Average Annual Soil Erosion by Water on
Cultivated Cropland as a Proportion of the Tolerable Rate (T), 1997
Description
This map shows the 1997 actual soil erosion due
to water as a proportion of the tolerable rate
(T) for each 8-digit hydrologic unit. Four
legend categories show areas that are less than
the tolerable rate, from one to two times the
tolerable rate, two to four times the tolerable
rate, and four or more times tolerable rate.
Actual soil erosion rate is determined using the
Universal Soil Loss Equation (USLE). This map
excludes areas where cultivated cropland is less
than 5% of the total surface area. Areas with
95% or more Federal area are shaded gray.
Cautions for this Product:
NRI maps reflect national patterns rather than
site-specific information. This map does not
show total erosion. Data are not collected on
Federal land. Data are not available for Alaska
or the Pacific Basin. Data for Puerto Rico and
the U.S. Virgin Islands are aggregated by 6-
digit hydrologic unit.
Sources
Source:
National Resources Inventory, 1997
Distributor:
USDA-NRCS-RIAD
Reliability:
NRI sample data are generally reliable at the
95% confidence interval for state and certain
broad substate area analyses. Generally,
analyses that aggregate data points by smaller
geographic areas and/or more specific criteria
result in fewer data points for each aggregation
and therefore less reliable estimates. NRI maps
reflect national patterns rather than site-
specific information.
Layers
Aggregate Layer:
8 Digit Hydrologic Unit Areas with Federal Land
Other Layers Displayed:
States, Rivers
Definitions
Cultivated Cropland:
Includes land identified as being in row or
close crops, summer fallow, aquaculture in crop
rotation or other cropland not planted --
including cropland in short-term set-aside
programs; or land in horticulture that is double
cropped; or land in hay which had at least one
of the three previous years in row or close
grown crops; or land in pasture which had at
least one of the three previous years in row or
close grown crops.
Erosion:
The wearing away of the land surface by running
water, waves, or moving ice and wind, or by such
processes as mass wasting and corrosion
(solution and other chemical processes). The
term "geologic
erosion" refers to natural erosion processes
occurring over long (geologic) time spans.
"Accelerated
erosion" generically refers to erosion that
exceeds what is presumed or estimated to be
naturally occurring
levels, and which is a direct result of human
activities (e.g., cultivation and logging).
[NSSH-96]
Gully erosion:
The erosion process whereby water concentrates
in narrow channels and, over short periods,
removes the soil from this narrow area to
considerable depths, ranging from 1 to 2 feet to
as much as 75 to 100 feet. [NRI-97]
Sheet and rill erosion:
Removal by runoff water of a fairly uniform,
usually imperceptible, thin layer of soil often
accompanied by formation of many small eroding
channels. Rills are only a few inches deep and
do not hinder farm machinery. Tillage erases
them, but they tend to recur after heavy rain
during the growing season, especially where
cover is limited. [SSM]
T-Factor:
The soil loss tolerance which can be used with
the USLE or the WEQ. It is the maximum rate of
annual soil erosion that will permit crop
productivity to be sustained economically and
indefinitely. [SSM]
Universal soil loss equation (USLE):
This equation estimates average annual soil loss
from sheet and rill erosion. Location specific
data for the field in which the NRI point falls
or that portion of the field surrounding the
point that would be considered in conservation
planning are used in the NRI calculations. The
equation is: A = RKLSCP, where A is the
computed soil loss per unit area, R is a
rainfall factor, K is a soil erodibility factor,
L is a slope length factor, S is a slope-
steepness factor, C is a cover and management
factor, and P is a conservation practice
factor. [NAM]
Water Erosion:
The process of detachment, transport and
deposition of soil in which the primary agent is
water. This may include sheet, rill and gully
erosion; however, for the purposes of this
analysis, unless otherwise stated, water erosion
refers only to sheet and rill erosion and
excludes gully erosion.
Product Information
Product ID:
5153
Production Date:
1/25/01
Product Type:
Map
For additional information
contact the Resources Inventory and Assessment Division.
Please include the Product ID you are inquiring about.
nri@wdc.usda.gov
or 1400 Independence Avenue SW - P.O. Box 2890 -
Washington D.C. 20013. If you use our analysis products,
please be aware of our disclaimer.
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