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5 Assessing Conservation Practices and Land Classification Schemes One of the most important uses of the NFIs is to analyze the diversity of erosion conditions across the country. The inventories are also used to determine the effectiveness of conservation practices in mitigating erosion problems. This chapter describes new information on the dis- tribution of conservation practices that were recorded in the 1982 NRI. It also focuses on the analytical component shared by most new policy initiatives the classification of cropland according to its susceptibility to erosion-induced damages. Such damages may affect the land itself or water bodies to which it is tributary. Accurate identification of crop- lands especially susceptible to erosion is dependent upon a sound land classification scheme. Conservation Practices The 1982 NRI contains data on the conservation practices employed by farmers on all types of land. The survey was designed to record up to three practices per acre. Many acres are treated with combinations of practices: terrace systems in conjunction with contour farming, and conservation tillage with stripcropping, for example. The 1982 NRI results support three basic conclusions regarding con- servation practices that first emerged from analysis of the 1977 NRI (U.S. Department of Agriculture, 1981~. First, nearly 50 percent of the intensively cultivated cropland in the United States is treated with some conservation practice. Second, many of the practices currently in 75
76 SOIL CONSERVATION Miles of grass-backed, tile outlet terraces conserve soil and moisture on this farm in Montgomery County, Iowa. Conservation tillage is also used on the farm to control erosion. This is an example of a highly effective conservation system involving C- (cover and management) and P- (supporting soil conserva- tion practices) factor practices in accordance with a conservation plan. Credit: U.S. Department of Agriculture, Soil Conservation Service. use again, about half are used on land not subject to excessive ero- sion losses. Third, much of the land most in need of erosion control as defined by the USLE is not treated with any practice. Table 5-1 shows the distribution and frequency of application of con- servation practices according to ranges of erosion potential. Ideally, the frequency of application should increase as inherent erosion potential increases. This does not appear to be the case. For the nation as a whole, the percentage of acres treated with one or more conservation practices appears to decline with successively higher potential erosion. A slight increase is suggested for the Corn Belt, Iowa, and the highest potential erosion in Georgia. The percentage of acres treated appears relatively constant across the range of potential erosion. Practices on land with erosion potential less than 10 have probably been adopted for reasons other than erosion control, such as fuel and labor savings. The acreage treated with conservation tillage methods is increasing (see Chapter 1~. The purpose of such tillage practices is to leave crop residues sufficient to cover a minimum of 30 percent or more of the soil
TABLE 5-1 Acreage Treated with Consenration Practices In the Unfed States, the Corn Belt, and Selected Statesa Acres (millions) Treated with Given Number of Practices 220 93 55 20 17 8 7 92 44 27 11 4 48 26 25 14 18 9 8 4 8 4 2 2 4 2 3 > 150 Iowa 82 30 16 6 5 3 2 1 2 1 0.4 0.3 0.08 0.04 17 7 5 2 2 1 1 0.4 0.4 0.2 0.3 0.1 0.2 0.07 0.06 0.02 0.02 0.02 32 12 15 7 5 0.9 0.8 Erosion Potential (tons/acre year) United States 0-<10 10-<20 20-<40 40-<60 60- < 100 100- < 150 > 150 Corn Belt 0-<10 10-<20 20- < 40 40-<60 60- < 100 100- < 150 > 150 Georgia 0-<10 10-<20 20-<40 40-<60 60-<100 100- < 150 Total Acreage (millions) None One Two Three One or More One or More (percent) 129 49 27 9 8 0.4 4 0.3 4 58 53 50 47 47 47 47 4 1 22 46 44 48 49 50 49 51 2 1 11 2 0.9 1 0.6 0.5 2 0.8 0.7 0.3 0.2 0.1 0.08 0.2 0.09 0.1 0.03 0.04 0.01 0.01 0.01 0.002 0.001 4 2 2 0.6 0.7 0.4 0.1 0.1 0.03 0.02 30 28 28 36 34 30 39 0- < 10 9 4 4 1 0.2 5 56 10-<20 6 3 2 0.7 0.2 3 50 20-<40 4 2 1 0.6 0.2 2 50 40- < 60 2 1 0.7 0.4 0.1 1 50 60- < 100 3 1 0.8 0.5 0.2 2 66 100-< 150 2 0.6 0.5 0.3 0.1 1 50 > 150 2 0.6 0.4 0.3 0.1 1 50 Ohio 0- < 10 6 3 3 0.4 0.08 3 52 10-<20 3 2 0.8 0.2 0.02 0.1 35 30-<40 2 1 0.4 0.08 0.02 0.5 33 40- < 60 0.6 0.4 0.2 0.05 0.01 0.2 40 60- < 100 0.5 0.3 0.1 0.05 0.005 0.2 39 100- < 150 0.3 0.2 0.1 0.03 0.01 0.1 40 > 150 0.4 0.2 0.1 0.04 0.005 0.2 46 aStates were selected to illustrate regional differences. In addition, the numbers in this table have been rounded for the convenience of the reader. The precise numbers generated from NRI data are the statistical summations of all acreage represented by sampling points; they should be used for fur- ther technical analyses. SOURCE: 1982 NRI.
78 SOIL CONSERVATION surface. Erosion control benefits are proportional to the degree of cover left on the surface. Figure 5-1 shows the variability in the degree of protection afforded by conservation tillage practices. Sixty-six percent of the 100 million acres treated with conservation tillage in 1982 had C-factor values between 0.1 and 0.30. However, a considerable amount of the acreage reported in conservation tillage had C-factor values that were as high or higher than those expected under conditions resembling continuous plantings of corn, with the land plowed in the fall or spring. This finding reflects the diversity of practices that can be classified as conser- vation tillage, some of which leave limited residue cover. Erosion con- trol benefits on such lands are minimal. It also suggests a possible need to more carefully define and use the term conservation tillage. The committee believes that improved field estimates of surface resi- due cover should be used in future NRIs. Surface cover, incorporated crop residue (that near the surface), and roughness factors can be esti- mated and have a major effect on C-factor values. 40 10 o , i . .; iiii . 39.3 ~ 26.7 . 1 .6 ~ I I I I 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 o.s 1.0 C-FACTOR VALUE FIGURE 5-1 Distribution of C-factor values on cropland treated with conser- vation tillage, 1982.
ASSESSING CONSERVATION PRACTICES 79 Regional variations in the distribution of conservation tillage prac- tices are shown in Table 5-2. For example, MLRA 103, located in south- ern Minnesota and northern Iowa, contains about 14 million acres of cropland. Of that, about 13 percent was in conservation tillage in 1982. About 70 percent of the Cropland had an RKLS value of 10 tons/ acre~year or less, a very small potential for sheet and rill erosion. About 12 percent of these nonerodible or slightly erodible acres were treated with conservation tillage. Ideally, the percentage of acres in a given MLRA treated with conser- TABLE 5-2 Four MLRAs: Distribution of Acreage by Crop, Potential Erosion Class, and Conservation Tillage Percent Acres in Potential Erosion Class and in Conservation Tillagea Cropland Conservation Crop (1,000 Tillage (RKLS~n tons/acre year) and MLRA acres) (% acres) < 10 10-20 > 20 - MLFUt 103 Corn 7,162 14 69 (13) 18 (16) 13 (18) Soybeans 5,854 13 72 (12) 16 (16) 12 (16) Wheat 388 10 76 (11) 17 (7) 8 (3) Cropland 14,443 13 7o (12) 17 (15) 13 (16) MLRA 105 Corn 3,166 28 13(19) 15(25) 72(30) Soybeans 198 15 31 (5) 21 (19) 47 (20) Wheat * * * * * Cropland 5,820 23 11 (14) 12 (23) 77 (24) MLFUk 134 Corn 445 21 6 (14) 29 (15) 65 (24) Soybeans 4,293 16 4 (9) 44 (13) 52 (19) VV heat 552 23 2 (12) 33 (21) 65 (25) Cotton 860 4 7 (3) 43 (7) 50 (3) Cropland 7,469 14 4 (7) 44 (13) 52 (17) MLFUY 136 Corn 804 12 7 (8) 20 (20) 73 (10) Soybeans 1,149 18 3 (12) 15 (11) 82 (20) Wheat 744 18 3 (56) 14 (11) 83 (18) Cropland 4,223 12 4 (17) 15 (11) 81 (12) aPercent conservation tillage is given in parentheses. *Number of sampling points too small. SOURCE: Pierce et al., 1986.
80 SOIL CONSERVATION The soil is relatively undisturbed when the no-till method of conservation tillage is used (Montgomery County, Maryland). The field will be sprayed for grass and broadleaf weeds after planting. Erosion will be reduced to less than one-tenth the rate expected from a comparable field under conventional tillage. Credit: U.S. Department of Agriculture, Soil Conservation Service.
ASSESSING CONSERVATIONPRACTICES 81 vation tillage should increase significantly in land groups with progres- sively higher inherent erosion potential. This is not the case in MLRA 103; it is true in MLRA 134 (the southern Mississippi Valley silty uplands of Mississippi, Tennessee, and Kentucky), where the propor- tion of land treated with conservation tillage increases substantially from 7 percent in the lowest RKLS groups to 17 percent in groups with RKLS values greater than 20. These and other regional variations dem- onstrate the existence of widely differing rates of adoption of conserva- tion tillage technologies on erosion-prone cropland. Several studies based on 1977 NRI data indicated that much of the land in conservation tillage had a modest potential for sheet and rill erosion before the practice was adopted (American Farmland Trust, 1984~. Two-thirds of the land treated with conservation tillage in the Corn Belt in 1977 had an inherent sheet and rill erosion potential of less than 20 tons/acre~year, which is below the national average RKLS value of 21.7 tons/acre~year reported for all cropland. Concentration of Sheet and RiZ! Erosion on CropZand One of the most significant practical applications of the NRI is likely to be new research leading to further documentation of the extent and geographical characteristics associated with the concentration of exces- sively erodible land on a relatively small portion of the land base. The concentration of sheet and rill erosion was pronounced on land used to produce close-grown crops in 1982. About 84 percent of that land (97.6 million acres) had sheet and rill erosion rates less than 5 tons/ acre~year; only about 3 percent had rates greater than 15 tons/acre~year. It is important to note, however, that much of the acreage planted to close-grown crops, wheat in particular, is in areas where wind erosion is the chief soil conservation problem. Nevertheless, land in row and close-grown crops that erodes at rates above 15 tons/acre~year accounts for more than 40 percent of the total sheet and rill erosion of land in those uses. Table 5-3 illustrates the relatively large proportion of cropland with modest potential for sheet and rill erosion. About 277 million acres of cropland and 133 million acres of the land used for row crops about two-thirds of the total acreage in these uses had an inherent potential for sheet and rill erosion of less than 15 tons/acre~year. Under average farming conditions (C-factor value of 0.30, P-factor value of 0.91), sheet and rill erosion rates would average less than 4 tons/acre~year on this land. The remaining 140 million acres of cropland had an inherent potential erosion rate of more than 15 tons/acre~year; about 52 percent of that acreage was used for row crops.
82 SOIL CONSERVATION TABLE 5-3 Cropland Uses by RKLS Factor, United States, 1982 (million acres) Potential Erosion, RKLS Row Close-Grown Other (tons/acre year) Crops Crops Hay Crops Total 0- < 5 44.8 38.0 12.0 19.4 114.3 5- < 10 56.7 28.0 6.5 13.5 104.8 10- < 15 31.6 16.6 3.0 6.7 58.0 15-<20 18.1 9.3 2.6 4.1 34.1 20-<20 10.8 6.1 1.5 2.4 20.8 25- < 30 7.3 4.2 1.4 1.8 14.7 30- < 35 5.4 2.9 1.0 1.3 10.5 35- < 40 4.0 1.9 1.0 1.0 7.8 40- < 50 6.0 2.6 1.4 1.5 11.5 50- < 75 8.9 3.2 2.1 2.3 16.6 75- < 100 4.7 1.4 1.4 1.3 8.8 > 100 7.9 1.6 3.4 2.7 15.6 Total 206.3 115.6 37.5 58.1 417.5 SOURCE: 1982 NRI; adapted from Rosenberry and English, 1986. The committee has given special attention to lands with extremely high potential rates of sheet and rill erosion (50 tons/acre~year or more). About 41 million acres, or 9.8 percent of total cropland, falls into this highly erodible category. On lands susceptible to high rates of sheet and rill erosion, it is generally difficult and costly to devise effective conservation farming systems. Implications for Policy and Program Administration The 1982 NRI has advanced the understanding of the concentration of soil erosion and the distribution and effectiveness of soil conserva- tion practices. The papers included in Volume 2 of Soil Conservation: Assessing the National Resources Inventory are representative of analytic capabilities that are possible using the 1982 NRI data. Their major con- clusions have important implications for government policy and pro- grams and for the design of on-farm conservation strategies and the mitigation of adverse offsite effects of erosion. Because of state and federal budget constraints, new public and pri- vate initiatives are needed to enhance the cost-effectiveness of soil conservation investments.
ASSESSING CONSERVATIONPRACTICES 83 Targeting It is probable that traditional USDA conservation pro- grams will receive significantly less funding in the future. Budgets for technical assistance and cost-sharing programs will probably remain near current levels in nominal dollars. Some states are considering ways to compensate for reduced federal investment in conservation. For example, Missouri voters recently enacted a special tax to generate income for conservation cost sharing Johannsen, 1986~. Presentations based on analysis of data from the 1977 NRI were instrumental in convincing the Missouri legislature of the merits of this special tax. More reliable techniques should be developed for targeting public and private soil conservation investments according to the potential to affect on-farm productivity, offsite damages stemming from soil ero- sion, or both. In particular, future targeting directed to onsite damages increasingly should be based on indices of the relationship between erosion and productivity. Models such as the Productivity Index (PI) and the Erosion-Productivity Impact Calculator (EPIC) have the poten- tial to quantify more precisely the physical and economic effects of erosion damage and identify land that is acutely vulnerable to erosion damage. Application in the field will require an appropriate data base that includes information from soil surveys. Soil conservation activities, public and private, should be systematically targeted or concentrated much like erosion toward those lands that are most susceptible to soil erosion damage or that contribute most significantly to seri- ous offside pollution problems. The committee recommends that the USDA expand the scope and shorten the timetable of targeting soil conserva- tion programs toward the most fragile cropland, rangeland, forestland, and pastureland. Under current fiscal constraints it is likely that, to be effective, targeting will need to be more selective than previously expe- rienced in the conservation field. If erosion is to be controlled, long- term land diversion programs designed to convert highly erodible cropland into stable forage- or forestry-based land uses are needed. Erosion Reduction Goals Estimates of soil erosion are imperfect indi- cators of actual on-farm or off-farm damage. A better understanding of erosion processes is clearly desirable to enhance the ability to quantify and predict the effects of different forms of erosion on soil productivity and environmental quality. At the same time, increases in the sophisti- cation of predictive models and the data bases necessary to apply them often come at exponentially higher costs. The committee believes that the practical conservation benefits likely to result from expanded data sets and new modeling exercises should
84 SOIL CONSERVATION be more systematically appraised in establishing budgetary priorities. The sophistication and cost of some models have already become pro- hibitive for national applications. Identification of the appropriate mix of research on the fundamental process of erosion and the role of improved models and data for future conservation policy and pro- grams should be more clearly articulated to maximize benefits from limited funds. Modeling and data needs will change as the scientific understanding of conservation needs changes. In the last few years, for example, optimism over the success of conservation tillage in controlling erosion in many parts of the United States has been tempered by concern and supported by very limited information suggesting that conservation tillage, coupled with common fertilizer and pest management prac- tices, might increase the level of pollutants entering ground and surface waterways. In response, however, considerable research is currently under way to reduce the reliance on chemicals in conservation tillage. Careful analysis is needed to ensure that future NRIs, new empirical models, and new methods will provide the information necessary to meet the dual challenge of soil and water conservation, both qualita- tively and quantitatively. The committee believes that reducing erosion and enhancing soil productivity while conserving water and protecting water quality in the broadest sense must become the dominant objec- tive in soil conservation policy. The USDA and state conservation agencies should continue to emphasize gross erosion reduction goals and accomplishments in program evaluation, design, and administration. Whenever possible, erosion reduction goals relat- ing to productivity should be redefined in accordance with available, reliable indicators of a soil's susceptibility to erosion damages. Recent work on productivity indices and the ways that erosion may influence potential productivity losses over time is beginning to pro- vide more sound quantitative measures of the impact of erosion. The complexity and variability of the processes were noted in Chapter 3. The ratio of inherent erosion potential to estimates of soil loss tolerance limits proposed by the USDA Fragile Soils Work Group has the merit of recognizing the importance of including both erosion and a measure of productivity in an indicator of damage potential. At the same time, uncertainties inherent in the estimates of T values remain, and the ratios are surrogates rather than measures that attempt to capture the impact of erosion on potential productivity over time. With continuing research, these processes can be further refined. They should be made an integral part of USDA program evaluation and management as soon as possible.
ASSESSING CONSERVATIONPRACTICES The Land Capability Class System 85 For many years soil information has been collected for most of the intensively cropped regions in the country through the National Coop- erative Soil Survey Program. Most of the acreage surveyed was assigned to one of eight classes and four subclasses in the SCS Land Capability Class System (LCCS), following subjective criteria and the judgment of regional experts. The LCCS classes of lend range frombest (class I) to most limited for agricultural production (class VIII). The numbers designate the severity of the problem for crop uses. The letters en w, s, and c indicate whether the problem is caused by erodibility; wetness; stoniness, shallowness, or drought; or climate, respectively. Like any classification system, the LCCS was designed to satisfy specific objectives. The variables selected and the range of classes were dictated by the objectives and the scale of intended use. Interest was focused on a number of factors related to agricultural production. The LCCS is a valuable tool, but it was not designed to provide quantitative estimates of either actual or potential erosion rates. In addition, when this system is used, it is often difficult to distinguish the physical char- acteristics of the land at the time of mapping from the effects of manage- ment practices that have been used in previous years. The committee does not focus on the broad, possible uses of the LCCS, but only on limitations of the LCCS specifically related to the delineation of erosion and alternative approaches that might better meet this objective. By using the erosion data in the 1977 and 1982 NRIs, particularly the USLE information, land management practices can be taken into account. Analyses of 1977 and 1982 NRI data raise questions about the utility of the LCCS for applications involving the calculation of erosion rates before or after applications of soil conservation practices. The LCCS has been used for more than 30 years; alternative schemes might be better suited to contemporary applications concerned with land erosion and conservation management. Factors of Inconsistency Several factors probably contribute to LCCS incongruities with respect to inherent potential and actual erosion rates. The system was essentially devised and implemented prior to the development of quantitative methods of estimating erosion in the field. Another impor- tant source of variation within the system is the local, subjective nature of LCCS determinations by SCS field personnel. When the system was developed 50 years ago, there was no need for categories of land group-
86 SOIL CONSERVATION ings that would be consistent across the country. However, the marked variation in erosion conditions, even within localized areas such as MLRAs, remains today. The LCCS was devised to categorize land according to its physical characteristics, without taking into account management practices such as conservation tillage in use at the time of classification. For the purposes at hand, the LCCS could be improved by reclassifying soils according to estimates from the soil erosion equations. This could be accomplished by basing reclassification on the USLE and Wind Erosion Equation (WEE) factors that reflect unchanging climate and soil charac- teristics: the RKLS product for sheet and rill erosion and the I, C, and L factors for wind erosion. The NRIs provide very useful information for updating LCCS classifications and developing alternative classification schemes. Rates of Erosion: A Basis for Classification The observation that suggests a need for a new land classification scheme emerged from analysis of the distribution of erosion rates on land classified within given LCCS land classes and subclasses, which was reported by Heimlich and Bills (1986~. They found that there is a wide variation of erosion and inherent erosion potential on lands classi- fied within the same class and subclass. This is true for most regions and throughout the country. Moreover, land categorized within speci- fied ranges of potential erosion with RKLS products between 20 and 30, for example also is often erratically distributed across several dif- ferent land classes and subclasses in the LCCS. For example, about 59 million acres of the total cropland acreage were classified in LCCS class IIIe in the 1982 NRI (Table 5-4~. Cropland classi- fied as IIIe is generally considered suitable for intensive cultivation, with appropriate conservation measures. Surprising variation, how- ever, is found in NFl values for the inherent potential for sheet and rill erosion on class IIIe land nationwide. Land within that class (and oth- ers) differs substantially in its vulnerability to erosion. At one extreme, class IIIe cropland includes about 21 million acres that have an inherent erosion potential (RKLS product) of less than 10 tons/acre~year, according to 1982 NRI data. Wind or ephemeral gully erosion might be significant problems on some of this land; however, sheet and rill erosion are not. Under average management conditions (represented by a C-factor value of 0.30) average annual erosion rates of less than 2 tons/acre~year would be expected. This class IIIe acreage with a low erosion potential contrasts dramati-
87 cn ~ C~ CN C~ o o o o o o o _ _ =~o<>ooo ~ : ~ =; o U ~ _ _ C~ ~ d~ 45 ;~] ·~ P ~ ¢ o E~ ~ CN ~ ~ o o o o o o o o . . . . . . oo o o o o o o o ~ o C . . o o C ~D d~ ~ CN ~ O O O ~ ~ ~ ~ o o o o Lr) o o o o o o o . . . . . . . . . . . . o o o o o o o o o o o o e~ ~ ~ ~ ~ o o o o o o o . . . . . . o o o o o o C~ o o ° 1 o ~ ~ ~ o o o o C o o o o o o o ~ C oo o o ~ o o o o Lr) ~ ~ o o ~ o 0 0 0 0 0 ~ o 0 0 0 0 0 0 0 di ~ ~ ~ C~ ~D ~ O O ~ O O O O O O O ~D ~ O O O O ~ o o o o o oo o o o o o o o o ~D ~ ~ CN ~ ~ ~ o o o ~ o o o oo o o o o ao ~ o o o o . . . . . . . . . . . O ~ O O O O O di ~ O O O O O O r) o o o o o o ~: . . . . . o o o o o ~ C ~ C~ o o , o ~ ~ C . . o ~ ~ o o C ·n ~ O O ~ ~D . . . . o o o o C~ ~ ~ ~ oo C ~ Lr d~ ~ ~ ~ ~ o o o o o o . . . . . o o o o o o Lr, o o o o o o . . . . . o o o o o ~ o C . . d~ ~ ~ O O C C~ o ~ di o o o o o o o o o o . . . . . . . . o o o o o o o o ~ ~ oo o o o ~ ~ ~ U~ o o o o o o o o ~ ~ ~ . . . . o o o C ,,> , oo O ~ ~ ~ di ~ O ~D L~ ~ ~ ~ 0 0 0 0 0 0 . . . . 00 ~ ~ 0 0 0 0 ~ ~ 0 0 ~ 0 0 0 0 ~ ~ ~ 0 0 0 0 0 V) 0 0 0 0 O O O O O ~ O O O O O LO C-~ \o ~ ~-4 ~ ~ =4 ~ ~ ~ V V V V V V <5,, V V V V V V 0 0 _ ~ 0 0 0 0 0 0 0 ·S: ~ ~ ~ t-1 ~ ~ ~ ~ ~ ~ A , - ~ A Ct ._ C~ ._ ~_ z ~o ~4 U) ~5 a~ u a, ~ it .= . _ ~ ~, ~ o E~ U) _ o ,= U, o > ~ ·` Z ~ h~ ._ .= ~o ~ o ~ CO ~ ~ · CD ,~ ~ cn ~ co ~ ~ ~ ~ Z ~ O O ~ ·. - CL ~ .. e~ E~o ~ t=~ Ct ~ O
88 SOIL CONSERVATION catty with another 11 million acres, also classified IIIe, that has an RKLS value of more than 60 tons/acre~year. Farmed under average condi- tions, this land 18 percent of the total IIIe class would erode at rates of 15 tons/acre or more annually. U. S. cropland classified as IIIe would exhibit large variations in sheet and rift erosion rates under comparable cropping and management conditions. The climatic and physical characteristics that affect sheet and rill erosion, as reflected by the RKLS product, are not homogene- ous within the LCCS class IIIe nationwide. Similar variability in inher- ent sheet and rill erosion potentials is observed within all land capability classes and within most subclasses. The committee considered the hypothesis that wind and ephemeral gully erosion problems might explain why cropland with low RKLS values might be classified as IIIe. The NRIs offer no insights into the ephemeral gully erosion question, because the inventories do not con- tain estimates of that form of erosion (see Chapter 3~. The committee believes that it is unlikely that ephemeral gully erosion accounts for much of the classification discrepancy, because the severity of ephem- eral gully erosion also appears to be correlated with slope gradient, slope length, and rainfall- critical factors in the USLE. However, the presence of wind erosion problems on class IIIe crop- land can be investigated through the NRIs. Analyses of the 1977 NRI indicate that wind erosion often is a problem on land that has a low RKLS value and is located mainly in the western United States (Ameri- can Farmland Trust, 1984~. The critical USLE factor in this combination is R (rainfall), the value of which is very low in areas prone to heavy . . . wind erosion. Regional Analyses Wind erosion probably plays a minimal role in accounting for LCCS inconsistencies according to actual rates of erosion; the inconsistencies are also evident in several MLRAs where wind erosion has never been a significant problem. Comparisons of the highly erodible acres in a given region by sub- class show similar inconsistencies. For example, serious erosion prob- lems exist in MLRA 136, the southern Piedmont of the southeastern United States, when row and close-grown crops are produced on land that has sheet and rill erosion rates in excess of 60 tons/acre~year. Yet, as indicated in Table 5-5, more land with an RKLS product greater than 60 tons/acre~year is classified as class lie than as class IVe (185,000 com- pared with 109,000 acres, respectively). Subclasses lie and IIIe com-
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So SOIL CONSERVATION bined have almost four times as much row and close-grown cropland in the 60 tons/acre year and over RKLS group than is reported in class IVe. Recent SCS staff reports have investigated the usefulness of the LCCS to define erosion potential on cropland with data from the 1982 NRI for 17 MLRAs. One study (Lee and Goebel, 1984) divided 97.5 million acres of cropland into four groupings, following the RKLS- based system developed by Heimlich and Bills (1986~. The analysis found considerable variation in RKLS values for individual land capa- bility classes. For example, about 64 percent of the class IIIe cropland acreage in the 17 MLRAs studied (about 10 million of 15.8 million acres) had an inherent erosion potential exceeding 50 tons/acre~year and esti- mated erosion rates greater than 5 tons/acre~year. About 4.7 million acres (80 percent) of the 5.9 million acres classified as IVe fit this cate- gory. About 18 percent of the class lie cropland (3.9 million acres) also fit this RKLS-based definition of highly erodible. Thus, the definition highly erodible based on the LCCS would inappropriately classify a considerable portion of cropland. Some land that belongs in this cate- gory would be excluded, and some land with effective erosion control would be included. These observed incongruities do not seriously compromise the use- fulness of the LCCS as a tool for farm-level conservation planning. A major value of the system is that the subclass e designation signals the landowner and the conservationalist that there is a need for some type of soil conservation treatment. Typically technicians from the SCS or local conservation districts present the land owner with conservation options, tailored to specific fields, that would control erosion to varying degrees at varying costs. This type of planning usually involves estima- tion of the sheet and rill erosion rates that would be expected before and after the use of select conservation practices. Accordingly, conservation planning generally reflects particular field conditions, regardless of the existing LCCS classifications. Changes in Land Use Based on the committee's review of data on the inherent erosion potential of cropland and the distribution of conservation practices, it is clear that new policies and programs deserve study. In addition, new initiatives might be needed to discourage conversion of more erosion- prone cropland to cultivated uses. The committee believes that more attention should be directed toward encouraging desirable land use changes in shaping cost-effective future policies. The maintenance of permanent vegetative cover on land with a high potential for sheet, rill,
ASSESSING CONSERVATIONPRACTICES 91 and wind erosion should figure prominently in future program design, especially during periods of surplus production. Because of the concentration of soil erosion on the acreage support- ing major crops, including wheat, feed grains, soybeans, and cotton, the committee suggests that permanent vegetative cover be used as a tool for conservation strategy. As shown earlier in Table 5-3, a relatively small portion of cropland has very high levels of inherent potential for erosion. Effective conservation systems on such lands are often prohibitively expensive; thus, the land may be unsuitable for intensive crop production from a resource management perspective. Furthermore, such land generally provides lower and more variable yields, even if well-designed soil conservation and other management practices are applied. Recent analysis of pastureland, hayland, and other land converted to cultivated crop uses between 1979 and 1981 indicates that about 19 percent of this land (about 2.1 million acres) is highly erodible (Heimlich, 1985~. (Heimlich defines highly erodible land as land that erodes above its tolerance value, even under the best management; this is a condition that is assumed to reflect adoption of the most effective, feasible conservation system.) In contrast, 7.1 per- cent of all cropland, nearly 30 million acres, meets this definition of highly erodible. When in stable grass or forestry uses, these lands generally erode at very low rates, about 1 to 3 tons/acre~year. Permanent vegetative cover might be used as a conservation option on particular lands. Alternative Land Classification Schemes Enormous pressure exists today to substantially reduce federal farm program expenditures that have cost over $15 billion annually in recent years and are projected to remain over $10 billion annually throughout the next several years. Several options to reduce these program costs through new conservation initiatives have been studied. One option, the so-called sodbuster provision that was proposed as part of the 1985 Farm Bill, would deny specific USDA program benefits to farmers who cultivate highly erodible land that has not been culti- vated for a period of five years. Another option, the conservation reserve, would offer land rental payments to farmers who voluntarily retire erosion-prone land currently in cultivation and put it to long- term, soil-conserving uses. Findings from analyses of the NRI data illustrate the high levels of erosion concentrated on limited land areas. To make the adoption of such options more administratively feasible and more cost-effective,
92 SOIL CONSERVATION the committee concludes that an alternative land classification system should be developed that more accurately classifies cropland according to its susceptibility to erosion and erosion-induced damages. Proposed Options to the LCCS National and regional analyses of the 1982 NRI demonstrate that the choice of a land classification scheme and criteria are of critical impor- tance in considering new conservation policy initiatives. The commit- tee believes that the 1982 NRI the USLE data, in particularcon- stitutes an adequate technical basis for the design and implementation of alternative classification schemes. A number of new schemes have been proposed in recent years, based on combinations of the factors in the USLE, soil loss tolerances, or the WEE (Heimlich and Bills, 1986~. The committee recognizes that these are among a number of possible options that could be formulated. They are noted here because they illustrate the kinds of information needed, the element of judgment involved, and the degree to which the geogra- phy and size of areas designated in various erosion classes depends upon the data and equations used. One approach, originally advanced by the American Farmland Trust, categorizes land into one of three groups based upon progressively higher ranges of inherent potential for sheet and rill erosion (RKLS value) and proposes comparable groupings based on the potential for wind erosion. Another system, developed by Heimlich and Bills (1986), uses RKLS ranges that differ from those in the American Farmland Trust system and different assumptions regarding C- and P-factor val- ues. Table 5-6 contains a comparison of the total cropland acreage that would be considered highly erodible nationwide under several alterna- tive systems. Depending on the criteria and system applied to the 1982 NRI, analyses showed that between 24 million and 89 million acres of cropland would be considered erosion prone, or highly erodible. The scheme under most intensive study by USDA would include between 32 million and 65 million acres of land considered eligible for a conser- vation reserve program, depending upon whether the criterion is RKLS/T > 10 or RKLS/T > 15 and whether the actual erosion exceeds twice the relevant soil loss tolerance limit (D. G. Burns, USDA, per- sonal communication, 1985~. The USDA approach improves upon RKLS-based systems only to the extent that T-factor values accurately reflect a soil's susceptibility to erosion. Notwithstanding the known shortcomings of T values, as
ASSESSING CONSERVATIONPRACTICES 93 TABLE 5-6 Acreage of Highly Erodible Cropland as Calculated Under Alternative Land Classification Criteria Land Classification Acres Option Criteria (million) USLE > 2T 50.9 WEE > 2T 35.4 LCCS (IVe, VIe, Vile, VIII) 49.4 AFTa RKLS > 75 24.0 RKLS/T or CI/T > 10 89.0 RKLS/T or CI/T > 15 49.7 USOA RKLS/TorCI/T > 10, eroding >T 64.6 RKLS/T or CI/T > 10, eroding > 2T 52.7 RKLS/TorCI/T > 15, eroding >T 36.9 USDA RKLS/TorCI/T > 15, eroding >2T 32.0 American Farmland Trust. SOURCE: Based on 1982 NRI data. noted earlier, the committee believes that the inclusion of concepts relating potential erosion to productivity is a step in the right direction. However, improved calculations of T values, incorporating the results of the PI and EPIC models, are needed. In particular, the common range of T-factor values (currently 2 to 5 tons/acre~year) should be refined in accordance with the true susceptibility of soils to erosion-induced pro- ductivity losses. Improving the accuracy of the relationship will improve such classification schemes. An advantage of the proposed USDA system is that it can be devel- oped and implemented for most policy purposes by using existing information in soil surveys and the 1982 NRI, supplemented by mini- mal field work. In addition, improvements in the system such as those expected from improved estimates of the relation of erosion to potential productivity can and should be readily incorporated into the system without requiring new policies or altering the impact of new conservation policy initiatives. The committee believes that further analysis is needed to fully evaluate the suitability of alternative land classification schemes used to identify highly erodible or fragile lands, particularly in relation to policy initiatives. Improper classification of croplands could seriously undermine the effectiveness of a program initiative. For example, con- fidence limits must be calculated for NRI acreage estimates at the MLRA and smaller levels of aggregation. The geographic distribution of land that would be subject to alternative definitions should be assessed. The
94 SOIL CONSERVATION implications of a provision that would exempt land cultivated even once during a designated grace period from sodbuster sanctions requires additional study. Similar concerns arise in consideration of conservation reserve pro- grams. Like the sodbuster provision, to the extent that erosion is of major concern, the eligibility criterion for placing land into a conserva- tion reserve should employ a land classification system based on the inherent potential and actual erosion of the land rather than a land capability class and subclass designation. Classification schemes based on erosion equations are imperfect. Their basic shortcoming is that gross erosion rates do not always accu- rately reflect the effects of erosion on soil productivity or the degree of offsite damages from erosion. Ideally, future classification systems for erosion hazards can be based on more explicit criteria, such as PI and EPIC model results. In addition, similar explicit criteria will be needed to classify land, at least in part, according to its potential to cause nonpoint pollution problems.
