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4 On-Fann and Off-Farm Consequences of Soil Erosion After several decades of relative quiescence, many new, innovative research studies on soil erosion control have been initiated. Some stud- ies (Larson et al., 1983; Williams et al., 1981) have focused on the effects of erosion on soil productivity. Less research is currently under way on offsite damages associated with soil erosion, which was a major area of interest in the 1930s and 1940s. Offsite damages are now an issue that some suggest will likely represent the most serious social consequence of soil erosion on much of cultivated U.S. cropland (Crosson, 1984; Clark et al., 1985~. The committee believes that a strong case can be made to initiate more work on offsite damages. New research might include assessments of the chemical composition of runoff and the effects of chemical constituents and sediment derived from erosion on water quality and the ecology of rivers, streams, and reservoirs. General findings of recent research on the major on-farm and off- farm consequences of soil erosion, focusing on research applications of NRI data, are summarized in this chapter. This research much of which was possible because of the availability of NRI data attempts to quantify basic reasons for conserving soil: reducing the impact of soil erosion on farm production costs and productivity in terms of yield and mitigating costs associated with offsite damages caused by erosion. The committee believes that both on-farm and off-farm costs of ero- sion are important. In some areas, erosion causes onsite productivity losses and also contributes to nonpoint pollution problems. In other areas, erosion might cause only one of these problems. These distinc- tions are critical to the design of effective conservation policies. 62
ON-FARM AND OFF-FARM CONSEQUENCES OF SOIL EROSION 63 Effects of Erosion on Production Costs Short-Term Costs Soil erosion and associated water runoff increase short-term farm production costs per unit of harvested crop in a variety of ways. Water runoff and sediment loss frequently displace fertilizer nutrients and pesticides from the area of original application. Yields may be reduced in these areas as a result of nutrient deficiencies, lack of sufficient mois- ture, or weed and insect problems. Farmers might be able to correct some or all of these problems, but not without incurring additional production costs. The severity of these problems is rarely uniform within individual farm fields. It is often impractical for farmers to identify and efficiently correct soil erosion and runoff problems, because they usually follow a common management routine on a given field. Problems on portions of fields are generally tolerated because of the prohibitive cost of more precise management practices. In most cases, to correct runoff prob- lems farmers have three options: (1) use conservation practices to bring erosion under control; (2) change land use, crop rotation, or both; or (3) alter field boundaries. The last two options are often not economically feasible considering current world market prices. Erosion can also directly damage crops, especially newly planted crops. The damage might be confined to an area of concentrated flow, where seedlings are washed away or inundated with sediment. Or it might affect substantial areas. This is often the case in the Great Plains where wind-blown soil particles bury or abrade germinating crops. In areas of rapidly developing severe gully problems, farm production costs can be increased through damages to farm equipment from cross- ing gullies and the greater fuel and labor requirements needed to farm around gullies. Except in areas of concentrated flows or where wind abrasion is severe, short-term effects of erosion on farm production costs and profits can be gradual and subtle. Researchers and farmers can find it difficult to accurately diagnose the effects of erosion on productivity or to distinguish them from other positive and negative influences such as weather and changing technology. Conservationists can often charac- terize erosion-induced problems in physical terms, but they cannot provide reliable estimates of the short-term economic costs associated with them. The effects of erosion occur over time; thus, short-term costs are modest on most cropland. The committee believes, however, that bet- ~r
64 SOIL CONSERVATION Severe erosion from the concentrated flow of water appears on this unpro- tected cropland after spring rains (Montgomery County, Iowa). Soil erosion and associated water runoff can increase short-term farm production costs in several ways, including loss of plant nutrients and damage to machinery incurred when crossing the channels left by runoff. Credit: U.S. Department of Agriculture, Soil Conservation Service. ter methods are needed to estimate these costs on cropland that is subject to high erosion rates. Improved methods would help farmers integrate the full economic costs and benefits of conservation practices into their economic planning. Long-Term Costs Long-term increases in the cost of farm production occur when ero- sion severely, and sometimes permanently, alters the productive capacity of a soil to support physical and economic crop production. The adverse effects of erosion on the depth and nature of the rooting zone available to plants are probably the most pervasive long-term
ON-FARM AND OFF-FARM CONSEQUENCES OF SOIL EROSION 65 cause of soil productivity losses. The most serious impacts occur when erosion reduces the depth of already shallow topsoils underlain by inhospitable clay subsoils or other material unfavorable to plant growth, or reduces shallow soils underlain by bedrock. Long-term consequences are often subtle. Mixing of subsoil that gradually becomes a more important part of the rooting zone a result of progressive erosion rarely makes the soil totally inhospitable to plants. Such eroded sites usually have less capacity to supply soil mois- ture or plant nutrients in forms readily available to growing crops. With time, farm production costs increase because more fertilizer, more care- ful tHlage operations, or other changes in management are required to attain a given crop yield. The most serious and ubiquitous long-term consequence of erosion, however, appears to be a reduction in the amount of water in the root zone and increased susceptibility to drought. The forces of wind and water can alter the physical properties of surface soils themselves. Organic matter in the surface soil is selectively eroded by wind and water. As subsoil material low in organic matter is mixed with topsoil by tillage, the organic matter content of the topsoil is diluted. The common result is a reduced capacity of surface soils to retain nutrients and moisture. The impact of raindrops on bare soil, especially if it is low in organic matter, can cause the formation of a crust or pavement. This and other structural changes in the soil can impede infiltration of rainfall, causing increased runoff and accelerated ero- sion. As is the case with short-term erosion damages, long-term damages maybe confined to fairly small portions of a given field, which often are easy to spot because of physical changes such as soil color and chroni- cally lower yields. Rather extensive potential damage, however, can go unnoticed. Changes in agricultural technology and management prac- tices confound recognition and quantification of long-term costs of ero- sion. In fact, crop yields have increased substantially over time in many areas where erosion rates are high. This paradox has led some observers to discount the importance of on-farm productivity losses from erosion. Extended periods of surplus crop production have rein- forced this conclusion. Erosion probably exerts minimal adverse impacts on productivity in eroding fields with deep topsoil layers; rich, productive subsoils; or a combination of both. In some regions, western Iowa and the Palouse in the Pacific Northwest, for example, many soils possess these favorable characteristics. In areas such as the southern Piedmont region that stretches through several mid-Atlantic states, almost all soils are vul-
66 SOIL CONSERVATION nerable to long-term productivity losses because of relatively shallow topsoil layers and subsoils that are unfavorable to root development. The relatively few studies that have focused on a variety of soils in the United States have been helpful in establishing the range of vulnerabil- ity to different erosion rates. However, many of these past studies are less useful in assessing contemporary erosion-productivity relation- ships (see Separating the Effects of Erosion and Technology). Separating the Effects of Erosion and Technology Yielcl observations over time are influenced by several interactive forces. Where soil erosion occurs, the relationship between erosion's adverse effects on productivity and technology's yield-enhancing effects poses a complex research challenge. Yielcl increases brought about by technological change may not only mask the effects of ero- sion, but failure to account for the ways that erosion may have influ- enced the rate and cost of technological advances can lead to inaccurate assessments of damage caused by erosion. Erosion damage cannot be measured solely as a simple function of yielcl. A correct assessment requires the clear separation of the pro- jected effects of erosion and technology. For example, crop yield mightbe increased bytechnological acivancesthatare inclepenclentof inherent soil productivity, such as hybrid seed and improved chemi- cal weed-control systems. To evaluate the effects of erosion in such instances, it is necessary to focus on the impact of erosion damage on attainable yield levels, taking into account the effects of technological advances. The question is: How much higher would yielcis be with new technology if soil had been conservecl? Even on cropland with creep, friable subsoils where technology boosts yields uniformly throughout the range of yield responses, in spite of the extent of erosion damage to the field, assessment of impaired productivity on land that has benefited from technology requires a measure of erosion damage based on a with conservation versus withoutconservation comparison of yielcis (Walkerand Young, 1 9861. The only valid way to assess erosion effects in regions experiencing substantial technological change the majority of U.S. agricultural areas is to compare actual yields and profitability under conditions of contemporary management with those under conditions of con-
ON-FARM AND OFF-FARM CONSEQUENCES OF SOIL EROSION 67 trolled erosion. Empirical studies that have attempted to disaggregate erosion from technology effects have demonstratecl that yield response to a given new technological acivance on conserved soils can be greater than that on eroded soils (Walker and Young, 1986~. Research has been initiated in several parts of the country to develop new analytical methods for more accurately quantifying complex ero- sion-technology interactions. For many years technology has been thought of as a mask or com- pensation for erosion. The availability of NRI data has helped analysts recognize that the relationship of erosion and productivity is compli- cated by technology. Understancling the interactions of erosion, pro- ductivity, and technology is a necessary step in reliably estimating the true costs and benefits of erosion-control investments. {Dng-Term Productivity The degree of impact of erosion on long-term productivity varies widely. Many different concepts and experimental designs have been advanced in an effort to quantify these impacts. Proper interpretation of the stated hypotheses and the experimental results is important. Long-term productivity losses might come about through reduced optimal yields or through higher costs to attain optimal yields. Erosion can also adversely affect a soil's responsiveness to future yield-enhanc- ing technologies. An important question is whether yield increase over time will be greater in the future if less topsoil is displaced by erosion (Walker and Young, 1982~. Alternatives must be placed in an economic and social context. A basic goal of soil conservation is to maintain the full potential of the soil to achieve a high level of production at per unit cost over a long period. Practical and reliable techniques for estimating the long-term eco- nomic costs of soil erosion at specific locations are not available. The standard approach of farm-level conservation planning is to identify the physical magnitude of an erosion problem, not its economic conse- quences, and present the farmer with a series of alternative conserva- tion systems for reducing the volume of soil displacement. The systems vary in cost and effectiveness. Some might be impractical, depending on the farmer's complement of machinery or operation. Ideally, the farmer selects a system that will make his operation more profitable and achieve the greatest reduction in erosion per dollar spent. Choices among conservation alternatives, however, are made without the aid of
68 SOIL CONSERVATION sound estimates of short- and long-term benefits. Costs, on the other hand, are generally known and are usually significant. Current research that identifies the effects of erosion on productivity will improve the general scientific understanding of the magnitude and extent of erosion damages. The development of practical methods for field estimation of both short- and long-term erosion damages to pro- ductivity would augment current research. Such new methods would be useful to farmers and conservationists. In developing models and procedures for assessing the effects of erosion on land productivity, the USDA should reconsider the current balance between farm-level basic research on the modeling of erosion processes and more applied, aggregate-level research. (Aggregate-level research is designed to provide technical information to support USDA and state soil conservation activities as well as the conservation planning undertaken by individ- ual farmers.) The secretary of agriculture should direct a team drawn from the SCS, the Agricultural Research Service, the Extension Service, the Cooperative State Research Service, the Economic Research Service, and other relevant research and administrative organizations to review and periodically report on current research and methods in the area of erosion-productivity interactions. The purpose should be to identify promising techniques for applying such research in the field, to assist extension workers and conservationists in applying research results to farm-level economic planning, and to coordinate future research to enhance its applications in the field and in program development. Erosion-Productivity Models Most of the research on the effects of soil erosion on agricultural productivity has been in the form of empirical studies at specific geo- graphic locations (Crosson, 1984; Larson et al., 1983~. Results have helped researchers quantify the impact of on-farm erosion damages and have drawn attention to the need for improved soil conservation practices. Empirical studies have also helped researchers identify the soil characteristics and management practices that most significantly influence erosion-productivity damage under many field conditions. However, research methods differ greatly from one study to the next, and the results of these studies are often pertinent to relatively few soils experiencing different rates of soil erosion. As a result, researchers have not been able to generalize about the effects of erosion on produc- tivity for broad geographic areas or to consistently compare effects from one area to another. Since the release of the 1977 NRI data, however, several mathematical
ON-EARM AND OFF-FARM CONSEQUENCES OF SOIL EROSION 69 models have been developed that attempt to quantify erosion-induced productivity effects for much broader geographic areas, including the entire United States. Two are discussed here: the Productivity Index (PI) model and the Erosion-Productivity Impact Calculator (EPIC) model. The PI model uses smaller amounts of input data and less technical data. In that sense it is limited, but it is relatively simple and inexpensive to use. The EPIC model is more complex and goes beyond the PI model in attempting to analyze the physical and chemical rela- tionships affecting erosion-productivity. These models and others draw upon the findings of empirical studies of erosion-productivity relationships, data contained in the NRIs, and other data sources. Thus far, the models mainly have been used for policy analysis and program evaluation at the national, regional, and state levels. With further refinement the results of erosion-productivity models should have important application in conservation planning on the farm. The PI Mode] In the late 1970s, researchers at the University of Missouri developed a model of productivity, the PI model (Kiniry et al., 1983~. As refined by a research team at the University of Minnesota, the PI model allows comparisons of soil productivity between an ideal soil, in which bulk density, pH, aeration, and available water storage capacity are optimal for root growth, and any other soil for which values for these parame- ters are available (Pierce et al., 1983~. The computerized data base com- posed of county-level soil surveys, Soils-5, contains such information for thousands of soil profiles. The NRI design permits cross-tabulation with the Soils-5 data base. Information recorded on most NRI sample points can be cross-referenced via computer with detailed Soils-5 infor- mation about similar soils. Once a PI has been constructed for a particular soil profile, changes in productivity that would be expected as a result of erosion can be simu- lated by evaluating the PI for progressively lower portions of the soil profile the portions that become the zone of plant root growth as erosion strips away the surface soil. The depth of the soil, its quality as a medium for plant growth, and the amount of erosion (rate multiplied by time) become the determinants of future soil productivity. The PI model assumes that the availability of plant nutrients will not be a limiting factor on crop production. Damage estimates are generally conservative, because the model assumes that any direct reduction in nutrient levels or nutrient storage capacity sustained from erosion can be overcome by the addition of increasing amounts of fertilizer. Table 4-1 shows some characteristics and PI model results for land in
70 SOIL CONSERVATION TABLE 4-1 Relationships Between Erosion and Measures of Soil Productivity in Six MLRAs for Land Planted in Row Crops Measures of Productivity Impact Change in Rates of Erosion (tons/acre year) 100 years MLRA Potential Actual Tolerablea PIb vc (percent) - 105 71.6 11.4 4.7 0.84 0.23 5.6 109 45.1 15.2 3.9 0.79 0.17 6.9 113 25.4 8.0 3.4 0.72 0.21 4.4 103 11.2 4.1 4.9 0.88 0.27 1.9 108 28.9 8.8 4.9 0.91 0.16 2.4 115 32.0 10.2 4.6 0.83 0.14 3.6 aSoil loss tolerance (T) value. Productivity index: Ratio of soil productivity after an increment of erosion in contrast to uneroded state. (PI does not equal 100 for any soils in the initial period.) Approximates loss in productivity from erosion. Vulnerability value: Approximates the rate of change over time for a given soil in the PI value. High V values indicate relatively high susceptibility to erosion-induced produc- tivity loss. SOURCE: 1982 NRI and Soils-5; adapted from Runge et al., 1986. six Major Land Resource Areas (MLRAs) that were reported as planted to corn or soybeans in the 1982 NRI. In general, MLRAs 103 (the central Iowa and Minnesota till prairies), 108 (the Illinois and Iowa deep loess and drift), and 115 (the central Mississippi Valley wooded slopes), which have fairly low average potentials for sheet and rill erosion (RKLS product values), exhibit high average levels of soil productivity (PI values). The V value approximates the rate of reduction in the PI value for uniform, incremental reductions in soil depth resulting from erosion. It is an indication of the average vulnerability of the land to erosion in these MLRAs (Pierce et al., 1984~. Whereas the PI value is a static measure of productivity, the V value is an indicator of how rapidly a soil's productivity can be reduced by erosion. Vulnerability increases with increasing V values. Table 4-1 indicates that there are exceptions to the general observation that high rates of erosion are associated with significant soil productivity damages. MLRA 103 has the lowest poten- tial for erosion of the six MLRAs listed in Table 4-1 and a fairly high productivity rating, but it is also the MLRA most vulnerable to a given amount of erosion, as indicated by the higher V-factor value for the soil.
ON-FARM AND OFF-FARM CONSEQUENCES OF SOIL EROSION 71 Uses ofthe PIModel Some researchers have suggested that V values, or similar measures of a soil's vulnerability to erosion damage, might be applied to refine conventional soil loss tolerance limits (Pierce et al., 1984; Runge et al., 1986~. The committee believes that such approaches should be used to improve the conceptual and empirical basis of T values with respect to erosion-productivity relationships. This could be accomplished within a reasonable time and at acceptable develonmen- tal and implementation costs. . ~ . . . . . . . Analytical tools such as the PI model can be used to investigate the ways that soil erosion influences productivity across the country over time. Table 4-1 shows the predicted effects on the average PI over 100 years for six MLRAs, assuming that the average sheet and rill erosion rates reported in the 1982 NRI continued. Again, the predicted decline in productivity incorporates the assumption that damage to the plant nutrient regime, or any weed or pest problems exacerbated by erosion, will be corrected through adjustments in input levels, management decisions, or both. MLRAs with high inherent potential for erosion as well as high actual erosion rates MLRAs 105 (the northern Mississippi Valley loess hills), 109 (the Iowa and Missouri heavy till plain), 113 (the central claypan areas of Missouri and Illinois), and 115 would sustain the most serious productivity damages. MLRAs 105, 109, and 113 exhibit greater suscep- tibility to damage, in some cases more than twice the average decline in PI. However, the average reduction in PI value is weighted heavily by modest PI reductions in MLRAs 103 and 108, which together account for more than 60 percent of the acreage evaluated in the table. Geographic Variation Broad geographic assessments of erosion-pro- ductivity damages, whether for the entire United States or smaller aggregations, provide useful indications of overall conditions. But these aggregations can obscure important variations observed among regions or in local areas within regions such as MLRAs. Erosion-pro- ductivity problems are not uniformly distributed across the country, within regions, or in individual fields of cropland. Assessments of erosion damages that emphasize average reductions in crop yields or other measures of productivity can underplay the more serious, geo- graphically concentrated instances of severe erosion-induced produc- tivity damages. The committee believes that the highly variable nature of erosion-related productivity should be more systematically consid- ered in studying erosion and in the design and implementation of erosion control programs.
72 The EPZCModeZ SOIL CONSERVATION The Resource Conservation Act (RCA) mandates that the USDA per- form an appraisal of soil and water resource conditions and trends every five years. As part of the 1985 RCA appraisal, an interagency research group involving the SCS, the Economic Research Service, and the Agricultural Research Service developed a mathematical model for estimating erosion impacts on crop yields and costs of production (in the form of additional requirements for such production inputs as fertil- izer and water). The EPIC model is a sophisticated descendent of the Yield Soil Loss Simulator, the model used to evaluate erosion-produc- tivity effects in the 1980 RCA appraisal. The model draws upon NRI erosion estimates to develop baseline conditions and can simulate ero- sion rates and long-term productivity effects that would be expected from alternative cropping and conservation management conditions. In view of the importance of testing the interactive effects of technol- ogy and erosion damage on crop yields and other measures of produc- tivity, the EPIC model, used in conjunction with erosion information from the NRIs, is a potentially useful tool for scientific research and policy analysis. The EPIC model is more complex and demanding of data than the PI, and like the PI model, EPIC will provide insights into the erosion-productivity relationship. The committee notes that until the full impacts of erosion on all factors contributing to agricultural productivity can be quantified, it is likely that the short- and long-term agronomic and economic impacts of erosion will continue to be noorlv understood. Soil Erosion and Water Quality ~ , The most serious damages of erosion occur, in some cases, after sedi- ment and runoff have left the field. A review of offsite erosion damages conducted by the Conservation Foundation estimates that the social cost of these damages may have totaled $3 billion to $13 billion in 1980 alone, excluding biological damages (Clark et al., 1985~. Damages appraised in the study include impaired recreation and damage to water storage facilities, navigational impacts, property values, and commercial fisheries. Also included are damage to water conveyance systems, water treatment facilities, municipal and industrial facilities, and flooding. Field studies and monitoring activities cited earlier (see Chapter 2) suggest the serious nature of offsite erosion problems. More recently, attention has been focused on the potential impacts of agricultural
ON-FARM AND OFF-FARM CONSEQUENCES OF SOIL EROSION 73 l .. .... ', __ Spring runoff on a plowed field, Brown County, Wisconsin. Such runoff often transports sediments, animal wastes, plant nutrients, and pesticidesall causes of nonpoint water pollution. Credit: U.S. Department of Agriculture, Soil Conservation Service. practices on groundwater quality, particularly the use of fertilizers and pesticides. The committee believes that strategies for protecting the quality of surface water and groundwater will have to consider the possibility that groundwater pollution can be aggravated by techniques intended to mitigate erosion problems. Conservation practices often rely more heavily on agricultural chemicals than conventional manage- ment and at the same time increase infiltration of water, which may aggravate pollution of groundwater. Potential Uses of NRIData There are major gaps in the scientific data on the basic processes of offsite damages and pollution of surface water and groundwater caused by erosion and agricultural management practices. These prob- lems may be serious in many parts of the country.
74 SOIL CONSERVATION Currently, the usefulness of NRI data for assessing offsite erosion damages is hampered by the lack of basic scientific understanding of the effects of soil erosion and runoff on surface water and groundwater quality. The Universal Soil Loss Equation (USLE), for example, was not designed to evaluate sediment delivery into water bodies or other non- point pollution problems. NRI estimates of sheet and rift erosion do provide an indication of potential sediment loads from agricultural land. The difficulty is in translating these potential loads into actual sediment levels observed in watercourses and then assessing the dam- age caused by the added sediment. NRI values for select physical factors of the USLE can be helpful in analyzing surface water quality when properly integrated into mathe- matical models such as those mentioned in Chapter 3. These models focus on the linkages between erosion, soil, and chemicals carried by runoff at the field's edge and additional changes that occur as runoff of water and sediment moves into or through small streams. Additional work is needed to better define the hazards posed by infiltration of nutrients or chemicals into groundwater in diverse regions that are subject to a variety of agricultural practices (Crosson and Brubaker, 1982) and hazards associated with sediments and chemical constitu- ents that are transported and deposited in water systems. As noted earlier, the committee believes that sufficient evidence exists to conclude that environmental hazards and social costs associ- ated with water pollution derived from agricultural lands are signifi- cant. Improved control measures and strategies are needed; better information is essential to devise such strategies. As such information is developed, current and new data from the NFI can contribute to the assessment of the offsite impacts of agricultural activities.
