides, or any combination of sources. Shallow water standing or flowing that is not concentrated as local runoff during or shortly after rain or snowmelt is excluded from the definition of flooding. Standing water (ponding) or water that forms a permanent covering is also excluded from the definition.

Flooding hazard is expressed by flooding frequency class, flooding duration class, and time of year that flooding occurs. Not considered here, but nevertheless important, are velocity and depth of floodwater. Frequencies used to define classes are generally estimated from evidence related to the soil and vegetation. They are expressed in wide ranges that do not indicate a high degree of accuracy. Flooding frequencies that are more precise can be calculated by performing complex analyses used by engineers. The class "very frequent" is used for areas subject to daily and monthly high tides.

The following flooding frequency classes are used in these fields:

None - No reasonable possibility of flooding; one chance out of 500 of flooding in any year or less than 1 time in 500 years.

Very Rare - Flooding is very unlikely but is possible under extremely unusual weather conditions; less than 1 percent chance of flooding in any year or less than 1 time in 100 years but more than 1 time in 500 years.

Rare - Flooding is unlikely but is possible under unusual weather conditions; 1 to 5 percent chance of flooding in any year or nearly 1 to 5 times in 100 years.

Occasional - Flooding is expected infrequently under usual weather conditions; 5 to 50 percent chance of flooding in any year or 5 to 50 times in 100 years.

Frequent - Flooding is likely to occur often under usual weather conditions; more than a 50 percent chance of flooding in any year (i.e., 50 times in 100 years), but less than a 50 percent chance of flooding in all months in any year.

Very Frequent - Flooding is likely to occur very often under usual weather conditions; more than a 50 percent chance of flooding in all months of any year.

Ponding

The percentage of the map unit that is subject to water being ponded on the soil surface, expressed as one of four classes: 0-14%, 15-49%, 50-74% or 75-100%

FromPart 618 of the National Soil Survey Handbook produced by NRCS:

Ponding is standing water in a closed depression. The water is removed only by deep percolation, transpiration, evaporation, or by a combination of these processes. Ponding of soils is classified according to depth, frequency, duration, and the beginning and ending months in which standing water is observed.

The susceptibility of soils to ponding is important for homes, building sites, and sanitary facilities. Time and duration of the ponding are critical factors in determining plant species. Ponding during the dormant season has few if any harmful effects on plant growth or mortality and may even improve growth.

Slope

Slope is the ratio of the elevation difference between two points and the distance between the points expressed as a percentage. SSURGO provides both the average slope of the map unit's components and the slope of the map unit's dominant component.

Depth to Bedrock and Water Table Graph

The graph at the bottom of the pop-up displays the values for depth to bedrock and two values for depth to water table in centimeters. Depth to bedrock is the distance from the soil surface to bedrock for the shallowest component in the map unit. Depth to water table is the depth of the water table for the shallowest component in the map unit and is reported for the annual water table and the water table during the months of April to June.

Other Resources

Esri hydro blog post on Hydrologic Group and Hydric Classification services.

Data from the Natural Resource Conservation Service's Soil Survey Geographic Data Base.