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Field Estimates of C Factors: How Good Are They and How Do They Affect Calculations of Erosion? E ~ Pierce, ~ E. Larson, and R H. Dowdy Techniques for predicting average annual soil erosion have been available since about 1940. The most notable of these has been the Universal Soil Loss Equation (USLE) which has found wide use for the last 25 years. Of the many influences on the degree of erosion, the most important one that individual farmers can control may be the condition of the soil surface and the vegetative cover at the time the potential for erosion by water exists (Renard and Foster, 1983). The coverage and management factor (C factor) of the USLE accounts for this. The ability of the C factor to represent these effects and its influences on the estimate of water erosion is the subject of this paper. This discussion of C factors considers the definition, context, and calculation of C factors; their importance in the context of conservation policy issues; how reliably and accurately they were used in the 1982 National Resources Inventory (NRI); and their effect on erosion calculations in relation to conservation policy issues THE CONTENT AND CALCULATION OF THE C FACTOR . - A basic objective of the 1982 NRI was to provide data on the extent and distribution of soil erosion in the United States. As with the 1977 NRI, the USLE was used to estimate soil erosion by water. The USLE was designed to predict long-term average soil losses in runoff from specific field areas in specified cropping and management systems (Wischmeier and Smith, 1978). The USLE has primarily been used to inventory erosion under current conditions and to guide in the development and application of conservation plans (Foster, 1982a). 63
64 The USLE is an empirical model that estimates water erosion as a function of six factors, one of which is the C, or cover and management, factor. (For a discussion of the physical factors, RKLS, see Heimlich and Bills, this volume). The C factor is the ratio of soil loss from an area with specified cover and management to that from an identical area in tilled continuous fallow. It measures the effect of canopy and ground cover on the hydraulics of raindrop impact and runoff; of cover and management on the amount and rate of runoff; of coverage and management on soil structure, organic matter, soil filth, evapotrans- piration, and other soil characteristics; of carryover from previous land use when land use changes; and of roughness from tillage or other disturbances (Foster, 1982a). These effects are evaluated from soil loss ratios-- the ratio of soil loss from a particular practice at a given crop stage on a given soil to that from a unit plot of the same soil. Soil loss ratios vary during the year with crop canopy, ground cover, primary tillage, seedbed preparation, and harvest. A value for C is a weighted ~ , soil loss ratio based on the distribution of rainfall erosivity over the year. For cropland, the figures are based on extensive data from natural runoff plots. Ratios from conservation tillage and construction sites are based on data from rainfall simulators. For undisturbed lands such as rangeland and forestland, C factors are based on subfactor relationships for separate effects. The subfactor method for calculating C factors has been described by Wischmeier (1973, 1975). The C factor is determined by many variables, including weather, that are influenced by management, such as crop canopy, residue mulch, incorporated residues, tillage, and land use residuals. Table 1 illustrates how an annual value for C is calculated (in this case, for continuous corn production). Column 3 lists the cumulative per- centage annual erosion index (EI) for the lower peninsula of Michigan; for any given period, this is a numerical measure of the erosive potential of rainfall. Column 5 gives the fraction of EI that occurs during the event in column 1. For example, 39 percent of the annual EI occurs during crop stage 3. The summation of column 7 (the product of columns 5 and 6) is the annual C factor value for continuous corn for this area of Michigan--0.37. Refer to Agriculture Handbook No. 537 (Wischmeier and Smith, 1978) for further details of the calculation.
65 ,1 A Ail £ Cal o o ·,, ~5 o ~4 U] :' o A. 8 set o o C) o o . - c) U] . L' o U] o C) C) In a Q o ~ · - · - O ~ U) at; - ~ O ~ · - ~ L. H 0) A: _ a) ~Q U] ~5 a, 0- O Ll C) ~_ · - ~ - 1`J ~ ~S S.l H p] _ a, u ~cn CO ~ 00 C0 ~ U~ O O O O O · · 1 · · · 1 1 o o 1 o 0 0 1 1 0 U. ~ ~ O d. ~ 1 In ~) ~ 1 1 · . 1 · · · 1 1 OD ~ 1 ~ 11~ ~ o 1 ~1 ~1 1 ~1 _I ~) 1 ~ 1 [4 U] 1 _~ ~ ~ ~ t~ 1 In ~ ~ ~ ~ 1 ~ ~ u~ ~ a, ~n 00 0 ~ u~ 0 ~ ~ ~ 0 0 O 003 P ~Q C) Y ,q ~ dP d~ d~ 54 _ ~ - - ~ o ° ~ ~ ~O ~ a P4 ~ ~ ~ ~ h E~ o V x · - O · - U] o a o tn ' ~Q U] a; :' o s~ 11 11 oo - s . - CO o V s O c) 11 a a) s O s 3 ~, - V ·` ..Q a, u, a .,, 1 U] 1 o . - U] o S~ a · ~ U] s s o 11 - a cn U a) ] _ U2 a, o Ll ~ o ~ ~ u H ~ ~ 4 ~ 4) H ~V · - S ~Q . - ·,' S ·,4 o ·,' tQ a) Q · - .,, s U) U U] · - U] U U :D o U]
66 In general, C value tables are prepared by people experienced in the calculation procedures. Users of the equation then select C factors most appropriate for conditions in the field. IMPORTANCE OF C FACTORS IN CONSERVATION POLICY Of the various physical and management factors incor- porated in the USLE, the C factor may be the most important, for several reasons. Its range of possible variation affects computed soil loss more than any other USLE factor (Foster, 1982a). C values range from 0.001 for undisturbed forestland with 100 percent cover to 1.0 for clean tilled fallowed land. It is the factor most easily changed through soil management to control erosion. The soil loss ratio for corn in a no-till sod-based system is given in Agriculture Handbook No. 537 as 0.01. Considerable efforts in research and field programs have thus been directed at management practices that affect the C factor. Public policy concerns have focused in recent years on issues related to the C factor, specifically, conservation tillage practices. The Agricultural Stabilization and Conservation Service (ASCS) currently provides financial assistance to farmers to adopt conservation tillage practices. Lastly, the C factor is important because it is probably the factor most in need of revision, especially in the areas of the effectiveness of crop residues in controlling erosion (Cogo et al., 1983, 1984; Laflen et al., 1981) and C factors for rangeland (Foster, 1982b; Osborn et al., 1977) and forestlands (Dissmeyer and Foster, 1985). How Good Are the C Factors? Over the last 25 years the USLE has evolved as its users gained a better understanding of erosion processes and control. Still, as an estimation procedure the USLE is imperfect and subject to specific limitations. Sources of error include: · the empirical relationship itself; · measurement of the parameters that affect the equation parameters;
67 · the application of the USLE in the field; and · the application of the equation to situations for which it has not been substantiated. The latest revision of the USLE was detailed in the Agriculture Handbook No. 537 (Wischmeier and Smith, 1978), which provided the basic guideline for estimating sheet and rill erosion in both the 1977 and 1982 NRIs. Although these guidelines included information on the variables that determine C, they do not reflect current knowledge, for example, on the effectiveness of conservation tillage in erosion control. The context of conservation tillage in relation to C factors is used here to illustrate sources of error in the use of C factors. Consider errors associated with the empirical relation- ship itself. The effectiveness of leaving crop residues on the soil surface to control soil erosion is well established. Although the impact of crop residue manage- ment is included in the C factor relationship used in the 1982 and 1977 NRIs, more recent information indicates that the importance of residues in erosion control has been underestimated. C values are currently selected on the basis of tillage system, spring residue weights, and crop residue cover after planting (Laflen et al., 1981). A C value for conventional tillage at a particular crop stage is multiplied by a residue factor based on percentage residue cover. The residue or mulch factor is illustrated in Figure 1. The relationship is described by: F = e-bM where F = the mulch factor; M = residue cover, percent; and b = a coefficient. The residue cover relationship presented in Agriculture Handbook No. 537 corresponds to a value for b of 0.025. Laflen et al. (1981) reported values for b in the litera- ture ranging from 0.016 to 0.074. Their suggested average value of 0.05 for b is also plotted in Figure 1. Cogo et al. (1984) found b to vary in their study from 0.015 to 0.103. Their findings showed that the b value varied with soil surface roughness and with type and incorpora- tion of residue. The effect of b on the mulch factor F is measurable (Foster, 1984). At 50 percent residue cover, the mulch factor is 0.5 when b is 0.014 and declines to 0.01 when b equals 0.10. The mulch factor is 0.29 when b is 0.025
68 1 0.8 lo 1 ° 0.6 IL - 0.4 :3 0.2 o 0 20 40 60 80 100 Residue Cover- M (%) \ \ \ A, - b = 0.05 \ F -bM \40.025 - - - FIGURE 1 The relationship between percentage residue cover (M) and the mulch factor (F) with b of 0.025 and 0.05 (adapted from Laflen et al., 1981). (Agriculture Handbook No. 537) and is 0.08 when b is 0.05 (Laflen et al., 1981). Two conclusions are clear. First, the effect of residue cover and conservation tillage on erosion control is quite variable. Second, residue cover is more effective in controlling sheet and rill erosion than was considered in the C factors used in the 1977 and 1982 HRIs. The mulch factor relationship also illustrates a second source of error--that associated with measurement. The importance of the measurement of residue cover becomes clear in the curves in Figure 1. At low levels of residue cover, a small change produces a relatively large change in the mulch factor. Richards et al. (1984) reported on the variation in measurement of residue cover using the line intercept method. Six observers measured residue
69 cover on eight strings placed in a field. The authors reported that the variation among strings was greater than that among observers. The mean residue cover ranged from 26 to 44 percent among strings and from 31 to 43 percent among observers. With a b value of 0.025, the mulch factor would range from 0.52 to 0.33 among strings and 0.46 to 0.34 among observers. Using b of 0.05, the mulch factor ranges from 0.27 to 0.11 and 0.21 to 0.12, respectively. Colvin et al. (1981) reported the range of residue cover for various tillage systems. For spring tillage, chisel plowing left residue cover that ranged from 40 to 85 percent. Disking in the spring left residues that provided 42 to 73 percent cover. Corresponding mulch factors for b of 0.025 would range from 0.37 to 0.12 for chisel plowing and from 0.35 to 0.16 for dishing; for b of 0.05, F ranges from 0.14 to 0.01 and from 0.12 to 0.03, respectively. Application of the USLE in the field is a third potential source of error. The 1982 NRI was completed over 3 years, and primary sampling points were visited annually throughout the field season. Do the C values in the 1982 NRI accurately reflect the conditions in the field? C values were selected by field personnel from values tabulated for cropping practices in the particular application area based on present practices. Thus, a corn field with a particular production level tilled using a particular conservation tillane system would have a set C factor. A recent study by Peter Nowak (University .. . . . Of Minnesota, personal communication, 1984) of three watersheds in Iowa may reveal a source of error. Of the 200 farmers interviewed, 78 percent claimed to be using conservation tillage in 1982. Yet only 7 percent of the corn acres of those farmers and 26 percent of their soybean acres had the residue cover recommended by the SCS (Soil Conservation Service) to be categorized as conservation tillage. (The recommended rate of corn residue for conservation tillage is 2,000 lbs/acre or more and for soybeans, 1,000 lbs/acre or more.) The implication is that the erosion control attributed to conservation tillage already on the land may be less than indicated in recent estimates of the extent of this tillage. A fourth source of error is that associated with the application of the USLE to situations for which the equation has not been validated. The USLE has been applied to a wide variety of situations over the years.
70 Of particular concern has been the quality of C values for undisturbed lands, particularly rangelands (Foster, 1982a,b; Osborn et al., 1977) and forestlands (Dissmeyer and Foster, 1981). The data base used to develop soil loss ratios used to calculate C is mainly for cropland situations (Renard and Foster, 1983). Rangelands are the largest single land classification in the United States. Osborn et al. (1977) described the problems associated with the application of the USLE to rangeland conditions as expressed in the Walnut Gulch Watershed in Arizona. They identified the determination of the C factor as the greatest uncertainty in the application of the USLE in the Southwest and suggested that when only rangeland vegetation is considered, ground cover is very low and C is very high. Erosion pavement (the concentration of coarse particles at the soil surface resulting from selective erosion of finer particles) present in Walnut Gulch protects the soil from direct raindrop impact and surface runoff erosion and should be considered in erosion estimates. Conversely, although it is valuable as surface protection, the pavement allows runoff to be concentrated between pebbles, thereby increasing erosion Potential. Foster (1982b) questions whether the effects of pavement on erosion potential properly belong in the K factor of the USLE or the C factor. Gullies (channels) apparently play a strong role in sediment yield on even the smallest rangeland watersheds, a factor not considered in the USLE. Osborn et al. (1977) suggest that a possible channel factor (Ec) be included in the USLE. The fact that single storm events can dominate soil loss from rangelands presents a major consideration when applying the USLE to them. Trieste and Gifford (1980) assessed the applicability of the USLE to rangelands on a per-storm basis and concluded that where sediment yields are dominated by single storm events, the USLE does not explain soil loss and may give misleading rather than useful results. While Foster et al. (1981) disagree with Trieste and Gifford's conclusions, the applicability of the USLE to rangelands remains an issue. Dissmeyer and Foster (1981) used the subfactor approach to develop a procedure for estimating C factors for forest conditions in the southeastern United States and, more recently (Dissmeyer and Foster, 1985), discussed the application of the USLE to other forest, range, and wildland conditions where data to develop C factors are limited or unavailable. This new information was not
71 available in the 1982 or 1977 NRIs, but will certainly be included in future estimates. As the USLE is extended to new situations and reflects new information, however, various NRIs may not be comparable. C FACTORS IN THE 1982 NRI Data compiled in the 1982 NRI provide some information as to the reliability of the C factors. Data from the 1982 NRI were summarized nationally and for four Major Land Resource Areas (MLRAs) in the United States (USDA, 1981). Table 2 lists this summary data on land use, potential for erosion, and C factors nationally and for the four NLRAs. The distribution of C factors for cropland identified in MLRA 105 as cropped to corn for the 4 years reported in 1982 is given in Figure 2. As would be expected, the majority of sampling points without conservation tillage practices falls in a single class, 0.35 to 0.40, and most of those with conservation tillage are in the class 0.15 to 0.20. The broad range of values under each category and the occurrence of a substantial number of sampling points with C factors in the lower classes was unexpected, given that during the last 4 years the crop was corn. The higher C values for conservation tillage might be explained in terms of differences in tillage practice and residue management. Low C values for cropland not in conservation tillage are surprising and warrant further investigation. Variation in C values with potential for erosion may be indicative of the effectiveness of conservation practices. On a national basis, the average C factor varied little with land's inherent potential for erosion, as estimated by the RKLS product of the USLE (see Figure 3). These data suggest that, on the average, conservation tillage practices have not been used on soils with high erosion potentials as much as they could be. The trends vary with region (see Figure 4). For MLRA 105, C values were quite low compared with other regions and decreased significantly with increased erosion potential. This, combined with a similar decrease in P factor with erosion potential, resulted in a diminished slope (0.13) of the estimated versus potential erosion line, relative to other areas. Little change occurred in the C values in MLRAs 103, 134, and 136, and the slope of this line was considerably higher for these areas (0.29, 0.2S, and
72 ~n a) o o ,' :' Q · - s~ U] ·,. U] o C) h ~q s" a, 3 o U] ~; C.) U] O ~ o RS ~ o 3 V o ns o d ,~ ~: o c) s . ... U] 3 ~ C.) E~ p, a) aJ 0 S ~ C) ·,' a.' ~ g <o ·e U] 3 P: 3 S O ~ ~ ~ · - '- C' 3 U] ~ U] a) u' s~ O O ) U] O ~ · - O ·- - o V .,, Q} o P4 a) S o tl? 1 ~ O U] ~ 3 3 o ~o C!) a:, 0 a, ~ · . · ~ o o o o o o . o d~ r~ ~ oo r d~ o ~ · . · .~ o ~ ~ l - ~ u) D · · · . ~ O ~ O o (D ~o oD · . · . ~ o .- ) o o ~ ~ ~ ~ ~ ~z
73 MLRA 105 500 450 an LLJ ~ 400 On 350 300 250 200 100 _ O WITH CONSERV. TILLAGE ~ W/O CONSERV. TILLAGE CLASS RANGES 1 = .00 - .05 2 = .05 - .1 0 3 = .1 0 - .1 5 4 = .15 -.20 5 = .20 - .25 6 = .25 - .30 7 - .30 - .35 8 = .35 - .40 9=.40-.45 ~ 10=.45& UPS _ ~ ~ - · 1 ~ ~ I ~ _ 1 2 3 4 5 6 7 8 9 10 C FACTOR CLASSES FIGURE 2 The distribution of C factors for land cropped to 4 years of corn in HLRA 105 expressed in terms of the number of point samples within a range of C values as summarized from the 1982 NRI. 0.29, respectively). Nationally, the slope of this line was 0.22. Table 3 summarizes acreage and C factors nationally by crop. The major crops (corn, soybeans, wheat, and cotton) account for 82 percent of the 323 million acres in row and close-grown crops. The average C factors were high relative to that obtainable with conservation tillage. The potentials for erosion, as indicated by the RKLS product of the USLE, vary with crop (see Table 3) and show that nationally a considerable portion of cropland soils have a low potential for erosion. This seems especially true for cotton and may explain the high C values for land planted in this crop. Soils with low potential for water erosion, however, may be highly susceptible to wind erosion. Since the C factor, high C factors may indicate a low degree of protection from wind. Table 4 gives the potential for erosion and the percentage of land in conservation tillage by crop for the four MLRAs. About 70 percent of the cropiand in MLRA 103 had a potential for sheet and rill erosion of less than 10 tons/acre/year. Of all cropland in conservation tillage in MLRA 103, some 65 percent was on land with
74 NATIONAL SU M MARY 20 40 _ 30 _ P-FACTOR / 25 _ / ~EROSION J Fly 1\f - _ , . . . . C- FACTO R - 0 30 60 90 120 150 180 AVERAGE RKLS IN CLASS t/ac/yr 1.0 0.8 0.6 O 11 0.4 0.2 0.0 FIGURE 3 Plots of the 1982 NRI weighted average erosion rate (tons/acre/year), C factor, and P factor versus the potential for erosion (tons/acre/year) as expressed by the RKLS product of the USLE. Data are summarized nationally. RKLS of less than 10, and an additional 20 percent was on cropland with an RKLS of 10 to less than 20 tons/acre/ year. For MLRAs 105, 134, and 136, however, 77, 52, and 81 percent of the cropland had an erosion potential of over 20 tons/acre/year, with 24, 17, and 12 percent of that land, respectively, being in conservation tillage. This does not suggest that conservation tillage is not effective on land with low water erosion potential. Table 5 shows that for land with a low potential for erosion (RKLS less than 10 tons/acre/year) in MLRA 103, wind erosion increases as the C factor rises, indicating that conservation measures are effective in controlling wind erosion. However, wind erosion rates are much lower than water erosion rates on sloping lands. There is no apparent explanation for higher wind erosion rates under conservation tillage. The wisdom of considering wind and water erosion estimates cumulatively, which is so often done, is called into question here. The Land Capability Class System (Klingebiel and Montgomery, 1961) is extensively used by the SCS and is
75 25t 20 1 C 1C C o con30 - O 25 En: LLJ 20 15 _ 1n . MLRA 103 P- FACTO Rat / EROSION - I? Aft C-FACTOR it, , . . !oo 0 20 40 60 80 100 MLRA 1 34 WON _ ~ ~C-FACTOR 1.0 30 0.8 25 _ 0.6 _ 0.4 _ 0.2 ~ 1.0 · j0.8 lo6 ~ 1°4 lo.2 20 _ 15 _ 10 ~ 5 _ red _ 3OI 2 20 . 15 10 5 _ MLRA 1 05 _ -__ P-FACTOR - _ 1 1 .o .8 6 EROSION ;~TO h V 0.0 1 ~20 40 60 80 100 o 11 _ 1.0 ~ 0.8 On' O 0.6 MLRA 1 36 ._ P-FACTOR /ROSIO N '' C-FACTOR ho .1°.4 n.2 0.4 _ 0.2 O 1 1 ' I 1 1 0.0 ° _ , , , , o.o 0 20 40 60 80 100 0 20 40 60 80 100 AVE. RKLS IN CLASS t/ac/yr FIGURE 4 Plots of the 1982 NRI weighted average erosion rate (tons/acre/year), C factor, and P factor versus the potential for erosion (tons/acre/year) as expressed by the RKLS product of the USLE for MLRAs 103, 105, 134, and 136. included in the 1982 NRI. One subclass of this system, e or erosion subclass, identifies lands for which erosion is a severe limitation to land use. Directing conser- vation practices at this land should be reflected in its C values. The change in C factor with increase in RKLS for selected land capability classes is given in Figure 5. Nationally, class I land showed the highest C factors when potential erosion was less than 60 tons/acre/year and the lowest C factors when RKLS exceeded that level. There was little change in C for class IIe land. Class IIIe and IVe land showed declines in C at lower potential erosion and a slight increase in C with further increases in RKLS. It appears that conservation measures are considered important on the more erosive class I land and less important on the intermediate erosive class I land
76 En hi; it; s .,. 3 a) a) P4 U] a) o o A' Q ·,1 U] .,, ·. U1 o o C) V A , O · - a) ~4 t) ~ a: P. O U] Pi ~ o Ed U] · o V m 0 ~ d a) a: ~ 0 . - ~d c; ~; s .,, 3 U) o V o a) o V tQ O ~1 O ~ ~ :~: C) - o C) 00 ~ d4 er C~ ..... Z o o o o o o C~ ~ r- cn [_ Oo 0 ~r ~r ~ u~ U) ~ a~ vo ~ ~ cx' ~O ~ ~o eq a) O ,1:: ~ ~ 0 R ~ cr, ~ :>1 ~ ~, a) ~ ~ O o 0 0= 0 0 O cl] t.) U] 3 ~E~ H Pi z C~ cr ·. := o U]
77 v o o ,, .,, S~ U] .,, a ·e u, a. a) ~ .,, ~ E~ v - ~ 0 0 . - U] a P: al ~ 0 0 ~ ·m c: U] ~S U] 0 ~ c) U] ~ 0 E~ ~_ 0 ~ P~ 0 O ~E~ C~ q~ o S 4~ ~q . - u' 3 U] U C O . - 0 q~ O O ,4 C o P4 a, C a, - o o C~ o V a) C C ~, O a ~4 p. 4J _ ~ ~U] k4 ~ n5 U] C O . - ~ U E~ O C o - o U] P. o O U - _ _ _ _ _ _ _ _ _ a:~ ~ _ kD O O ~r ~ o~ u~ _ rn ~ ~c ~ ~ ~ ~ _ _ _ _ _ _ _ _ _ _ _ oo ~ ~ l ~ 1- ~ 1 ~- - - Lr' ~ U) o tD U~ tD U~ U~ C~ CO oo oo _ _ __ _ _ _ _ _ _ _ _ - ~Lr) ax ~u~ ~ ~ _ ~0 _I, - 1- ~ ~1 ~ ~ ~- ~ ~ - 1 _ _ _ _ _ _ _ _ _ _ _ _ _ _ I c ~ ~ ~ rn ~0 u~ ~r . ~ ~ ~I ~ e tD O ~) _1 1 _I ~ ~ ~ r- ~r- (D ~ ~1 ~r ~ 0 ~ u ~1 ~_I ~ ~ ~ CO 0O ~ ~ I ~ _1 1 ~ _I O1 ~ _ ~_~ _~ _I _l c~ ~ ~ ~ co 0 ~ ~ c~ 0 a ~er a, ~r u~ ~ ~ a ~ o~ U) tD (D O ~ ~r 0O ~ ~ ~ ~ U) CO ~CO ~ ., ~ t- ~1 U ~t-_. _1 1 U) ~U] ~U) ~q~ C C C C CC ~C , ~ l ~Q ~ ~C ~ ~to o a, ~ ~ o a' ~ ~/ ~a' ~ 0 ~ ~a' ~ ~ C Q Q1 Q, ~' C Q ~ ~, -1 C Q /0 4~ Q. ~ C Q ~ a, ~ ~ a' 0 ~ ~ a) 0 ~ ~ a) ~ 0 ~ ~ G) 0 <: O o S ~ ~ O o S ~< O o S o ~ ~ O o S 3 ~ ~ ~ ~ 3 Q~ ~ ~ 3 Q~ ~ ~ ~ 3 a ,' o ,' a, U] C o U] a, q~ o a) ~ ~1 a ~ u] Ld ~ o P4 0 4~ ~q c U] ~rl ~ o oO s 4 c aJ c ~l u] z z O Q C H _I ~ Z U] W o a: as ·. o U)
78 TABLE 5 Major Land Resource Area 103: Acreage with Low Potential Erosion, and Wind Erosion Rates by Conservation Tillage Use Potential Eros ion (1,000 Acres Wind Erosion Rates (Tons/Acre/Year) with RKLS <10 Conservation No Conserva C Class Tons/Acre/Year) Tillage Used tion Tillage Overall <0.1 162 0.9 0.8 0.7 0.1--<0.2 463 3.7 1.8 1.6 0.2--<0.3 911 4.2 3.3 3.3 0.3--<0.4 3,741 4.4 3.9 3.7 0.4+ 4,798 5.6 6.1 6.0 SOURCE: 1982 NRI. (RKLS 30 to 60 tons/acre/year). For land in the erosive subclasses IIe, IIIe, and IVe, conservation practices are not emphasized on lands with greater erosion potential. Although Figure 5 shows that a large portion of subclass e land had a low potential for water erosion, a signifi- cant portion of the land with high erosion potential is not receiving conservation treatments. As earlier indicated, the situation varies by area. (see Figure 6). For MLRA 103, C was high (O.51) for class I land with high erosion potential (although the acreage was small). There were over a million acres of class IIIe land in this area in row and close-grown crops (7 percent of the total), most of which had a high potential for erosion. The C factor averaged 0.37 for class IIIe land in MLRA 103 and did not decrease with increase in RKLS. C factors for MLRA 105 were low for row and close- grown crops, averaging 0.21, and were lowest for IIIe and IVe land. They tended to decrease even further with increasing erosion potential. Class IIe, IIIe, and IVe land accounted for 70 percent of the land in row and close-grown crops in MLRA 105. MLRA 134 showed a linear decrease in C with increasing potential for erosion for class I land in row and close- grown crops. C factors for class IVe land started high but decreased with increasing erosion potential for RKLS less than 40 tons/acre/year. C factors for high-erosion- potential class IVe land and for class IIe and IIIe land did not decline with increasing RKLS and averaged about 0.30.
79 NATIONAL SU M MARY 0.4 0.3 O ~ c' 0.2 _ LL _ class I class I le ~ · class Ille 0~0 class 1\/e 0 30 60 90 120 150 180 AVE. RKLS IN CLASS t/ac/yr FIGURE 5 Plots of the weighted average C factor versus potential for erosion (tons/acre/year) as expressed by the RKLS factor of the USLE for land capability subclasses I, IIe, IIIe, and IVe. Data were summarized nationally from the 1982 NRI. In NLRA 136, class IVe cropland with medium potential for erosion (RKLS 20 to 40 tons/acre/year) had lower C factors than cropland in other classes. For the most part, C factors for land in row and close-grown crops in this area did not change with increasing erosion potential. There is no one pattern in the use of conservation measures on class I cropland. Farmers consistently did not increase their use of conservation measures in proportion to increasing erosion potential, especially on lands in the eroded subclasses. This can be a substantial problem considering the acreage in capability classes IIe, IIIe, and IVe (see Figure 7). Both nationally and in MLRA 103, class IIIe land is the primary problem, while in MLRAs 105, 134, and 136, class IIe as well as class IIIe land in row and close-grown crops is important. The C values in the 1982 NRI indicate three things: (1) a considerable tillage has a low potential for water erosion, (2) conservation tillage practices are not adequate on land with medium to high potential for water erosion, and (3) some C values in the 1982 NRI are unexpected and suspect. portion of the land in conservation
80 Or 0.8 0.6 0.4 MLRA 1 03 class I class I le class Ille O class IVe _~ 1 1.0 0.8 _ 0.6 0.4 0.2 ~0.2 _ o (at) 0.8 1.0 0.6 0.4 0.2 O.oL I l I 20 40 60 80 MLRA 134 . ~ ~3 0.0 I ) 20 40 60 80 MLRA 105 'it o.o 1.0 0.8. 0.6 0.4 0.2 40 60 80 0 20 MLRA 1 36 0.0 I 1 1 0 20 40 60 80 AVE. RKLS IN CLASS t/ac/yr FIGURE 6 Plots of the weighted average C factor versus potential for erosion (tons/acre/year) as expressed by the RKLS factor of the USLE for land capability subclasses I, IIe, IIIe, and IVe for MLRAs 103, 105, 134, and 136. The Effect of C Factors on Calculation of Erosion Given the the current state of knowledge about the USLE, what effect would reducing C on cropland have on the extent and degree of soil erosion by water? In a related question, what would be the effect on water erosion of bringing potential cropland into production under various management practices? Figure 8 plots the percentage of acres nationally with erosion rates as estimated with the USLE under three assumed C factors for land currently in row and close-grown crops, land with a high potential for cropland conversion, and land with a medium potential for conversion. With a C factor of 0.3, 73 percent of the land with row and close-grown crops would have erosion rates under 5 tons/acre/year. This compares with the 75 percent actually estimated in 1982 with a calculated
81 20 rl MLRA 103 6 MLRA 105 ~ ~ class 1 5 _ J. 15 ~ lA tciassile / V \ ~ class ille 4 _ / S A= o~431 0 20 40 60 800 20 40 60 80 5: 0 6 ~ MLRA 1346 _ MLRA 136 264 0;~ 0 20 40 60 80 AVERAGE RKLS IN CLASS 0 20 40 60 80 FIGURE 7 Distribution of land in land capability subclasses I, IIe, IIIe, and IVe by RKLS class for MLRAs 103, 105, 134, and 136. average C factor of 0.3 for row and close-grown crops (see Table 1). With an assumed C factor of 0.1, 93 percent of U.S. acreage in row and close-grown crops would have eroded less than 5 tons/acre/year. For land with a high potential for conversion to cropland, the comparable figures are 60 and 89 percent, respectively. And for land with a medium conversion potential, the values are 57 and 82 percent, respectively. These numbers suggest that some latitude exists for reducing erosion through conservation on both existing and potential cropland. Some 7 percent of the cropland would require additional erosion control practices. Again, the situation varies considerably throughout the country (see Figure 9). The situation in MLRA 103 is similar to that nationally. But in MLRA 105, the erosion situation is improved by increased use of conservation practices. However, the erosion potential is so great for these soils that practices such as conservation tillage are not enough to solve the erosion problem. The
82 NATIONAL SUMMARY loo o en 80 o - V Oh ~ _ 60 40 _ 20 o __> ROW & CLOSE GROWN HIGH POTENTIAL MEDIUM POTENTIAL I I 0.1 0.2 0.3 C FACTOR (ASSIG N ED) FIGURE 8 Percentage of acres nationally with USLE erosion rates <5 tons/acre/year at assumed levels of C factors for land in row and close-grown crops in 1982. data also suggest that potential cropland from this area is less desirable than that from MLRA 103 when viewed from strictly an erosion perspective. Land in MLRA 134 is intermediate between MLRAs 103 and 105. Notice, however, the steepness of the curves. This suggests that the benefits of conservation tillage systems on these soils should be dramatic and very visible. The situation for MLRA 136 is similar to MLRA 105 in position and to MLRA 134 in terms of slopes. In all cases, land that has a medium potential for cropland conversion is always below current cropland and high potential land, and the curve generally slopes less. It is clear that reducing the C factor through manage- ment practices can significantly affect soil erosion. But erosion control measures beyond conservation tillage need to be explored and promoted on the land. CONCLUSIONS Foster et al. (in press) described the USLE as "the world standard for an equation to estimate sheet and rill erosion," saying that "no other current equation or procedure for estimating erosion approaches, as a whole,
Inns 80 _ z O 60 cn 40 o Do 20 o.< ) 0.1 - LO 1 nn V Oh IL - o o 80 60 40 An 83 MLRA 103 ROW & CLOSE GROWN ~ HIGH POTENTIAL -- MEDIUM POTENTIAL loor 80 _ 60 _ 40 _ __ 20 _ ~~~_ MLRA 105 . ~o , . . 0.2 0.3 0.0 0.1 0.2 0.3 o MLRA 134 100 MLRA 136 _o, ^< 2 O ' ' ' I o , 0.0 0.1 0.2 0.3 0.0 0.1 0.2 C FACTOR (ASSIG N ED) FIGURE 9 Percentage of acres in MLRAs 103, 105, 134, and 136 with USLE erosion rates >5 tons/acre/year at assumed levels of C factors for land in row and close-grown crops in 1982. the USLE in ease of application, breadth of application, and accuracy." What can be concluded from this brief look at the reliability and accuracy of the C factor in the USLE? A few things are apparent. First, conservation tillage may be more effective in controlling erosion than previously considered. This would support public policies that promote the use of conservation tillage to control erosion. Second, conservation tillage is currently concentrated on land with low potential for erosion. Third, there is probably less crop residue management on the soil surface than recent data on the extent of conservation tillage imply. From a policy standpoint, these two items would suggest that technology transfer or the extension of information regarding conservation tillage to the land user is not adequate and that more effort needs to be directed toward farmers who have land with medium and high potential for erosion.
84 Fourth, conservation tillage is not the sole solution for all soils and landscapes needing erosion control. It is critical that efforts not be limited to a few select control measures. Fifth, C factors will be improved in future NRIs. Revisions of the USLE are currently being done at the National Soil Erosion Laboratory. Last, as C factors change, comparisons to earlier NRIs will become complicated. There are often attempts to compare NRIs in hopes of discerning a change in soil erosion attributable to conservation policy. A measure of caution should accompany such endeavors. In closing, it is far easier to criticize than to construct. The USLE has been an important tool and is reliable when data are available and the equation has been evaluated. Erosion technology has been greatly advanced under the umbrella of the USLE (Dissmeyer and Foster, 1985). Erosion prediction will undoubtably change and improve as the knowledge to do so is gained, in part to the credit of the USLE. REFERENCES Cogo, N. P., W. C. Moldenhauer, and G. R. Foster. 1983. Effect of residue, tillage-induced roughness, and runoff velocity on size distribution of eroded soil aggregates. Soil Sci. Soc. Am. J. 47:1005-1008. Cogo, N. P., W. C. Moldenhauer, and G. R. Foster. 1984. Soil loss reductions from conservation tillage practices. Soil Sci. Soc. Am. J. 48:368-373. Colvin, T. S., J. M. Laflen, and D. G. Erbach. 1981. A review of residue reduction by individual tillage implements. Pp. 102-110 in Crop Production with Conservation in the '80s. Proc. Am. Soc. Agric. Eng., December 1-2, 1980. PUbl. No. 7-81. St. Joseph, Mich.: American Society of Agricultural Engineers. Dissmeyer, G. E., and G. R. Foster. 1981. Estimating the cover-management factor (C) in the Universal Soil Loss Equation for forest conditions. J. Soil Water Conserv. 36:235-240. Dissmeyer, G. E., and G. R. Foster. 1985. Modifying the Universal Soil Loss Equation for new situations. In Proc. of the Second International Conference on Soil Erosion and Conservation, Honolulu, Hawaii, January 16-23, 1983. Ames, Iowa: Soil Conservation Society of America.
85 Foster, G. R. 1982a. Special problems in the application of the USLE to rangelands: C and P factors. Pp. 96-100 in Proc. of Workshop on Estimating Soil Erosion and Sediment Yield from Rangelands. ARM-W-26. Washington, D.C.: USDA Agricultural Research Service. Foster, G. R., 1984. Plan for revising Agriculture Handbook 537 to update the Universal Soil Loss Equation (USLE). Unpublished, April 15, 1984. Foster, G. R., J. R. Simanton, K. G. Renard, L. J. Lane, and H. B. Osborn. 1981. Discussion of Application of the Universal Soil Loss Equation to rangelands on a per-storm basis," by Trieste and Gifford in Journal of Range Management 33:66-70, 1980. J. Range Manage. 34:161-165. Foster, G. R., J. M. Laflen, and C. V. Alonso. In press. A replacement for the Universal Soil Loss Equation (USLE). In Proc. of the ARS-SCS Natural Resources Modeling Workshop. Washington, D.C.: USDA Agricultural Research Service. Klingebiel, A. A., and P. H. Montgomery. 1961. Land Capability Classification. Agriculture Handbook No 210, USDA Soil Conservation Service. Washington, D.C. U.S. Government Printing Office. Laflen, J. M., W. C. Moldenhauer, and T. S. Colvin. 1981. Conservation tillage and soil erosion on continuously row-cropped land. Pp. 121-133 in Crop Production with Conservation in the '80s. Proc. Am. Soc. Agric. Eng., December 1-2, 1980. Publ. No. 7-81. St. Joseph, MiCh.: American Society of Agricultural Engineers. Osborn, H. G., J. R. Simanton, and K. G. Renard. 1977. Use of the Universal Soil Loss Equation in the semiarid Southwest. Pp. 41-49 in Soil Erosion: Prediction and Control. Special Publ. No. 21. Ankeny, Iowa: Soil Conservation Society of America. Renard, K. G., and G. R. Foster. 1983. Soil conservation: Principles of erosion by water. Pp. 155-176 in Dryland Agriculture. Agronomy Monograph No. 23. Madison, Wis.: American of Society of Agronomy. Richards, B. K., M. F. Walter, and R. E. Muck. 1984. Variation in line transect measurements of crop residue cover. J. Soil Water Conserv. 39:60-61. Trieste, D. J., and G. F. Gifford. 1980. Application of the Universal Soil Loss Equation to rangelands on a per-storm basis. J. Range Manage. 33:66-67.
86 USDA Soil Conservation Service. 1981. Land resource regions and major land resource areas of the United States. Agriculture Handbook No. 296. Washington, D.C.: U.S. Government Printing Office. Wischmeier, W. H. 1973. Conservation tillage to control water erosion. Pp. 133-144 in Proc. of the National Conservation Tillage Conference, Des Moines, Iowa, March 28-30, 1973. Ankeny, Iowa: Soil Conservation Society of America. Wischmeier, W. H. 1975. Estimating the soil loss equation's cover and management factor for undisturbed areas. Pp. 118-124 in Present and Prospective Technology for Predicting Sediment Yields and Sources. USDA Agricultural Research Service. ARS-S-40. Washington, D.C.: U.S. Government Printing Office. Wischmeier, W. H., and D. D. Smith. 1978. Predicting Rainfall Erosion Losses: A Guide to Conservation Planning. Agriculture Handbook No. 537, USDA Science and Education Administration. Washington, D.C.: U.S. Government Printing Office. Discussion William C Moldenhauer The National Soil Erosion Laboratory is very much aware of the problems that Pierce and Larson pose concerning the accuracy of erosion measurement and effectiveness of cropping and management, especially with the added com- plication of surface residue. Many of the problems cited can be corrected by building a sufficient data base. Unfortunately, the field is not moving as fast as it should to build this data base. Considerably more research is needed on the roughness-cover-soil type interaction. The soil erosion laboratory's work and the tillage research conducted by John Laflen of the Agricultural Research Service at Ames, Iowa, certainly imply that residue is more effective in controlling erosion than USDA Agriculture Handbook No. 537 shows. But much more data are needed before it can be said with confidence that these numbers can be extrapolated over a wide area. Also, concentrated flow cuts the erosion control effectiveness of mulch drastically by either undercutting or floating the mulch away. The National Soil Erosion Laboratory has done some work on this, but more is needed.
87 The percentage of residue left at planting time on tilled land in the Corn Belt is certainly very dis- appointing. The percentage of mulch cover left for overwintering is quite good on many fields, but by planting time it has been reduced to the point of ineffectiveness in many, if not most, cases. _ . . Bled measurement ot mulch cover is not an exact science at the present time, as Pierce and Larson point out. There is a great deal of human error involved. The National Soil Erosion Laboratory is working with an image analyzer to try to perfect a standard against which measuring techniques can be compared. Lowery et al. (1984) compare different techniques. A standard for practice using the technique is essential, and goes a long way toward making estimates more uniform. However, an accurate standard is essential, or there is a tendency to become biased toward the leader, whose estimates may or may not be the most accurate of the group. Adding a channel factor to the USLE (Universal Soil Loss Equation) is being investigated, because of the concentrated flow erosion or ephemeral sullying common on cronland (see Barfield: Foster, this volume). This _ investigation may help in applying a channel factor on rangeland. A decision must be made on how to predict erosion with consideration of surface stoniness, a condition that has been encountered in Indiana on reclaimed strip-mined soils. There are many such areas in the United States and elsewhere. Box (1981) has discussed the effect on erosion of surface stoniness on cropland soils in the U.S. Southeast. Collinet and Valentin (1984), in West Africa, found C to go from 0.52 with 5 percent fragment cover to 0.005 with 80 percent cover. It is difficult to visualize C factors of 0.05 to 0.30 with moldboard-plowed continuous corn unless the plowing is extremely rough. It is much easier to visualize high C values with conservation tillage because of the very broad definition of this type of tillage. These values should be examined for an explanation. It also seems unlikely that wind erosion is higher with conservation tillage unless residue cover is minimal and flat. In this case, surface roughness caused by the moldboard plow may be more effective than this sparse, flat residue. Fryrear has described this situation in the High Plains of Texas (Moldenhauer et al., 1983). It was surprising to find virtually no decrease in the C factor [except in MYRA (Major Land Resource Area) 105] as RKLS increased. This seems to reflect a much lower C
88 factor than is necessary for erosion control on the low RKLS situations and that conservation tillage is being done for reasons other than erosion reduction. This is a very significant finding of this study and shows that professional conservationists may be taking more credit than they deserve for reducing erosion through conserva- tion tillage. Much work must be done to convince people to use conservation tillage as an erosion reduction measure on erodible land. But, just as a great break- through occurred in the use of conservation tillage when all farmers in the Corn Belt bought a chisel plow to work their soybean land, many then found they could also use it on corn stalks. If farmers with low RKLS (erosivity/ erodibility) situations can make conservation tillage work and are using it, it will be much easier to convince farmers with high RKLS (erosivity/erodibility) situations to use these systems too. Farm operators adopt conservation practices for a variety of reasons. Pierce and Larson should be applauded for looking at the big picture and showing the difficul- ties of applying effective conservation to the land. This helps to focus research and education efforts where they will be most effective. The impatience young scientists feel with the slow pace of solving some of our erosion control and estimation problems is understandable. Until as recently as 10 to 15 years ago, very few farmers would even talk about changing their cropping systems and tillage practices. Yet much progress has been made in 30 years, due to the USLE and early tillage research. Real appreciation of this equation comes only with the perspective engendered by a time when it was impossible to put any quantitative value on soil erosion. Research has been brought to current levels by pioneers in the area of erosion prediction--Dwight Smith, George Browning, Austin Zingg, Walter Wischmeier, and G. W. Musgrave--and by W. E. Larson's pioneering paper on tillage modeling in 1964. Much of the basis of these early efforts was expert intuition. Without these early efforts, scientists might still be stumbling around with no suitable quantitative estimates of any of the factors involved in soil erosion or tillage. Today's modelers are building on these efforts with tools undreamt of in the 1940s and 1950s. Progress is being made faster than anyone could have thought possible just a few years ago. But expert intuition should not be ignored: It is what scientists used in place of computer models, and it is still essential today.
89 REFERENCES Box, J. E., Jr. 1981. The effects of surface slaty fragments on soil erosion by water. Soil Sci. Soc. Am. J. 45:111-116. Collinet, J., and C. Valentin. 1984. Evaluation of factors influencing water erosion in West Africa using rainfall simulation. Challenges in African Hydrology and Water Resources. Proc. of the Harare Symposium, Publ. No. 144. Difford, Wallingford, England: International Association of Hydrological Sciences Larson, W. E. 1964. Soil parameters for evaluating . tillage needs and operations. Soil Sci. Soc. Am., Proc. 28:118-122. Lowery, B., T. M. Lillesand, D. H. Mueller, P. Weller, F. L. Scarpace, and T. C. Daniel. 1984. Determination of crop residue cover using scanning microdensitometry. J. Soil Water Conserv. 39:402-403. Moldenhauer, W. C., G. W. Langdale, W. Frye, D. K. McCool, R. I. Papendick, D. E. Smika, and D. W. Fryrear. 1983. Conservation tillage for erosion control. J. Soil Water Conserv. 38:114-151.