Process_Description:
The dominant rock fragment modifier class for each of 11 standard layers was determined for each map unit of each state using the STATSGO Component and Layer tables. The standard layers were introduced because of the wide variation in the number, thickness, and depth to top and bottom of soil layers in the STATSGO data from one soil component to another, even within the same map unit.
The Layer table entries for each component use the TEXTURE1 variable for each layer to record the dominant rock fragment and texture class in a combined description (e.g. "extremely cobbly-sandy loam"). The Layer table also records the depth to the top and bottom of the each layer. The Component table entries for each component of each map unit use the COMPPCT variable to record the percent of the area of the map unit which is covered by that component; this table also reports the minimum and maximum depth-to-bedrock for the component.
Determining the dominant rock fragment class for each of a set of standard layers required three main steps:
Translating the combined texture description in TEXTURE1 to a standard rock fragment modifier class.
For each component, determining the contribution of each component layer to the 11 standard layers.
For each mapunit, combining the contributions of all components to determine the dominant rock fragment class for each standard layer.
The translation from the combined texture description to one of 10 standard rock fragment modifiers was carried out using a look-up table. If the TEXTURE1 variable contained no rock fragment specifier, the fragment class was coded as "no rock fragments". In addition, TEXTURE1 values specifying water, organic materials (peat, muck, etc.), and other non-soil materials were translated simply to "water", "organic material", and "other", respectively. A "bedrock" class was also introduced to indicate that the depth-to-bedrock entries in the Component table implied the presence of bedrock; as noted below, this is often misleading. Including these additional classes in the final data product provides maximum flexibility to the modeler for making decisions with regard to the use of the dataset. Additional lookup tables or the re-classification functions found within standard GIS software environments may be used to reinterpret or aggregate these classes.
To determine the contribution of each component of a given map unit to the standard layers, the layers defined in the Layer table for the component were compared with each standard layer. If the standard layer was entirely included within one of the component layers, the fragment class associated with the TEXTURE1 value for the layer was multiplied by the COMPPCT value to determine the fractional contribution of the fragment class to the standard layer. If the standard layer overlapped two or more component layers, the fragment classes for each component layer were first weighted in proportion to the amount of overlap before multiplication by the COMPPCT value.
After all components for the map unit were processed, the fragment class for which the total fractional contributions from all components in the map unit was largest was determined for each standard layer and entered as the dominant fragment class for that layer. It must be emphasized that other fragment classes are often present in the layer -- in some cases, the dominant class may be representative of less than half the map unit's area.
Many Component table entries for depth to bedrock used 60 inches (1.5 m) to infer that bedrock was not encountered within this distance of the surface. In many cases, the layer table for such a component included a non-rock layer extending below the specified maximum depth-to-bedrock. When the Layer table did contain layers extending below the depth to bedrock reported for the component, these layers were used, and the bedrock was assumed to actually start immediately below the deepest such layer. When the bottom of the deepest layer was above the reported bedrock, the bottom layer was extended to the bedrock depth or the bottom of the deepest standard layer (2.5 m), whichever was less. In a large proportion of cases, however, an entry of "bedrock" in the two lowest standard layers (1.5 to 2.5 m) is misleading, since the soil or rock was not examined to that depth.
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).