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Excessive Erosion on Cropland, 1997

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Description

This dot density map shows acres where excessive erosion from wind and water is occurring on cropland. Each red dot represent 5,000 acres of highly erodible land eroding excessively (57.3 million acres). Each yellow dots represent 5,000 acres of non-highly erodible land eroding excessively (50.5 million acres). Data are aggregated by 8-digit hydrologic units. Excessive erosion is defined as erosion greater than the tolerable rate (T). Highly Erodible Land is defined as land where the erodibility index is greater than or equal to 8. The Universal Soil Loss Equation (USLE) is used to calculate water erosion. The Average Annual Wind Erosion Equation (AAWEQ) is used to calculate wind erosion. A total of 108 million acres are eroding excessively resulting in 1.3 billion tons of erosion. In instances where both sheet and rill and wind erosion were classified as excessive, acreage was counted only once to avoid double counting. Therefore, acreage comparisons with tables for sheet and rill and wind erosion with regard to T will be greater than the 108 million acres in this analysis. Areas with 95% or more Federal area are shaded gray. Excess erosion leads to water quality concerns from sediments, nutrients, and pesticides as well as air quality in wind erosion areas of the West, Midwest, Northern Plains, and Southern Plains. Excess erosion is also an indicator of forgone opportunities for improving soil, water, and air quality, sequestering carbon dioxide, and helping in goals to reduce greenhouse gases in the atmosphere.

Cautions for this Product:
Erosion by water includes sheet and rill erosion and excludes gully erosion. Within an 8-digit hydrologic unit, dot counts represent acreage totals correctly plus or minus one dot to account for remainders. This map does not show rates of erosion or how much erosion has occurred. 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: Cross of States with 8 Digit Hydrologic Units and Federal Land
Other Layers Displayed: Rivers, States


Definitions

Cropland:
A Land cover/use category that includes areas used for the production of adapted crops for harvest. Two subcategories of cropland are recognized: cultivated and noncultivated. Cultivated cropland comprises land in row crops or close-grown crops and also other cultivated cropland, for example, hayland or pastureland that is in a rotation with row or close-grown crops. Noncultivated cropland includes permanent hayland and horticultural cropland. [NRI-97]

Erodibility Index (EI):
The soil erodibility index (EI) provides a numerical expression of the potential for a soil to erode considering the physical and chemical properties of the soil and the climatic conditions where it is located. The higher the index, the greater the investment needed to maintain the sustainability of the soil resource base if intensively cropped. It is defined to be the maximum of (RxKxLS)/T (from the Universal Soil Loss Equation) and (CxI)/T (from the Wind Erosion Equation), where R is a measure of rainfall and runoff, K is a factor of the susceptability of the soil to water erosion, LS is a measure of the combined effects of slope length and steepness, C is a climatic characterization of windspeed and surface soil moisture and I is a measure of the susceptability of the soil to wind erosion. Erodibility Index scores equal to or greater than 8 are considered highly erodible land. (NRI, 1992)

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]

Highly Erodible Land (HEL):
Land that has an erodibility index of eight or more (NFSAM).

Hydrologic units:
A hierarchical system developed by the U.S. Geological Survey that divides the United States and the Caribbean into 21 major regions, 222 subregions, 352 accounting units, and further subdivided into 2,150 cataloging units that delineate river basins having drainage areas usually greater than 700 square miles. [USGS]

Non-Highly Erodible Land:
Land that has an erodibility index of less than eight. NRI, 1997

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.

Wind erosion:
The process of detachment, transport, and deposition of soil by wind. [NAM]

Wind erosion equation (WEQ):
An erosion model designed to predict long-term average annual soil losses from a field having specific characteristics (NAM). E= f(IKCLV) where E is the estimated average annual soil loss expressed in tons per acre per year; I is the soil erodibility; K is the soil ridge roughness factor; C is the climatic factor; L is the equivalent unsheltered distance across the field along the prevailing wind erosion direction; and V is the equivalent vegetative cover. [NAM]


Product Information

Product ID: 5083
Production Date: 12/7/2000
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.