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, Vol 289, Issue 5477,248-250, 14 July 2000
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[DOI: 10.1126/science.289.5477.248]
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U.S. Soil Erosion Rates--
Myth and Reality
LAND USE:
Stanley W. Trimble and Pierre Crosson*
irst
oil erosion in the United States has been a
matter of public concern since the 1930s.
Conditions were improved by the 1960s,
很爱很爱你日文版although no one knew just how much ().
Starting in the 1970s, however, veral studies
concluded that erosion was high. Although a
few studies have been skeptical of the high
rates(,), most have suggested that soil
erosion is an extremely rious environmental
problem, if not a crisis(-). Quantification of
the problem has been elusive, and average
annual U.S. cropland soil erosion loss have
been given as 2 billion (), 4.0 billion (, ),
4.5 billion (), 4.8 billion (), 5 billion (), or
6.8 billion tons (). The U.S. Department of
Agriculture (USDA) National Resource
Inventory (NRI), bad on models, gave high
values in the 1970s and1980s() but has
shown decreas in the past decade. Some
艾薇儿好听的歌曲sources have suggested that recent erosion is as
great as or greater than that of the 1930s, when
the soil conrvation
effort was begun(,,). Increas in
spending for soil conrvation have been many
billion dollars ().
S
1
23
47
8910
5116
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12
13
101114
15
Studies of the on-farm productivity effects bad on 1982 NRI cropland erosion rates indicated that if tho
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pullrates continue for 100 years, crop yields (output per hectare) would be reduced only 2 to 4% (). The results indicate that the productivity effects of soil erosion are not significant enough to justify incread federal outlays to reduce the erosion, but not all agree ().167The remarkable feature of all this discussion and attempted rectification is that it was bad mostly on models. Little physical, field-bad evidence (other than anecdotal statements) has been offered to verify the high estimates. It is questionable whether there has ever been another perceived public problem for which so much time, effort, and money were spent in light of so little scientific evidence. Here, we asss the techniques now ud to estimate erosion and the resulting off-farm movement of diment and suggest new directions for rearch that may provide more policy-relevant information.
Two models have been ud to estimate soil erosion (). The first, the universal soil loss equation [USLE ()], attempts to predict sheet and rill erosion by water. Although the USLE has been criticized, it is an excellent planning tool for estimating the relative values of varying land us and conrvatio
n measures. However, it only presumes to predict the amount of soil moved on a field, not necessarily the amount of soil moved from a field (). The latter is estimated by a diment delivery ratio [SDR ()], a simple empirical model that shows a highly generalized decrea of diment with increasing area. Implicit in this model is that only a small proportion of eroded soil leaves a field or stream basin. Some diment is presumed to be deposited by wind on the field, or downslope of the field along fencerows or in woods, or along streams as alluvium. In reality, not nearly enough is known about this diment delivery process, and using it for analysis is a continuing problem in fluvial geomorphology (). However, many investigators have termed the output of the USLE as "removed from the land" (). Another problem is that the potential variance of SDR has not been
appreciated. In Coon Creek, WI, for example, diment delivered to streams from about a 3-km drainage area in the 1970s was only about 8% of the amount estimated by the USLE; the difference was presumably diment stored as colluvium. In the 1930s, however, when gullying downslope from agricultural fields was common, the diment delivered was 123% of upland soil erosion as estimated by the USLE ().
遏制是什么意思
The Models 17181719207221For wind erosion, the wind erosion equation [WEE ()] has been ud, for which results are uncertain but often exaggerated (). Like the USLE, there is a mass continuity pr
oblem--even though soil may be eroded in one area, most of the particles are simply moved to other fields. During the 1930s when wind erosion was really a crisis, huge dust clouds from the Dust Bowl darkened the skies of the eastern United States and moved out over the Atlantic Ocean in the upper westerly winds. However, much wind erosion of the past few decades appears to be mainly local redistribution--some areas lo, others gain. But as is the ca with water erosion, there has been little scientific evidence.2223Whatever the limitations of each equation for predicting soil detachment, the obrvation that much of the soil remains clo by, and thus is not lost, is a concept clearly not taken into account (). Although large areas of the United States were proclaimed to have erosion rates >25 tons ha (), diment yields (efflux) were
usually on the order of 0.5 to 2.0 tons ha , and the yields were usually augmented by significant stream channel and bank erosion (). Expresd another way, total diment delivered to streams has been given as 2.7 to 4.0 billion tons (, , ), but the total diment yield is estimated to be only about 0.5 billion tons (). This huge disparity between presumed erosion and measured downstream diment yield means that large volumes of diment would have been stored in the watershed.
Sediment Budgets 17-113-1246162526To investigate the t of process linking erosion in upland areas with diment delivery downstream requires construction of a diment budget. For example,
consider an agricultural watershed of 100 km (10,000 ha) 22-1 -12
Areas of the United States having cropland erosion rates of >25 tons ha year as predicted by the U.S. Department of Agriculture in 1982 [modified from ()]. "Driftless Area" is approximately coincidental with Major Land Resource Area 105, Northern Mississippi Valley Loess Hills.
duplicate
where 90 km is cropped upland eroding at a rate of 20 tons ha year . The remaining 10 km is stream and flood plain subject to diment deposition. Of the eroded material, assume that 60% is conveyed to streams. Further assume a high diment yield (efflux) from the basin of 200 tons km year (2 tons ha year ). This would leave 8.8 x 10 tons of diment to be deposited on the 1000 ha of flood plain. At a typical bulk density of 1.3 tons m , this would cover the flood plain to an average depth of about 6.9 cm in only a decade. Such accretion is easily measurable, and even obrvable, since the root crowns of small trees would in places be buried. A specific example comes from the upper Mississippi River Loess Hills region (Driftless Area), which was designated a soil erosion problem region in the 1980s, when it ostensibly had cropland loss greater than 25 tons ha () (Fig. 1). However, a long-term diment budget for one stream in the region, Coon Creek, WI, showed that, of all upland erosion (including nonagricultural), only about 2 tons ha year reached the streams and much of that was deposited ().
-2-1 -1-15-3-1 13-1-127Fig. 1. Areas of cropland erosion.-1-113Indeed, measures of alluvial diment flux are usually better measures of basin process than are estimates of upland erosion or measurements of diment yield (, ). During recent decades, when soil erosion rates were ostensibly so high, rates of alluviation declined in various regions (,,). Studies of wind erosion mass budgets have been few, but the too show declining airborne dust (). Thus, although mass budget studies of diment and dust have been limited, much of the available field evidence suggests declines of soil erosion, some very precipitous, during the past six decades.282921273031Some asssments of U.S. erosion have warned that increasingly eroded soil profiles will allow less rainfall to be infiltrated and stored (). This process would logically result in incread overland flow, erosion, and flooding, process that might be occurring if the soils were eroding rapidly. However, detailed hydrologic studies in two large regions, the Southern Piedmont and the Driftless Area, indicate that just the opposite is occurring: Runoff is decreasing, flood peaks are smaller, and in some places, the ba flow is greater. The field studies show that more water is infiltrating into the soil and, in some cas, that significantly more water is being transpired by plants. Investigators attribute the changes to improved land u ().
Associated Resources 732Such hydrologic improvements, in turn, improve other resources. For exa
mple, the stability of tributary channels in the Driftless Area has been enhanced greatly over the past half century (Fig. 2), and channels have become smaller, reflecting the improved hydrologic regimes (). Perhaps the most dramatic and convincing change there has been that of fish habitat. At the time of European ttlement, streams were notable for large 33
in the Driftless Area, 1940 to recent times. Photo t from Bohemian Creek, La Cros County, WI. () Photo made by S. C. Happ in 1940 to depict a "typical" tributary of the period. Note the eroded, shallow channel compod of gravel and cobbles, with coar diment deposited by overflows on the floodplain. Such tributaries were described as rembling "gravel roads." () Remake of photo by S. Trimble in 1974. The stream channel is narrower, smaller, and more stable. The coar diment has been covered with fine material, and the floodplain is vegetated to the edge of the stream. This condition has continued and improved over the past 25 years.
numbers of brook trout, , which require high-quality water (). Degradation of habitat was evident in the late 1800s, so that by the 1930s, only exotic brown trout, , which had to be stocked, could survive the flooding, high diment concentrations, warmer water temperatures, and stream channel instability of that period. Indeed, floods were so frequent and violent that improvement of fish habitat was not practicable [(, ) and Fig. 2, top]. With the improved land u and soil conrvation measures
aboard
starting in the late 1930s, stream conditions had improved enough by the 1960s so that brook trout could be stocked. By the 1980s, stream conditions were suitable for natural reproduction in some areas, a condition now widespread in this agricultural region.
Salvelinus fontinalis 21Salmo trutta 3435Fig. 2. Improvement of tributary stream channel conditions Top Bottom CREDIT: (TOP) STAFFORD C. HAPP, (BOTTOM) STANLEY W. TRIMBLE
The foregoing discussion suggests that the general impression of vere soil erosion with deteriorating associated resources is not correct in some regions and, by implication, is open to question in all others. What is required now is the initiation of continuing field studies and monitoring bad on mass budgets. In humid areas of water erosion, baline data should be collected from small sample stream basins so that changes of colluvium and alluvium can be monitored. Initially, this should be by ground surveys, which are quick, cheap, and preci, but this might eventually be augmented with cosmogenic isotopic dating and high-precision remote nsing techniques. Water quality, especially diment concentrations, should be monitored. To more effectively measure annual diment yield (including bedload), sample basins should ideally terminate in a
虞美人赏析
Monitoring Soil Erosion and Associated Resources
rervoir to trap diment, including bedload. In some cas, existing dams could be ud. Basins with existing baline data; e.g., tho in the Vigil Network, would be especially valuable and are available for some regions (). Ideally, biological and chemical indicators should also be monitored. Erosion and diment fluxes should be studied annually in light of the land u and climatic conditions of that year.36Regions of wind erosion are more problematic, becau efflux can go in any direction. Although some obrvations of dust are being made (), it is important to have a better grasp of the size, concentration, and movement of dust clouds. Perhaps just as important are more measurements of dust deposition.37No problem of resource or environmental management can be rationally addresd until its true space and time dimensions are known. The limitations of the USLE and the WEE are such that we do not em to have a truly informed idea of how much soil erosion is occurring in this country, let alone of the process of diment movement and deposition. The uncritical u of models is unacceptable as science and unacceptable as a basis for national policy. A comprehensive national system of monitoring soil erosion and conquent downstream diment movement and/or blowing dust is critical. The costs would be significant; nevertheless, they would reflect efforts better focud on achieving better management of the country's land and water resources.
Conclusions References and Notes
1.R. Held and M. Clawson, (John Hopkins Univ. Press, Baltimore, 1965).Soil Conrvation in Perspective
2.S. Trimble and S. Lund, . , 355 (1982); S. Batie, (The Conrvation Foundation, Washington, DC, 1983); S. Trimble,., 162 (1985).J. Soil Water Conrv 37Soil Erosion: Crisis in America's Croplands Agric.Hist 59
3.P. Crosson [letter], , 461 (1995); "Soil erosion and its on-farm productivity conquences: What do we know?" (Discussion Paper 95-29, Resources for the Future, Washington,DC,1995);, 4 (1997).Science 269Environment 39
4. F. Steiner, (Johns Hopkins Univ. Press, Baltimore, 1990)Soil Conrvation in the United States: Policy and Planning
5. D. Pimentel . [respon, e ()], , 461 (1995).et al 3Science 269
6. D. Pimentel ., , 149 (1976).et al Science 194
7. D. Pimentel ., , 1117 (1995).et al Science 267
8.USDA, Soil and Water Resources Conrvation Act, 1980 Appraisal, Part I: Soil, Water, and Related Resources in the United States: Status, Conditions and Trends, and Appraisal; Part II: Soil, Water, and Related Resources in the United States, Analysis of Resources Trends (USDA, Washington, DC, 1981).
9.R. Beasley, (Iowa State Univ. Press, Ames, 1972); P. Raven, L.Berg,G.Johnson, (Saunders, Philadelphia, 1995).Erosion and Sediment Control Environment 10.General Accounting Office (GAO), "To protect tomorrow's food supply, soil conrvation needs priority attention" (CED 77-30, GAO, Washington, DC, 1977).11.T. Barlowe, in (Soil Conrvation Society of America, Ankeny, IA, 1979).Soil Conrvation Policies: An Asssment 12.J. Harlin and G. Barardi, (Westview, Boulder, CO, 1987).Agricultural Soil Loss 13.L. Lee, . , 226 (1984).J. Soil Water Conrv 3914. D. Popper and F. Popper, , 12 (1987); O. Owen, D. Chrias, J. Reganold, (Prentice-Hall, Upper Saddle River, NJ, 1998).Planning 53Natural Resource Conrvation 15.Although instituted only partially for soil conrvation, the Conrvation Rerve Program cost $22 billion from 1985 to 1999, and other federal conrvation costs, much for soil conrvation, were $17 billion for the same period (written communication, Natural Resources Conrvation Service, Resource Economics and Social Science Division, 18 November 1999. All figures are in constant 1999 dollars). None of this includes state and private expenditures, much of which were spent in conjunction with
federal expenditures.16.
F. Pierce ., , 131 (1984),(USDA, Washington, DC, 1989).et al J. Soil Water Conrv. 39The Second RCA Appraisal: Soil, Water, and Related Resources on Nonfederal Land in the United States 17.
National Rearch Council, (National Academy Press, Washington, DC, 1986).Soil Conrvation; Asssing the National Resources Inventory 18.W. Wischmeier and D. Smith, . (1978).U.S. Dep. Agric. Agric. Handb 53719.
Natural Resources Conrvation Service, , Section 3, Sedimentation (USDA, Washington, DC, 1983).National Engineering Handbook 20.M. Wolman, . , 50 (1977); D. Walling, . , 209 (1983).Water Resour. Res 13J. Hydrol 6521.S. Trimble and S. Lund, (1982).U.S. Geol. Surv. Prof. Pap. 1234 22. E. Skidmore and N. Woodruff, . (1968).U.S. Dep. Agric. Agric. Handb 34623. D. Gillette, in (), vol. 2, 129.1724.
J. Milliman and R. Meade, . , 1 (1983); R. Meade and R. Parker, . .(1985); R. Meade, T. Yuzyk, T. Day,in , M. Wolman and H. Riggs, Eds. (Geological Society of America, Boulder, CO, 1989), pp. 255-280.J. Geol 91U.S. Geol. Surv Water Supply Pap 225Surface Water Hydrology 25. D. Pimentel and E. Skidmore [letter], , 1477 (1999).Science 28626.W. Curtis, W. Culbertson, E. Cha, . (1973).U.S.
Geol. Surv. Circ 67027.S. Trimble, , (1999); [respon, e ()], , 1477 (1999).Science 285124425Science 28628.
L. Leopold, W. Emmett, R. Myrick, , 193 (1966); S.Trimble,, 1207 (1975), [respon], , 871 (1976); S. Schumm [letter],, 871 (1976); W. Dietrich and T. Dunne,. , 191 (1978); H. Kely,., 190 (1980); F. Swanson, F. Janda, T. Dunne, D.Swanston,Eds., (USDA, Forest Service General Tech. Rep. PNW 141, 1982); R. Meade, . , 235 (1982); P. Patton and P. Boison, ., 369 (1986); J. Phillips, ., 780 (1987); M. Church and O. Slaymaker,, 352(1989);J.Phillips,, 231 (1990); P. Ashmore,. , 190 (1993); A. Schick and J. LeKach, . , 225 (1993); O. Slaymaker,. , 305 (1993); S Trimble, in , A. Gurnell and G. Petts, Eds. (Wiley, Chichester, UK, 1995), pp. 201-215; L. Reid and T. Dunne, (Catena, Reiskirchen, 1997); S. Trimble, , (1997).U.S. Geol. Surv. Prof. Pap.352-G Science 188Science 191Science 191Z. Geomorphol. Suppl 29Geol. Soc. Am.Bull 91Sediment Budgets and Routing in Forested Drainage Basins J. Geol 90Geol. Soc. Am. Bull 97Am. J. Sci 287Nature 337Geomorphology 4Prog. Phys. Geogr 17Prog. Phys. Geogr 17Prog. Phys. Geogr 17Changing River Channels Rapid Evaluation of Sediment Budgets Science 278144229.
M. Wolman in , C. Jolly and B. Torrey, Eds. (National Academy Press, Washington, DC, 1993).Population and Land U in Developing Countries 30.
R. Hadley, in (Pub. 113, International Association of Scientific Hydrology, 1974), pp. 96-98; R. Meade and S. Trimble,(Pub. 113, International Association of Scientific Hydrology, 1974), pp. 99-104; S. Trimble,(Soil Conrvation Society of America, Ankeny, IA, 1974); R. Jacobson and C. Colman, . , 617(1986);J.Knox,. , 224 (1987); S. Miller ., , 309 (1993); W. Wolfe and T.Diehl,(1993); T. Beach,. , 5 (1994); J. Oppenheim, thesis, Univ. of Georgia (1996); A. Gellis ., .(1999); C. Craft and W. Cay in 1999 , K. Hatcher, Ed. (Inst. of Ecology, Univ. of Georgia, Athens, 1999), pp. 443-446.Effects of Man on the Interface of the Hydrological Cycle with the Physical Environment Effects of Man on the Interface of the Hydrological Cycle with the Physical Environment Man-Induced Soil Erosion on the Southern Piedmont, 1700-1970 Am. J. Sci 286Ann. Assoc. Am. Geogr 77et al Geomorphology 6U.S. Geol Surv. Water Resour. Inv. Rep . 92-4082 Ann. Assoc. Am. Geogr 84et al U.S. Geol. Surv. Water-Resource Inv.Rep 99-4010Proceedings of the Georgia Water Resources Conference 31.
R. Ervin and J. Lee, . , 430 (1994); N. Godon and P.Todhunter,., 1587 (1998); P. Todhunter and L. Cihacek, ., 543 (1999).J. Soil Water Conrv 49Atmos.Environ 32J. Soil Water Conrv 5432.S. Trimble, F. Weirich, B. Hoag, ., 425 (1987); K. Potter,. , 845 (1991); W. Gebert and W. Krug, ., 733 (1996); W. Krug, ., 745 (1996); P. Price, thesis, Univ. of Calfornia, Los Angeles (1998); J.Knox,in , A. Brown and T. Quine, Eds. (Wiley, Chichester, UK, 1999), pp. 255-282.
Water Resour. Res 23Water Resour. Res 27Water Resour.Bull 32J. Am. Water Resour. Assoc 32Fluvial Process and Environmental Change
