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Annual Soil Erosion by Water on Cropland as a Proportion of the (T), 1997

Average Annual Soil Erosion by Water on Cultivated Cropland as a Proportion of the Tolerable Rate (T), 1997

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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.


Source: National Resources Inventory, 1997
Distributor: USDA-NRCS-RIAD
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.


Aggregate Layer: 8 Digit Hydrologic Unit Areas with Federal Land
Other Layers Displayed: States, Rivers


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.

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]

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

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