STATSGO soil characteristics for the conterminous United States

The data were derived for use in regional (multi-state) and national studies
requiring estimates of soil characteristics. The georeferenced soil
characteristics were intended to be averaged over large areas, such as over a
river basin, and used (1) for watershed simulation models that require
spatially lumped estimates of soil hydraulic parameters and (2) as explanatory
variables in regional and national water-quality assessment studies.

This digital data release consists of an ARC/INFO grid and associated INFO
tables. The grid is called MUID and has STATSGO (U.S. Department of
Agriculture, 1994) soil mapping unit identifiers gridded on a 1-kilometer
resolution for the conterminous United States. The INFO tables have soil
characteristics data in them. The ITEMS in the tables are weighted average
values for several soil characteristics in the STATSGO data base. The weighted
average values were computed by aggregating the soil layers and components in
the data base.

The INFO tables are called MUID.LAYER, MUID.COMP, MUID.KFACT, MUID.TFACT, and
MUID.WEG. The INFO tables can be related or joined to the MUID grid or to
individual State coverages (grids or polygons) of MUIDs. Joining or relating
the tables to the MUID grid creates 1-kilometer resolution grids of the soil
characteristics for the conterminous United States.

The soil characteristics in MUID.LAYER are permeability (PERML and PERMH in the
STATSGO data base), available water capacity (AWCL and AWCH), bulk density (BDL
and BDH), and organic matter (OML and OMH). The soil characteristics in
MUID.COMP are slope (SLOPEL and SLOPEH), depth to seasonally high water table
(WTDEPL and WTDEPH), and depth to bedrock (ROCKDEPL and ROCKDEPH). The soil
characteristic in MUID.KFACT is the soil erodibility factor (KFACT), the soil
characteristic in MUID.TFACT is the soil loss tolerance factor (TFACT), and the
soil characteristic in MUID.WEG is the wind erosion group (WEG).

The MUID grid and INFO tables were created with a set of arc macro language
(aml) and Fortran programs. Send electronic mail to [e-mail] to obtain
copies of the computer code. (See Procedures_Used.)

Procedures used

The STATSGO data were processed on a Data General desktop computer using Data
General Unix operating system. Processing was accomplished using a set of alms
and Fortran programs. Contact [e-mail] for copies of the computer code.

Deriving MUID "average" values for land-surface slope, depth to the seasonally
high water table, and total soil thickness required that a weighted average
value be computed for all the soil components in a soil mapping unit. The
average was computed by weighting the component values by their percentage
composition of the MUID. For example, suppose MUID AL005 was 35 percent
component A, 25 percent component B, and 40 percent component C. Further assume
that the SLOPEL value for component A was 3 percent, the SLOPEL value for
component B was 6 percent, and the SLOPEL value for component C was missing.
The weighted average value (AVE) of SLOPEL for MUID AL005 would then be:

AVE = (0.35 * 3 + 0.25 * 6) / (0.35 + 0.25) = 4.25 inches per hour.

Deriving MUID average values for permeability, available water capacity, bulk
density, organic matter content, soil erodibility factor, soil loss tolerance
factor, and wind erodibility group required that a weighted average value be
computed for all the soil layers in a soil component, and then a weighted
average value be computed for all the soil components in a soil mapping unit.
First, the component average values were computed by weighting the layer values
by their thickness. Second, the MUID average was computed by weighting the
component average values by their percentage composition of the MUID. Each step
of the two-step weighting process was done exactly as described above, except
first the layer values were weighted by their thicknesses, and then the
component values were weighted by their percentage composition.