Determination of Surface Soil Erodibility Factor

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.

1. 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.
2. 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.

8/27/2001