A soil map is a geographical representation of the variety of soil types and/or soil properties (soil pH, textures, organic matter, horizon depths, and so on) in a given area. Soil maps are often used in projects involving land assessment, spatial planning, agricultural extension, environmental protection, and others.
Soils are assigned to one of four groups according to water infiltration rates when the soils are without vegetation, are thoroughly wet, and are subject to precipitation from long-duration storms. These groups are referred to as hydrologic soil groups in the United States. They are based on estimates of runoff potential.
When rain falls over land, a portion of it runs off into stream channels and storm water systems while the remainder infiltrates into the soil or returns to the atmosphere directly through evaporation.
Physical properties of soil affect the rate at which water is absorbed, and the amount of runoff produced by a storm. Hydrologic soil group provides an index of the rate that water infiltrates a soil and functions as an input to rainfall-runoff models that are used to predict potential stream flow.
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 consist of deep, well drained sands or gravelly sands with high infiltration and low runoff rates. Infiltration rate > 0.3 inch/hr when wet.
- Group B soils consist of deep well drained soils with a moderately fine to moderately coarse texture and a moderate rate of infiltration and runoff. Infiltration rate 0.15 to 0.3 inch/hr when wet.
- Group C consists of soils with a layer that impedes the downward movement of water or fine textured soils and a slow rate of infiltration. Infiltration rate 0.05 to 0.15 inch/hr when wet.
- Group D consists of soils with a very slow infiltration rate and high runoff potential. This group is composed of clays with a high shrink-swell potential, soils with a high water table, soils that have a clay pan or clay layer at or near the surface, and soils that are shallow over nearly impervious material. Infiltration rate 0 to 0.05 inch/hr when wet.
- Group A/D soils naturally have a very slow infiltration rate due to a high water table but will have high infiltration and low runoff rates if drained.
- Group B/D soils naturally have a very slow infiltration rate due to a high water table but will have a moderate rate of infiltration and runoff if drained.
- Group C/D soils naturally have a very slow infiltration rate due to a high water table but will have a slow rate of infiltration if drained.
The Soil Maps command of CivilGEO’s software allows the user to display global or regional (United States, Canada, Africa and Europe only) soil maps on the Map View which includes the soil data and soil survey of the defined area of interest. This command also allows the user to download the hydrologic soil groups of the United States as a shapefile data.
Soil Data and Soil Survey
Soil data is used to define infiltration parameters for rainfall runoff modeling within HEC-RAS or HEC-HMS. Soil data can also be used to define spatial sediment transport data for 2D sediment transport modeling.
A soil survey is a systematic description, classification, and mapping of soils in an area. In the USA, they are published by the USDA Natural Resources Conservation Service (NRCS), formerly known as the Soil Conservation Service.
Natural Resources Conservation Service (NRCS) is an agency of the United States Department of Agriculture (USDA) that provides technical assistance to America’s farmers, ranchers and forest landowners in order to conserve the nation’s soil, water, air and other natural resources.
Soil mapping is the process of delineating natural bodies of soils, classifying and grouping the delineated soils into map units, and capturing soil property information for the interpretation and depiction of soil spatial distribution on a map.
Each map unit is characterized by a distinct combination of soil components and/or miscellaneous areas in predictable proportions. Some map unit components may be highly contrasting from others. The mapping of a broad variety of landforms and landform segments gives a plethora of information from which it is possible to develop a range of resource strategies.
To display the soil map on the Map View, select the Soil Maps menu item from the Map Data ribbon menu.
Note that the user needs to assign a coordinate reference system (CRS) to the project before assigning soil survey data.
Note that simply clicking on the Soil Maps command icon will cause the software to open the Soil Maps dialog box, as given below. This is used to directly add the soil survey data layer of the USA on the Map View.
The following section describes how to interact with the Soil Survey Data and NRCS Soil Survey Data Download options.
Soil Survey Data
This section allows the user to display the hydrologic soil groups on the Map View based on the selected region.
Note that based on the current CRS of the project, the corresponding region in the Soil Survey Data menu will be enabled.
The following options are provided under the Soil Survey Data menu item:
- USA – This option contains the Soil Survey Geographic (SSURGO) maps by the United States Department of Agriculture’s Natural Resources Conservation Service (NRCS).
- Canada – This option contains the national scale thematic dataset displaying the classification of soils in terms of provincial Detailed Soil Surveys (DDS) polygons, Soil Landscape Polygons (SLCs), Soil Order, and Great Group.
- Africa – This option contains the harmonized soil map showing the soil classes at the continent scale produced for the Soil Atlas of Africa. The map has been derived from the Harmonized World Soil Database (HWSD).
- Europe – This option contains the European Soil Database (ESDB), the harmonized soil database for Europe, also extending to Eurasia.
- Global – This option contains the Hydrologic Soil Groups (HYSOGs250m) with worldwide coverage for curve number-based runoff modeling. This dataset from the Oak Ridge National Laboratory Distributed Active Archive Center (ORNLDAAC) provides a globally consistent, gridded dataset of hydrologic soil groups (HSGs) with a geographical resolution of 1/480 decimal degrees, corresponding to a projected resolution of approximately 250 x 250 meters. These data were developed to support USDA-based curve-number runoff modeling at regional and continental scales.
Clicking on any of the options listed above will cause the software to display the Soil Maps dialog box. The user can click on [OK] button to add the soil survey data layer of the selected region on the Map View.
NRCS Soil Survey Data Download
This section allows the user to download hydrologic soil groups as shapefile data.
Clicking this option causes the software to open the NRCS Soil Survey Data Download dialog box.
The following section describes how to interact with the above dialog box.
In this section, by default, the Layer group name is Soil Survey Data, which can be changed by the user. Click the […] button beside the Shapefile path entry to specify the directory location to which to save the downloaded shapefiles.
Soil Survey Data Boundary Limits
In this section, choose one of the three options for defining the geographic location:
- Current screen limits: The software will use the Map View screen display as a boundary limit.
- User-defined limits: The user can draw a rectangle on the Map View representing user-defined limits for the NRCS soil survey data to be downloaded. Click the [Pick] button to define the soil survey limits. The dialog box will temporarily disappear, and a message will be displayed on the status line. Click and drag a rectangular region to define the limits. After releasing the mouse, the user will be returned to the dialog box. A layer will be created with a rectangular box to represent the user-defined selected region.
- Model extents: If a project model has been defined, this option will create a bounding rectangular region to the extents of the defined model, plus an additional buffer boundary.
Clicking on [OK] button allows the user to download hydrologic soil groups as shapefile data.
Note that this command works only within the United States.
An example of NRCS soil survey data is shown below.