Soil Conservation An Assessment of the National Resources Inventory, Volume 2 (1986) / Chapter Skim
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1. An Improved Soil Erosion Classification: Update, Comparison, and Extension
1. An Improved Soil Erosion Classification: Update, Comparison, and Extension
Pages 1-20
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or classifications based on annual soil loss rates. The purpose of this paper is to (1)
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Comprehensive 1982 wind erosion estimates may allow wind-induced erosion to be added to these classification systems. EROS ION ASSESSMENT: AN UPDATE TO 19 8 2 Recent research has focused on an erosion classification based on physical erosion potential and the observed range in land management used by farmers in relation to commonly accepted soil loss tolerances tBills and Heimlich, 1984)
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If the shifts among erosion categories are taken at face value, however, the net increase in cropland over these 5 years was accomplished by substituting nonerodible for highly erodible land. Abandonment or improved management of moderately erodible land losing above 5 tons/acre/year also appears to have occurred.
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Moderately Erodible Non- Highly erodible <5 TAY >5 TAY Erodible Total <5 (1,000 acres) 1977 157,342 161,058 -- -- 318,400 1982 165,136 163,626 -- -a 328,762 5 to 13 1977 -- -- 56,99011,150 68,140 1982 -- -- 54,98810,026 65,014 14 to 24 1977 -- -- 5,7828,672 14,454 1982 -- -- 5,8728,809 14,681 >25 1977 -- -- 12411,852 11,976 1982 -- -- 8510,905 10,990 TotalC 1977 157,342 161,05862,89631,674 412,970 1982 165,136 163,62660,94529,740 419,447 <5 1977 1982 5 to 13 1977 1982 14 to 24 1977 1982 >25 1977 1982 (Percente )
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between an increase in cropland in the LCCS erodible subclasses and a decrease in cropland in erodible categories of this classification is apparent. The 1982 NRI data also preserve the inconsistency between the LCCS subclass e and the alternative classification of highly erodible cropland first shown with 1977 NRI data.
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In summary, the NRI evidence for 1977 and 1982 shows that sheet and rill erosion decreased from 1.9 billion to 1.8 billion tons/year in the face of an overall increase in acreage cropped. The absolute and relative importance of highly erodible cropland in U.S.
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Group 3 consists of highly erodible land for which conversion to permanent cover is probably the most cost-effective means of
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A long-term conservation reserve is proposed by AFT as the primary means for encouraging such conversion on group 3 land. The ranges of RKLS in AFT's system result from applying Normal farming conditions n to achieve specified ranges of erosion rates without traditional conservation practices.
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There is more nonerodible land in group 3 than there is highly erodible land. Thus, the proposed interim grouping has little to recommend it as a way to distinguish cropland resources requiring different kinds of conservation management because lands of all kinds are present in each group.
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Each incorporates an objective, scientific, quantitative measure of physical erosion potential, separate from the management currently applied to the land. Both utilize the concept of triage, borrowed from medical practice, in which three groups are defined: land that needs no erosion treatment because it has no erosion potential; land with so much erosion potential that no treatment will reduce erosion to acceptable levels; and the remaining land for which treatment is needed and will reduce erosion to acceptable levels.
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The impact of wind erosion on the classification developed earlier can be seen by arraying wind erosion rates against the RKLS-based erosion classes (see Tables 5 and 6)
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f(V) where E = potential average annual soil loss in tons/acre/year; I = soil erodibility, based on percentage of soil particles less than 0.84 mm in diameter; K = soil ridge roughness in relation to a 1:4 ridge height to spacing ratio; C = climatic factor, a function of average annual wind speed and the Thornthwaite precipitationevaporation index; f(L)
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Thus, it is equally unclear that cropland in such areas should be classed "highly erodible." Conversely, setting soil loss goals too high forces acreage that might suffer erosion damage into the nonerodible category. Under a 5 tons/acre/year goal, almost 20 Percent of cronland in the Northeast is considered nonerodible, while under actual T values (soil loss tolerance limit)
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Four Princinal conclusions can be drawn from the discussion. First, the LCCS is flawed when used in quantitative assessments of erosion potential on cropland because it fails to link land capability class-subclass designations with soil loss outcomes produced by the interaction of physical and management factors.
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The specification of highly erodible land depends on the level of conservation management it is reasonable to expect farmers to use. If soil loss tolerances cannot be achieved with feasible cropping systems, land should be taken out of crop production.
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1984. An improved soil erosion classification for conservation policy.
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It is possible to compare land capability classes with RKLS (physical erosion potential) classes, but not to substitute one for the other.
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But more important, the numbers increase as potential soil losses increase, making the relationship easier to comprehend and use in planning and making decisions on land use. Not only does the use of T values help to normalize the information, it also carries with it a subjective notion of the importance of soil loss in changing the soil environment and reducing long-term crop production Where sustainable production is influenced mainly by sheet and rill erosion rather than by wind erosion or by a combination of wind and water erosion, a classification based on RKLS/T appears to be a reasonable compromise and one that is feasible to implement at the field level.
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Perhaps up to one-third of the highly erodible land may someday be considered moderately erodible or at least controllable if improved crop cover and conservation practices are developed and adopted by farmers. This shift could occur if the best CP combination values were lower than at present.
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The agency is working closely with parts of the Soil Conservation Service to develop technology for the construction of data bases on soils information. A whole new technology is emerging in terms of geographic information systems that will allow users to combine this information, analyze it, and display it rapidly in many different forms.
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