The information below was compiled from the following web page: http://www.essc.psu.edu/soil_info/index.cgi?soil_data&conus&data_cov Determination of Surface Soil Erodibility Factor The STATSGO Layer table specifies two soil erodibility factors for each component layer, KFFACT and KFACT. The STATSGO documentation describes KFFACT as a soil erodibility factor which "quanitifies the susceptibility of soil particles to detachment and movement by water. This factor is used in the Universal Soil Loss Equation to caluculate soil loss by water." KFACT is described as a soil erodibility factor which is "adjusted for the effect of rock fragments." The average value of each of these soil erodibility factors was determined for the top (surface) layer for each map unit of each state. For each component of each map unit, the Layer table entries for KFFACT and KFACT were read for the surface layer only. These values were multiplied by the percentage of the area of the map unit covered by the component, given by Comp table variable COMPPCT, and the products summed over all components of the map unit. The sum of the products was then diveded by the sum of the CPMPPCT values for all components in the map unit, and rounded to the nearest 0.01. In most cases, this yields the mean surface values of KFFACT and KFACT over the map unit. However, three special cases required different treatment: If one or more components of the map unit were specified to be all water and there were also non-water components, the mean values were adjusted to include only the non water components. If the map unit was entirely covered by water, the surface KFFACT and KFACT values were set to zero. There were also a small number of components for which the depth to bedrock was entered as zero and no Layer table entries were found. These components were assumed to be all rock, and were assigned erodibility factor values of zero. There were also some components which contained apparently incorrect entries for the KFFACT value in the Layer table. There were two types of problems which required special consideration. Most states had a number of components (overall, about 1.4% of all components) for which KFFACT for the top layer was entered as zero while KFACT had a non-zero value. For these cases, KFFACT was assigned the value of KFACT. Rock fragments in the surface layer are expected to provide a surface-protecting effect, so the value of KFACT is expected to be no greater than KFFACT. However, there were 232 components (0.2%) in 206 map units (2% of map units) which had KFACT > KFFACT > 0. The values of KFACT and KFFACT may have been interchanged when they were entered into the STATSGO Layer table; on the other hand, there may be certain circumstances under which the presense of rock fragments does, in fact, increase the erodibility. Accordingly, no attempt was made to correct for this; the computation of soil erodibility factors for the map unit used the values entered in the Layer table. As a result, there were 69 map units for which, after averaging over all components, KFFACT < KFACT.

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 has been added by taking area-weighted averages over all the components 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

Process_Description:

Determination of Surface Soil Erodibility Factor

The STATSGO Layer table specifies two soil erodibility factors for each component layer, KFFACT and KFACT. The STATSGO documentation describes KFFACT as a soil erodibility factor which "quanitifies the susceptibility of soil particles to detachment and movement by water. This factor is used in the Universal Soil Loss Equation to caluculate soil loss by water." KFACT is described as a soil erodibility factor which is "adjusted for the effect of rock fragments." The average value of each of these soil erodibility factors was determined for the top (surface) layer for each map unit of each state. 

For each component of each map unit, the Layer table entries for KFFACT and KFACT were read for the surface layer only. These values were multiplied by the percentage of the area of the map unit covered by the component, given by Comp table variable COMPPCT, and the products summed over all components of the map unit. The sum of the products was then diveded by the sum of the CPMPPCT values for all components in the map unit, and rounded to the nearest 0.01.

In most cases, this yields the mean surface values of KFFACT and KFACT over the map unit. However, three special cases required different treatment: 

If one or more components of the map unit were specified to be all water and there were also non-water components, the mean values were adjusted to include only the non water components. 

If the map unit was entirely covered by water, the surface KFFACT and KFACT values were set to zero. 

There were also a small number of components for which the depth to bedrock was entered as zero and no Layer table entries were found. These components were assumed to be all rock, and were assigned erodibility factor values of zero. 

There were also some components which contained apparently incorrect entries for the KFFACT value in the Layer table. There were two types of problems which required special consideration. 

Most states had a number of components (overall, about 1.4% of all components) for which KFFACT for the top layer was entered as zero while KFACT had a non-zero value. For these cases, KFFACT was assigned the value of KFACT. 

Rock fragments in the surface layer are expected to provide a surface-protecting effect, so the value of KFACT is expected to be no greater than KFFACT. However, there were 232 components (0.2%) in 206 map units (2% of map units) which had KFACT > KFFACT > 0. The values of KFACT and KFFACT may have been interchanged when they were entered into the STATSGO Layer table; on the other hand, there may be certain circumstances under which the presense of rock fragments does, in fact, increase the erodibility. Accordingly, no attempt was made to correct for this; the computation of soil erodibility factors for the map unit used the values entered in the Layer table. As a result, there were 69 map units for which, after averaging over all components, KFFACT < KFACT. 

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).

kfact_grid

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