This dataset contains the percent of each mapunit which is in each of the hydrologic soil group (HSG) classes, as reported in the State Soil Geographic (STATSGO) soils data compiled by the Natural Resources Conservation Service (NRCS) of the U.S. Department of Agriculture. Hydrologic soil group classes are used in the computation of runoff by the SCS (now NRCS) Curve Number Method. Class placement is based on the minimum annual steady ponded infiltration rate for a bare ground surface.

Supplemental_Information:

The information below was compiled from the following web page:
http://www.essc.psu.edu/soil_info/index.cgi?soil_data&conus&data_cov

The components of a given STATSGO mapunit are not necessarily all in the same HSG class. Since the location of a component within a map unit is not specified, it is only possible to determine the relative amounts (percent) of the different HSG classes over an entire mapunit. For any given point within the mapunit, these percent values may then be interpreted as the probabilities that the soil at that point is in each of the possible classes. 

The computation of the percent of each HSG class in each mapunit used the variables HYDGRP (HSG class) and COMPPCT (percent of the mapunit's area covered by the component) from the STATSGO Component table. Possible HYDGRP values include A, B, C, D, A/D, B/D, and C/D. The percentage of each HSG class within each mapunit was calculated by totalling the COMPPCT values for all components in that class; mixed classes (A/D, B/D, and C/D) were converted to HSG D during processing. Since some mapunits contain components which are all water, the percent of water for each mapunit was added as a fifth class, HSG W. 

Hydrologic soil group classes are used in the computation of runoff by the SCS (now NRCS) Curve Number Method. Class placement is based on the minimum annual steady ponded infiltration rate for a bare ground surface. 
The components of a given STATSGO mapunit are not necessarily all in the same HSG class. Since the location of a component within a map unit is not specified, it is only possible to determine the relative amounts (percent) of the different HSG classes over an entire mapunit. For any given point within the mapunit, these percent values may then be interpreted as the probabilities that the soil at that point is in each of the possible classes. The computation of the percent of each HSG class in each mapunit used the variables HYDGRP (HSG class) and COMPPCT (percent of the mapunit's area covered by the component) from the STATSGO Component table. Possible HYDGRP values include A, B, C, D, A/D, B/D, and C/D. The percentage of each HSG class within each mapunit was calculated by adding up the values of the COMPPCT values for all components in that class; mixed classes (A/D, B/D, and C/D) were converted to HSG D during processing. Since some mapunits contain components which are all water, the percent of water for each mapunit was added as a fifth class, HSG W. If no HYDGRP value was specified for a component and it was not all water, a default HSG class of D was used.

To increase compatability with different types of data analysis software, the dataset is available in several different data file formats. These include both Arc/Info polygon format and a gridded version at 1 km resolution; the latter is available in both Arc/Info grid format and as a three-dimensional array of 8-bit unsigned binary integers. The description of dataset files provides additional details on formats, the data files asociated with each format, and instructions for file retrieval.

For the gridded version of the dataset, any 1-km grid cell which contains portions of two or more mapunits was assigned the HSG percent values for the mapunit which occupies the largest fraction of the cell. For the Arc/Info grid version, the HSG values may be accessed using the mapunit serial numbers associated with each STATSGO mapunit; the percent of each HSG class has been incorporated into the Value Attribute Table (VAT) entry for each mapunit.

It is important to emphasize that, in addition to the limitations associated with generalizing from detailed soil maps to representative soil profiles in the STATSGO data, another level of generalization by taking area-weighted averages over all the components is added in each STATSGO mapunit. Hence, for most mapunits, the average soil profile will not closely match any actual soil profile.

Miller, D.A. and R.A. White, 1998: A Conterminous United States Multi-Layer Soil Characteristics Data Set for Regional Climate and Hydrology Modeling. Earth Interactions, 2. [Available on-line at http://EarthInteractions.org] http://www.essc.psu.edu/soil_info/index.cgi?soil_data&conus&data_cov&ph

The components of a given STATSGO mapunit are not necessarily all in the same HSG class. Since the location of a component within a map unit is not specified, it is only possible to determine the relative amounts (percent) of the different HSG classes over an entire mapunit. For any given point within the mapunit, these percent values may then be interpreted as the probabilities that the soil at that point is in each of the possible classes. The computation of the percent of each HSG class in each mapunit used the variables HYDGRP (HSG class) and COMPPCT (percent of the mapunit's area covered by the component) from the STATSGO Component table. Possible HYDGRP values include A, B, C, D, A/D, B/D, and C/D. The percentage of each HSG class within each mapunit was calculated by totalling the COMPPCT values for all components in that class; mixed classes (A/D, B/D, and C/D) were converted to HSG D during processing. Since some mapunits contain components which are all water, the percent of water for each mapunit was added as a fifth class, HSG W. The 11 standard layers are : Layer Thickness Depth to Top Depth to Bottom 1 5 cm (2 in) 0 cm (0 in) 5 cm (2 in) 2 5 cm (2 in) 5 cm (2 in) 10 cm (4 in) 3 10 cm (4 in) 10 cm (4 in) 20 cm (8 in) 4 10 cm (4 in) 20 cm (8 in) 30 cm (12 in) 5 10 cm (4 in) 30 cm (12 in) 40 cm (16 in) 6 20 cm (8 in) 40 cm (16 in) 60 cm (24 in) 7 20 cm (8 in) 60 cm (24 in) 80 cm (31 in) 8 20 cm (8 in) 80 cm (31 in) 100 cm (39 in) 9 50 cm (20 in) 100 cm (39 in) 150 cm (59 in) 10 50 cm (20 in) 150 cm (59 in) 200 cm (79 in) 11 50 cm (20 in) 200 cm (79 in) 250 cm (98 in) The above selection of the number and depths of these standard layers reflects three main considerations: The wide variation of numbers, thicknesses, and depths of layers for different components means that there are no "natural" or "obvious" choices for the standard layers. Many models are particularly sensitive to the properties of the top few centimenters of soil; hence extra priority should be given to preserving all available information for this region. To minimize data volumes, layer thicknesses should not be much less than the thicknesses of "typical" component layers at similar depths. To aid in the selection of standard layers, therefore, the frequencies of depths and thicknesses of layers were tabulated for all components. This tabulation indicated that roughly 50% of components have surface layers thicker than 20 cm (8 inches); only about 4% of surface layers have a thickness of 5 cm (2 inches) or less, and about 16%, 10 cm (4 inches) or less. Deeper layers are in general thicker -- roughly 60% of all layers were at least 50 cm (20 inches) thick. The majority of components did not record layers extending below 60 inches (approximately 1.5 m); only about 10% include layers extending beyond 2.0 m (79 inches).

hsgpct_grid

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