Modeling - Field Sampling: Principles and Practices in Environmental Analysis - page 186

units. The R factor needed is obtained from this map and inserted into the equation.
The third factor in the equation is the erodability factor, or K. Because of differences in
infiltration and percolation rates and structural stability, different soils are more or less
resistant to erosion. This difference is reflected in the K factor. These factors have been
developed from field studies of numerous soils. Erosion by natural rainfall events from
field plots with definite area, length, and slope have been measured and related back to
soil characteristics. Either a soil will have an experimental K value or its K can be
calculated from the characteristics of the soils in the field.
The L factor is the length of the field and S is the slope. These two are combined to
give a single factor, LS. The body of the LS table contains the LS factors as a single
number. Factors are found by finding the slope on the left, length on the top, and LS
factor in the body of the table. (See Refs 8 and 9 for examples of a table.) From such a
table a soil with a slope of 4% and a length of 100 meters would have an LS of 0.64. A
soil with a slope of 16% and length of 200 meters would have an LS of 5.40.
Both the C and P factors are obtained from a table relating soil cover, organic matter,
vegetation, and management. A field in permanent pasture would have a C factor of
0.003, and with a corn-soybean rotation the factor would have a C of 0.53. The P factor is
the conservation practice or practices used in the field. Some examples would be grass
waterways, terraces, and contour strip cropping. Conservation practices control the
movement of water over the field or decrease the LS factor to lower erosion. A field with
a slope of 7% that is strip cropped has a P factor of 0.25, while this same field with 33-
meter-wide terraces without water outlets has a P factor of 0.6. All these factors can be
found on various Internet sites, such as those given in Refs. 8 and 9.
The appropriate factors from the tables are inserted into the equation and multiplied
together to obtain the estimated soil loss A. This is then compared to the T factor for the
soils in the field. If the A value is too high, management factors LS, C, and P are adjusted
so that a suitable soil loss is obtained. Note that the R and K factors cannot be changed
[8, 9]!
This equation can be used in a different fashion in field sampling. Here the information
needed is where the component of interest is most likely to be found. By knowing the rate
of erosion, the position and movement of the component along the surface can be
estimated. Places can be identified in which the component may be found deeper in the
soil profile because of the deposition of eroded soil. Areas in which the component is
closer to the surface because of the erosion of overlying soil can also be determined. The
likelihood and rate of soil and contaminant being eroded off the field can be calculated.
Also, if contaminated material is eroding onto the field the analytical results of field
sampling will constantly be changing, making the sampling program futile unless erosion
is taken into account.
The R and K factors can be used by themselves to estimate the amount of soil affected
by splash erosion. Splashing can lead to contamination of vehicles and buildings. It will
also expose sorbed contaminants to the atmosphere. This can lead to evaporation and loss
of material [10].
The RUSLE is the revised version of the USLE. This equation can easily be accessed
on the Internet. An explanation of the RUSLE is presented at this Internet site, along with
a computer program that allows the calculation of the A factor. The program is not