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1 Scope and Content of the 1982 NRI Between the spring of 1980 and the fall of 1982, the Soil Conservation Service (SCS) of the U.S. Department of Agriculture (USDA), as directed by Congress, conducted the most extensive inventory of land and water resources ever undertaken in the United States. SCS person- nel recorded over 70 observations on resource conditions and land use at each of about one million locations across the country. The collected data were entered into computers. The approximate cost of the project was $15 million. The product of this project is the 1982 National Resources Inventory (NRI): a computerized natural resource data base covering all nonfed- era1 land in the United States. The extensive and in some cases unique contents of the 1982 NRI make it a primary source of data for research- ers, government program administrators, and policymakers. Such an extensive survey will not be conducted again until 1987. The 1982 NRI is the most recent of a series of national resource sur- veys and inventories performed by the SCS, beginning with the Ero- sion Reconnaissance Survey of 1936. The first NRI was conducted in 1977 in anticipation of the passage of legislation directing the USDA to evaluate resource conditions and trends. The Soil and Water Resources Conservation Act of 1977 mandated that such an inventory continue on a five-year cycle. The most important inventory items in earlier surveys were land use, conservation treatment needs, and soil classification. The 1982 NRI and its predecessor, the 1977 NRI, include additional information on prime 1
2 SOIL CONSERVATION farmland; the physical and economic potential for future conversion of forest, pasture, and rangeland to cropland uses; incidence of types of wetlands; susceptibility of types of land to flooding; and incidence of soil conservation practices applied on land. New resource items were added for the 1982 inventory, including data on critically eroding areas, riparian vegetation, wildlife habitat diversity, and vegetative cover con- ditions on rangeland and forestland. (See the Appendix for a formatted listing of the basic data in the 1982 NRI.) The increased sampling density of the 1982 NRI substantially improves the statistical reliability of the data base and permits, for example, more reliable analysis of smaller geographic areas than was previously possible. Uses of the 1982 NRI The Committee on Soil Conservation Needs and Opportunities iden- tified five important applications for the data contained in the 1977 and 1982 NRIs. These uses demonstrate the value of this data source for investigating many aspects of land use and quality, especially soil ero- sion and conservation. Summaries of the five areas of application fol- low. Trend Analysis Direct comparisons of surveys are not always possible or advisable because of changes in inventory definitions, procedures, sample sizes, and estimation techniques (see Comparing Results from the 1977 and 1982 NRIs). Generally, however, the definitions and procedures per- taining to cropland and its use a major land use category for erosion studies are consistent. As a result, data on conversion of land into and out of cropland use between 1977 and 1982 are reliable. Such land use shifts often have a significant effect on soil erosion. To some extent, trends of this type can also be analyzed using the 1982 NRI alone, because for each sample point land use is identified for the three years prior to sampling. Although rotations were not adjusted to a common 1982 year, Ogg's analysis (1986) of 1982 NRI data suggests that land converted to crop use between 1979 and 1982 generally pro- duced lower yields and eroded at rates higher than average for all cropland in 1982. With some cautions and exceptions, erosion rates reported for major land uses in the inventories can be compared to indicate changes from 1977 to 1982.
SCOPE AND CONTENT OF THE 1982 NRI Classification of Soils According to Rates of Erosion Erosion estimates contained in the NRIs permit analysts to identify and investigate areas with particularly high or low erosion rates. As rudimentary as such information may seem, it has not been available until recent years. With the steadily declining buying power of most conservation program budgets, program managers would benefit from more reliable indicators of the most serious problems and from an assessment of the effectiveness of alternative strategies. Reliable indi- cators are vital to efforts now under way to target government pro- grams to the most immediate resource problems. 3 Identification of Needs and Opportunities The NRIs provide a record of the types of conservation measures in use, where they have been adopted, and their effects on erosion. Already, this information has had a notable effect on state and federal conservation policies and programs (American Farmland Trust, 1984~. The NRIs also enable researchers to correlate estimates of rates of erosion observed at thousands of points in the field to other data bases. Thus, they can test hypotheses within the limitations of these data- about the relationships of erosion to crop yields and sediment loads in waterways, and the effects of alternative conservation strategies. Data on sheet and rid erosion from the 1977 NRI have been used to investigate the relationship between soil erosion and crop yields across broad geographic areas. Some studies have been made possible by the capability to link NRI data via computer with detailed soil information contained in the computerized SCS soil survey file, Soils-5. (Soils-5, more formally called SCS-SOI-5, is a compilation of soil interpretations data and contains descriptions of the properties of all soils in the United States.) Investigations of erosion-productivity relationships have led to research on the amount of erosion that soils can withstand before seri- ous and perhaps irreparable damage is done to their capacity to sustain long-term agricultural production (Pierce et al., 1983~. Relating the NRI to Water Quality and Other Enz~ironmentat Issues Both inventories appear to have limited applications for direct analy- sis of water quality problems. But as this area of interest becomes
4 SOIL CONSERVATION increasingly important in the next decade, new uses might be found for NRI data, possibly in conjunction with new data added to future inven- tories or as an extension to the 1977 and 1982 inventories. There is a need to enhance future surveys by providing data, where possible, that are relevant to problems of groundwater and surface water contam~na- tion by dissolved and particulate substances. Erosion data from the NRIs might be combined with a number of mathematical models devel- oped to analyze water quality problems, particularly those associated with sediment from agricultural land uses (Christensen, 1986~. Inqumes made to USDA by various agencies indicate an interest in other potential research and policy applications of the NRI (G. Nord- strom, SCS, personal communication, 1984~. The National Oceanic and Atmospheric Administration plans to use the NFI to study land use trends in coastal areas. The Environmental Protection Agency has expressed interest in using the NRI in its studies of acid rain, nonpoint pollution, pesticide use patterns, and other environmental issues asso- ciated with land use patterns. And the U.S. Geological Survey has requested NRI data for use in analyzing observed trends in water qual- ity and for research on nitrate contamination of shallow groundwater. Comparing Results from the 1977 and 1982 NRls Table 1-1 shows the percentage of land in each of the major rural categories for the 1967 Conservation Needs Inventory (CNI) and the 1977 and 1982 NRls. Forestland increased by 1.5 percent between 1977 and 1982. However, since USDA did not have a uniform defini- tion for that land use until just prior to the 1982 NRI, some land classified as pastureland, rangeland, or other rural land in 1977 was reclassified as forestiand in 1982. Additional analysis, especially at the regional or Major Land Resource Area (MLRA) level, could help determine how much of this apparent increase is attributable solely to definitional changes. (MLRAs consist of geographically associated land resource units; groupings by patterns of soil, climate, water resources, land use; and type of farming. A state, for example, might have between 6 and 12 MLRAs.) The use of improved inventory procedures resulted in an apparent decline of nearly 16 million acres of land in built-up use between 1977 and 1982 (see Table 1-2~. Built-up areas include cities, villages, industrial sites, cemeteries, airports, golf courses, and similar areas. This 18 percent change does not reflect an actual decline in built-up
SCOPE AND CONTENT OF THE 1982 NRI TABLE 1-1 Use of Nonfecleral Rural Land (percent), 1967-1982 Land Use 1967a 1977b 1982' Cropland 29.9 29.5 29.8 Pastureland 35.26 9.5 9.4 Rangeland 29.1 28.7 Forestland 30.9 26.4 27.9 Other rural land 4.0 5.5 4.2 Total 100.0 100.0 100.0 aUnadjusted data, 1967 CNI. b1977 NRI. -C1982 NRI, preliminary data. Includes rangeland. SOURCE: 1977,1982 NRI; 1967 CNI. acreage during that time but an overestimate of built-up acreage in 1977. Some of this land was reciassifiecl as rural in 1982 as a result of more accurate expansion factors and other improved inventory pro- cedures app! fed retrospectively to the 1 977 survey resu Its. This reclas- sification and other instances of improved assessments of acreage and classification illustrate the importance of statistical clesign. The SCS is conducting a series of studies clesigned to rectify some of the problems caused by definitional and procedural changes. Until these studies have been completed, NRI users who want to compare data from 1982, 1977, or earlier years might choose to perform pre- liminary analyses of comparative data for discrete geographic areas that can be confirmed by other sources of information. This analysis can validate the accuracy of NRI-basecl methocis. TABLE 1-2 Nonfederal Lancl Uses (million acres), 1967-1982 Land Use 1967a 1977b 1982' Rural lands 1,438 1,401 1,414 Urban, built-up, rural transportation 61 90 74 Small water bodies 7 9 10 Total 1,506 1,500 1,498 Unadjusted data, 1967 CNI. b1977 NRI. NOTE: The decline in rural land from 1967 to 1977 is apparent, not real, and results from improved identifications in the NRI. C1982 NRI, preliminary data. Includes cropland, pastureland, rangeland, forestland, and minor land cover/uses. SOURCE: 1977,1982 NRI; 1967 CNI. 5
6 ToolforResearch and Policy Analysis The NRIs have had an important influence on agricultural policy and program administration. The availability of nationally consistent, sci- entifically derived estimates of soil erosion has made it possible for policy analysts to examine the distribution of national and regional erosion problems more effectively than ever before. For example, analyses of the 1977 NRI revealed that high rates of soil erosionand most of the total tonnage that was eroded were concen- trated on a small amount of land. This finding led USDA and others to propose a number of policy and program changes designed to target, or concentrate, federal conservation assistance to specific geographic areas. The inventories have also made possible analyses of the distribu- tion of soil conservation practices nationwide. As a result, policyma- kers now have a better understanding of the amount of conservation work that needs to be done and the techniques that are most likely to be effective. Information on erosion and conservation practices contained in the NFIs suggests the potential value of focusing new conservation pro- grams and policies on particular lands. A proposed soil conservation reserve, for example, would offer land rental payments to farmers who voluntarily retire erosion-prone land currently in cultivation. Erosion information has also been used to evaluate so-called sodbuster policies, which propose to eliminate a farmer's eligibility for federal commodity program subsidies if highly erodible land not currently in production is converted to crop uses that are covered by commodity programs. Iden- tification of highly erodible lands a source of sediment and other pol- lutants that can affect water quality may also contribute to selective policies for the control of pollutants. The NFIs have been used to develop and refine land classification schemes that can contribute to administering the conservation reserve and sodbuster proposals. The committee is convinced that the NRI is an important source of basic information about U.S. agricultural resource conditions and trends. The value and diversity of applications for the NRIs are certain to increase in the future. Soil conservation and related issues have begun to receive increasing attention from many public interest groups. Increasing interest is also evident within the agricultural community. Polls and surveys of farmers indicate a willingness to accept some forms of mandatory con- servation provisions as a requirement for participation in government farm programs (American Farmland Trust, 1984~. Public interest in soil SOIL CONSERVATION
SCOPE AND CONTENT OF THE 1982 NRI 7 conservation has reinforced efforts within the research community and posed new challenges for refining an understanding of soil erosion processes and their economic, ecological, and environmental conse- quences. As policymakers are more frequently drawn into discussions on conservation issues, the need for reliable information will increase. Information from the 1982 NRI The SCS began releasing data and analyses of the 1982 NRI on a variety of subjects in 1984. Major findings of the 1982 NRI pertaining to land use, soil erosion, and conservation practices that have been reported (see Soil Conservation: Assessing the National Resources Inven- tory, Volume 2) are summarized here. Detailed analyses of NRI data can be found in subsequent sections of this report. And Use and Soil Erosion Rates The acreage in each of eight major uses of nonfederal land is reported in Table 1-3. Of the total of 1.498 billion acres of nonfederal land in 1982, TABLE 1-3 Use of Nonfederal Rural Land, 1982a Land Use Rural land Cropland Pastureland Rangeland Forestland Minor land cover/usesb Acres (millions) 421.4 133.3 405.9 393.8 59.6 Subtotal Urban and built-up land Rural transportation Small water area Total 1,414.0 46.6 26.9 10.1 1,497.6 al982 NRI, preliminary data; excludes Alaska; includes Caribbean. bFarmsteads and ranch headquarters; other land in farms; mines, quarries, and pits; small built-up areas; and other rural lands. SOURCE: 1982 NRI.
8 SOIL CONSERVATION TABLE 1-4 Estimated Average Annual Sheet, Rill, and Wind Erosion (tons/acre~year~a Sheet and Wind Land Use Rill Erosion Erosions Cropland (total) 4.4 3.0 Cropland (cultivated) 4.8 3.3 Pastureland 1.4 0.0 Rangeland 1.4 1.5 Forestland (grazed) 2.3 .1 Forestland (ungraded) 0.7 0.0 al982 NRI, preliminary data. Estimates of absolute rates of wind erosion are subject to considerably greater uncertainty than those for sheet and rill erosion (see Chapter 3~. SOURCE: 1982 NRI. about 94 percent (1.414 billion acres) was classified as rural land. Crop- land, rangeland, and forestland are the dominant uses of rural land, each accounting for roughly 30 percent of rural acreage. Table 1-4 illustrates the substantial differences in erosion rates reported for each use. Cropland has the highest average rates of sheet, rill, and wind erosion. Annual sheet and rill erosion rates on the 421 million acres of Cropland average 4.4 tons/acre, almost twice as great as the next highest rate (2.3 tons/acre~year for grazed forestland). Wind erosion rates for cropland, although subject to much uncer- tainty, appear to be much greater than the rates for rangeland (see Chapter 3~. Erosion rates are yet higher for the 323 million acres of Cropland that is cultivated and used for row and close-grown crops or for vegetables, fruit, and other crops. Sheet and rill erosion rates aver- age 4.8 tons/acre~year on cultivated cropland. These averages obscure enormous variations in erosion rates within each land use (see Chapter 5~. But to indicate the subtlety of the erosion process, it is helpful to note that it takes about 100 years, on average, to form 1 inch of soil. The loss of 5 tons of soil from an acre in one year amounts to a layer of soil less than 1/30th of an inch deep, slightly more than the thickness of a dime. Soil Erosion in Relation to Soil Loss Tolerances A traditional way to gauge the significance of various rates of erosion is by comparison to the conventional SCS soil loss tolerances (T values)
SCOPE AND CONTENT OF THE 1982 NRI TABLE 1-5 Distribution of Cropland Acreage by Soil Loss Tolerance T value (tonstacre year) Cropland 1,000 Acres Percent 5 300,552 71.4 4 48,355 11.5 3 54,514 12.9 2 15,441 3.6 1 2,540 0.6 Total 421,402 100.0 Average T value (weighted by acreage): 4.55 tons/acre year SOURCE: McCormack and Heimlich, 1985. 9 estimated for most soils in the United States. The T value is defined as the maximum rate of annual soil loss (in tons/acre~year) that will permit crop productivity to be sustained economically and indefinitely. For cropland soils, T values have been estimated to range from 1 to 5 tons/ acre~year; 71.4 percent of these soils have been assigned the maximum value of 5 tons/acre~year, and another 11.5 percent have a T value of 4 tons/acre~year. For the generally shallower rangeland soils, T values range from 1 to 3 tons/acre~year (see Table 1-5~. Additional research is needed to increase the reliability and useful- ness of the overall concept of T values, as well as the values assigned to specific soils (see Chapter 4~. However, conventional T values do con- vey a sense of which soils are relatively vulnerable to erosion, because values less than T = 5 were assigned to reflect relatively shallow topsoil depth, less favorable geologic material, the relative productivity of top- soil and subsoil, and the historical amount of erosion. T values were intended to be indicators of the amount of erosion that can be sustained without causing on-farm productivity losses. Addi- tional work is under way to better define the relationship between erosion and potential productivity losses (see Chapter 4~. At the same time, recent research (Clark et al., 1985), which includes use of the 1977 and 1982 NRIs, indicates that this concept might be too narrow to represent the most serious consequences of erosion that involve off- farm impairment of water quality in some regions. To the extent that erosion contributes to offsite damages such as water pollution from runoff, for example, soil loss tolerance values in the future might reflect the on-farm and off-farm consequences of erosion.
10 SOIL CONSERVATION Soil on this 40 percent slope in the Palouse is eroding at a rate of 300 tons/ acre.year (Whitman County, Washington). Severe erosion occurs in early spring when the soil is unprotected. Credit: U.S. Department of Agriculture, Soil Conservation Service.
SCOPE AND CONTENT OF THE 1982 NRI 11 Some soils are not subject to rates of erosion that impair productivity under routine farming conditions; yet the off-farm effects from rela- tively low rates of erosion on such soils might still be important in setting tolerances. In such cases, the conventional definition of T values is of limited use, because it is unlikely that erosion could reach levels that threaten on-farm productivity. In other cases the T value needed to protect against off-farm effects might be very high, even higher than the inherent potential for erosion of certain soils. In these cases, the effects on on-farm productivity would be the binding constraint under- lying T values. The rate of erosion from sheet and rill erosion (4.4 tons/acre year) is roughly equivalent to the average weighted T value of 4.55 tons/acre year in Table 1-5. Where wind erosion is additive, total average soil loss may exceed these estimated tolerance levels. However, the interrela- tionship of wind and water erosion is uncertain. Seventy-five percent of U.S. cropland (315 million acres) was eroding below the tolerance level in 1982 (see Table 1-6~. In the Corn Belt about 60 percent of the acreage is estimated to be eroding below 5 tons/ acre~year; in Iowa the value is 54 percent. Altogether, cropland eroding at a rate below the T value accounted for over 506 million tons of soil displacement, about 27 percent of the total displacement on all crop- land. Another 13 percent (55 million acres) of the cropland had an erosion rate ranging from the tolerance level to twice the tolerance level (T-2T), averaging 6 tonslacre~year. This land accounted for 330 million tons of erosion, 18 percent of the cropland total. On an additional 51 million acres, cropland erosion measured in the 1982 NRI exceeded assigned T values by a factor of two or more. The average sheet and till erosion rate on this acreage was 20 tons/acre~year, a rate about four times greater than the national average. Next to cropland, the most serious sheet and rill erosion problems on nonfederal lands sampled in the NRI are the 353 million acres of range- land (see Table 1-6~. Tolerance values on rangeland soils are generally set at levels that are 1 to 3 tonslacre~year lower than or equal to those for soils in other regions, reflecting the belief among soil scientists that such lands are more sensitive to the consequences of erosion. The erosion rate on about 13 percent of rangeland was at least two times greater than the assigned soil loss tolerance. Erosion problems are less serious on other land uses, primarily because the soil is more protected by the vegetative canopy. Only 9 percent of pastureland (11.5 million acres) and 6 percent of forestland (25 million acres) had an erosion rate greater than the assigned soil loss tolerance.
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SCOPE AND CONTENT OF THE 1982 NRI Concentration of Erosion 13 One of the major findings of the 1977 NRI was the marked concentra- tion of all forms of erosion on relatively small portions of virtually all land uses. Analyses of erosion data from the 1982 NRI indicate similar phenomena, including variation in the concentration of erosion by land use. Concentration of erosion is most pronounced on cropland, as shown in Table 1-6. Table 1-7 shows the distribution of sheet and rill erosion grouped into classes according to the rates of erosion. Data in Table 1-7 are confined to the acreage planted to row and close-grown crops, the most impor- tant category in terms of extent of acreage, volume of erosion, and intensity of use. Corn, soybeans, cotton, and sorghum comprise the bulk of the row crop acreage. Major close-grown crops include wheat, oats, and barley. About 325 million acres of cropland were planted in row and close- grown crops in 1982. About 136 million of those acres (42 percent) had a calculated erosion rate of less than 2 tons/acre~year. A total of 107 mil- lion acres of land used for row and close-grown crops, 33 percent of the total, eroded at rates between 2 and 5 tons/acre year. About 75 percent of the most intensively used cropland in the United States (243 million acres) had sheet and rill erosion rates below the average assigned toler- ance level for all cropland. This 75 percent of cropland contributed only 30 percent of the total soil eroded (see Table 1-7~. At the other extreme is a relatively small proportion of cropland acreage with high erosion rates, where a disproportionate share of the total soil displacement occurs. On about 7 percent of the land in row and close-grown crops (23 million acres), the average rate of sheet and rill erosion is 15 tons/acre year or greater. Yet this 7 percent of the acreage accounts for 700,000 tons of soil displacement, or 41 percent of the total tonnage of sheet and rill erosion on cropland used for row and close-grown crops. Erosion by wind, although estimated with more difficulty and con- siderably less reliability than sheet and rill erosion (Gillette, 1986), appears to be similarly concentrated (see Table 1-8~. Following the pat- tern observed with sheet and rill erosion, wind erosion is also highly concentrated on a limited portion of cropland. Although the precise figures for gross erosion by wind are in doubt, the relative magnitudes in Table 1-8 are illustrative. Perhaps 5 percent, some 16.8 million acres, of land in row and close-grown crops has very high average annual wind erosion rates (more than 14 tons/acre year). Yet this 5 percent
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16 SOIL CONSERVATION accounts for about half (555 million tons) of the total estimated tonnage of soil displaced through wind erosion in an average year. While the reliability of wind erosion estimates in the NRI can be questioned (see Chapter 3), it is likely that improved estimation procedures and data will affirm the spatial distribution and concentration of erosion of crop- land by wind. In general the geographic locations of significant sheet, till, and wind erosion problems do not overlap in much of the United States. This results primarily because very different climatic conditions and pat- terns are associated with different forms of erosion. This may not be the case in some semiarid regions with sparse vegetation and intense rain- falls. However, current information is insufficient to define physical or geographic domains where wind and water erosion processes interact significantly. Soil Conservation Practices on CropZand For the 1982 NRI, field personnel were instructed to note the number of conservation practices in use (up to three) at each sample point. Four major cropland erosion control practices conservation tillage, terrac- ing, contour farming, and stripcropping were recorded. (Conservation tillage can be defined as any of a number of tillage systems that reduce loss of soil or water, compared with clean tillage practices that bury all or nearly all crop residues or cover crop. The terms reduced tillage, minimum tillage, and others are often used inter- changeably with conservation tillage. No-till is the ultimate form of conservation tillage; the new crop is seeded directly into existing crop residue, cover crop, or sod. Implements are used to make small slits in the soil to place seed. The land is not usually cultivated during crop production.) Conservation tillage is the dominant conservation practice; it is applied to 49 percent of row crop acreage and 24 percent of total crop- land acreage. Other traditional cropland conservation practices occupy a relatively small share of the row crop acreage and the total cropland. Terracing and contour farming, for instance, were reported on 14 and 17 percent, respectively, of row crop acreage and on significantly smaller proportions of the total cropland. Stripcropping was reported on less than 1 percent of cropland. It should be noted that because more than one practice could be recorded for each sample point, a certain amount of acreage has been
SCOPE AND CONTENT OF THE 1982 NRI To reduce the threat of erosion and conserve moisture, this new wheat crop is being planted in the residue of the previous wheat crop using a no-till drill (Whitman County, Washington). With this method, erosion will not exceed 5 tons/acre~year. Credit: U.S. [Department of Agriculture, Soil Conservation Service. counted twice. For example, most terraces are constructed along the contours of a field, more or less dictating the use of contour farming within each terrace interval. Thus, much of the acreage reported for terrace systems is also counted for contour farming. Less than 64 mil- lion acres were protected either by terraces or contour farming. The same double-counting is likely with other practices. Thus, the
18 SOIL CONSERVATION sum of acreages for two or more individual practices, and the total acreage for all four, is often an overestimate of the total acreage treated. In 1982 at least one of these four conservation practices was used on approximately 167 million acres. Conservation practices are described in detail in Chapter 5.
