Highly Erodible Land Defs
Highly Erodible Land Definitions
Highly erodible land is defined by the Sodbuster, Conservation
Reserve, and Conservation Compliance parts of the Food Security Act of 1985 and
the Food, Agriculture, Conservation, and Trade Act of 1990. Determinations for
highly erodible land are based on an erodibility index as defined in the
National Food Security Act Manual.
Official lists of highly erodible and potential highly erodible
soil map units are maintained in the NRCS - Field Office Technical Guide. Policy
and procedures for developing and maintaining the lists are given in part 511 of
the National Food Security Act Manual.
Highly Erodible Soil Map Units – A soil map unit with an
erodibility of 8 or greater is highly erodible land (HEL).
Calculating Erodibility Index – The erodibility index (EI) for a
soil map unit is determined by dividing the potential erodibility for the soil
map unit by the loss tolerance (T).
Sheet and Rill Equation: R x K x LS = EI
Soil Erodibility Factor (K)
Soils vary in their susceptibility to erosion. The soil
erodibility factor K is a measure of erodibility for a standard condition. The
soil erodibility factor K represents both susceptibility of soil to erosion and
the amount and rate of runoff, as measured under the standard unit plot
condition. Fine textured soils high in clay have low K values, about 0.02 to
0.15, because they are resistant to detachment. Coarse texture soils, such as
sandy soils, have low K values, about 0.05 to 0.2, because of low runoff even
though these soils are easily detached. Medium textured soils, such as silt loam
soils, have moderate K values, about 0.25 to 0.40, because they are moderately
susceptible to detachment and they produce moderate runoff. Soils having a high
silt content are the most erodible of all soils. They are easily detached and
they tend to crust and produce large amounts and rates of runoff. Values of K
for these soils tend to be greater than 0.4. In the Caribbean Area, K values
range from 0.02 to 0.32.
Soil Loss Tolerance (T)
It is defined as the maximum rate of annual soil erosion that
will permit crop productivity to be sustained economically and indefinitely. The
T factors are integer values from 1 through 5 tons per acre per year. The factor
of 1 ton per acre per year is for shallow or otherwise fragile soils and 5 tons
per acre per year is for deep soils that are least subject to damage by erosion.
The classes of T factors are 1, 2, 3, 4, and 5.
Erosivity Factor (R)
The rainfall-runoff factor, R, can be simply defined as the
erosive force of the rainfall. The erosivity of rainfall varies greatly by
location. For example, erosivity in the eastern interior zone of the Caribbean
Area is much higher than in the south coastal zone of the area. The R factor
represents these differences in erosivity among locations. Values for R computed
from weather records were used to produce the Caribbean Area data base map.
Slope Gradient Factor (S)
Slope gradient is the difference in elevation between two points
and is expressed as a percentage of the distance between those points. For
example, a difference in elevation of 1 meter over a horizontal distance of 100
meters is a slope of 1 percent.
Slope gradient influences the retention and movement of water,
the potential for soil slippage and accelerated erosion, the ease with which
machinery can be used, soil-water states, and the engineering uses of the soil.
Slope gradient is usually measured with a hand level or
clinometer. The range is determined by summarizing data from several sightings.
Enter the high, low, and representative values to represent the
range of slope gradient as a percentage for the map unit component. Entries for
high and low are whole number integers and range from 0 to 999.
Slope Length Factor (L)
Slope length is the horizontal distance from the origin of
overland flow to the point where either the slope gradient decreases enough that
deposition begins or runoff becomes concentrated in a defined channel. Reference
Agriculture Handbook 703.
Slope length has considerable control over runoff and potential
accelerated water erosion. Slope length is combined with slope gradient in
erosion prediction equations to account for the effect of topography on erosion.
Slope length is measured from the point of origin of overland
flow to the point where the slope gradient decreases enough that deposition
begins or runoff becomes concentrated in a defined channel. Pacing or measuring
in the field best determines the slope length.
Enter the high, low, and representative values for the range for each map
unit component. Enter a whole number that represents the length in meters from
the point of origin of overland flow to the point of deposition or concentrated
flow of the slope on which the component lies. The slope length may be fully
encompassed within one map unit or it may cross several map units. The minimum
value is 0, and the maximum value used in erosion equations is 305 meters.
For additional information on Caribbean Area Soils, please contact:
Carmen L. Santiago
at 787-766-5206 x. 127.