Soil Conservation An Assessment of the National Resources Inventory, Volume 2 (1986) / Chapter Skim
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2. Assessing Soil Erosion Productivity Damage
2. Assessing Soil Erosion Productivity Damage
Pages 21-62
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From page 21... ...
The erosion process is gradual, and annual yield variability from weather, disease, and pests obscures the inexorable reduction of crop yields from soil loss. In many regions, technical progress has boosted crop yields faster than erosion has reduced them, perhaps persuading some that erosion damage is of no consequence.
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From page 22... ...
develop and explain fundamental concepts for the correct assessment of productivity damage from soil erosion, (2) present empirical evidence from the Pacific Northwest Palouse region on the historical nature of technical progress and its interaction with erosion as both processes have influenced winter wheat yields through time, (3)
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From page 23... ...
Two possible comparisons are relevant. In a comparison of yield with erosion versus yield without, the basis for comparison is yield after zero erosion, i.e., with unchanged topsoil depth.
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From page 24... ...
Avoid Confounding Tillage Yield Penalty and Damage Even with a dynamic basis for comparison, where two different relevant tillage systems generate the two topsoil depths used for the yield damage assessment, one yield-topsoil depth response function must be used to measure yield at both the conserved and eroded topsoil depths. The conservation tillage response function should be used on the premise that ultimately conservation will be required to protect the soil.
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From page 25... ...
Conventional Tillage Yield Function Conservation Tillage Yleld Function _,Ye l I 17 ll l 5.2 15.4 Topsoil depth (inches) Avoid confounding erosion damage with tillage Distinguish Between Reparable and Residual Yield Damage It is useful to partition yield decline from soil erosion into two components, reparable damage and residual damage.
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From page 26... ...
made. From initial topsoil depth A, using the conservation practice for a number of years would reduce topsoil depth to E and yield to G -- providing the basis for comparison with the erosion alternative.
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From page 27... ...
Curve Yo illustrates the yield damage from the additional erosion with conventional tillage compared to conservation tillage over a 64-year period with static technology; that is, no technical progress in yields. In this example (identical to Figure 1)
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From page 28... ...
Technology shifts the yield function upward from Yo to Yn, increasing yield by an equal absolute amount at each topsoil depth. Land-neutral technical progress is most likely on cropland with deep, friable subsoils.
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From page 29... ...
Even though there has been an upward yield trend over time in this example, exogenous technology has not reduced erosion damage. Land-Complementary Technology Land-complementary technical progress boosts yields more at deeper topsoil depths as illustrated by the shift from Yo to Yn in Figure 5.
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From page 30... ...
It is important to incorporate technology projections in erosion damage assessment. Ignoring technical progress will result in unbiased damage estimates only with landneutral technical progress.
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From page 31... ...
for damage assessment with exogenous technology is yield with technology on conserved soil versus yield with the same exogenous technology on eroded soil. Yield damage is measured along the single technology-augmented yield function.
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From page 32... ...
FIGURE 7 Residual yield damage with induced technology. Damage assessment should be based on yield with conservation and unchanged technology versus yield with erosion and induced technology.
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From page 33... ...
The correct damage measure with induced and exogenous technology combined would be yield with conserved soil and exogenous technology at G versus yield with eroded soil and induced technology at C' . NATURE OF PAST TECHN ICAL PROGRESS: EMPIRICAL EVIDENCE FOR WINTER MEAT YIELDS IN THE PALOUSE REGION The discussion in the preceding section established the importance of projecting technology trends as well as erosion rates to assess erosion damage accurately.
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From page 34... ...
(1985) conducted a statistical evaluation of the impact of technical progress on winter wheat yields in the eastern Palouse region of southeastern Washington between the 1950s and the 1970s.
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From page 35... ...
(~.79) , where Y and D represent winter wheat yield in bushels per acre and topsoil depth in inches, respectively.
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From page 36... ...
. The empirical yield response functions in Figure 9 should also permit conclusions concerning residual yield damage, as described in the previous section.
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From page 37... ...
As shown earlier in this paper, exogenous, land-complementary technical progress increases erosion damage. Thus, technology in the Palouse, and perhaps in the rest of the nation, rather than mitigating the problem of erosion and yield damage, has actually intensified it.
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From page 38... ...
uses topsoil depth (depth of the mollic epipedon4) as a proxy for soil properties such as organic matter content and bulk density that are affected by erosion and are correlated with topsoil depth.
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From page 39... ...
and measures the present value of the lost income over 75 years from reduced future yields due to 1 year of erosion with conventional tillage in wheat. The price of wheat was $3.60/bushel and the initial topsoil depth was assumed to be 10 inches, a moderately to severely eroded soil in the Palouse.
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From page 40... ...
As described earlier, measuring erosion damage requires detailed information on crop yields by erosion status le.g., remaining topsoil depth) for different tillage systems.
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From page 41... ...
ADDITIONAL DATA NEEDS FOR DAMAGE ASSESSMENT Along with the technical information available from the NRI data base, regional or national erosion damage assessments will require assembling data (or assumptions) on: Static technology yield relationships -- Yield penalties for alternative management systems -- Crop yield impacts of erosion within a given technological era · Technical progress relationships -- Whether technical progress for various regions and crops is induced or exogenous with respect to erosion -- Whether technical progress for various regions and crops is land-substituting, -neutral, or -complementary -- Projected rates of technical progress, for various regions and crops
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From page 42... ...
either be estimated the Palouse wheat yield or synthesized using as the Erosion Productivity developed by Agricultural Temple, Texas (Williams et , _ ~ ~ Appropr~ate data sets for estimating empirical topsoil depthyield relationships are unlikely to be available or affordable for all major crops and production regions. Where appropriate data are available, these relationships should be estimated to validate and calibrate the general simulation models.
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From page 43... ...
If technology differs from this assumed uniform pattern for some crops and regions, substantial bias in damage projections could result. In future RCA appraisals, it would be desirable to elicit forecasts of technical progress patterns from agricultural scientists who are familiar with past technical advances in crop yields for major production regions and are knowledgeable about likely developments in the foreseeable future.
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From page 44... ...
distinguish between reparable and residual yield damage, and include both components in the cost of erosion damage; and (4) project the separate effects of erosion and technology to avoid errors in erosion damage assessment caused by confounding erosion and technical progress.
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From page 45... ...
The evidence on the exogenous, land-complementary technical progress affecting winter wheat yields in the Palouse confirms that technology has complemented, not substituted for, soil conservation. A prudent strategy for ensuring future productivity includes continued support both for basic research to promote future technical progress and for vigorous soil conservation programs.
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From page 46... ...
1982. Evaluating the Long Run Impacts of Soil Erosion on Crop Yields and Net Farm Income in th Palouse Annual Cropping Region of the Pacific Northwest.
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From page 47... ...
1985. Separating erosion and technology impacts on winter wheat yields in the Palouse: A statistical approach.
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From page 48... ...
In the general case of land-substituting technology, the new or improved input is applied at a uniform rate for all topsoil depths. The entire yield function shifts upward but in a fashion that reduces the slope of the restored-yield curve.
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From page 49... ...
Potential yield on conserved soil shifts with improved technology from G to G' because of the new or improved input associated with the technical advance. That same input level is applied for all topsoil depths along the constant-input yield curve, shifting it from GH to G'H'.
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From page 50... ...
Because of the greater application of the new or improved input, the shift II' exceeds the shift HH' and reparable damage increases with erosion-compensating technology, I' - H' I - H Residual damage decreases with erosion-compensating technology because the restored-yield curve shifts upward _ more at shallow topsoil depths, II' > GUI'.
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From page 51... ...
Reparable damage is the same as with exogenous technology because reparable damage is measured at the eroded topsoil depth, where the yield function shift from technology is the same whether it is induced by erosion or exogenous. Because induced technology reduces residual damage more than exogenous technology does while the effect on reparable damage is the same, induced erosion-compensating tech- nology reduces overall yield damage more than exogenous technology does, G - H' < G' - H'.
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From page 52... ...
Taking the time derivatives of Equation B-3 as topsoil depth approaches 0 and infinity, a' and (a' + b') , respectively, can be interpreted as the annual rates of change in wheat yields
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From page 53... ...
(B-7) Equation B-7 describes the combined impact of technical progress and erosion on wheat yields given the pattern of recent technical progress for winter wheat in the Palouse.
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From page 54... ...
.(B-8) Given the presence of technical progress, continuing soil erosion, and the restrictions on parameters imposed at the outset, the definitive sign determinations noted in Equation B-8 can be made.
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(B-ll) Given the presence of technical progress, continuing erosion, and the restrictions on the parameters imposed at the outset, we can make the definitive sign determinations noted in Equation Bell.
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From page 56... ...
. The period 1953-1913 happened to bracket a period of atypically rapid technical progress in wheat yields in the Palouse.
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From page 57... ...
. Differential calculus is used to solve algebraically in Equation B-13 for the effective rate of yield growth exhibited by Equation B-12 considering both technical progress and erosion: dYt/dt = (a' + b')
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From page 58... ...
. As an example, projecting eastern Palouse regional average winter wheat yield for 1982: Y1982 = [24~46 + 0.393(30)
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From page 59... ...
1983. Yield-topsoil depth response functions: Linear versus MitscherlichSpillman.
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From page 60... ...
}/(l + r) , where P equals crop price; Dt equals topsoil depth at the end of year t*
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From page 61... ...
Price is treated as an exogenous variable in this formulation. In applying the damage model for regional or national erosion damage assessments, it would be desirable to allow for endogenous changes in equilibrium crop prices with cumulative erosion over time.
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From page 62... ...
Therefore, yield damage is computed using the conservation yield function. The second bracketed term is the reparable damage due to current-year erosion.
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