Soil Conservation An Assessment of the National Resources Inventory, Volume 2 (1986) / Chapter Skim
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5. Wind Erosion
5. Wind Erosion
Pages 129-162
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From page 129... ...
In an effort to include wind erosion data with other kinds of soil erosion data for the entire United States, the Soil Conservation Service (SCS) estimated total wind erosion by using National Resources Inventory (NRI)
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From page 130... ...
. This paper includes an analysis of four of the five terms used in the WEE, a possible alternative WEE that would correct some perceived shortcomings of the original equation, a proposed provisional WEE (requiring more research for implementation)
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From page 131... ...
The proposed integral is composed of a function that expresses horizontal soil mass flux at the downwind edge of a field as a function of wind friction velocity, a probability density function for wind friction velocity at a given location, and a threshold friction velocity at which erosion starts.
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From page 132... ...
Substitution into Equation 2 of the mass flux function of wind speed (Equation 3) , the distribution of wind speeds, and the lower limit of wind speed at which the soil erodes gives an expression of expected wind erosion for a flat bare soil:
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From page 133... ...
Figure 1 must be regarded only as highly approximate. n Use of an expectation integral appears justified because of the progress that has been made in the determination of wind speed probability distributions, of the horizontal flux of soil as a function of wind stress, and of threshold friction velocities for the onset of wind erosion.
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From page 134... ...
Indeed, the significant differences of erodibility for the WEE and for the expectation integral based on threshold velocity would indicate an overestimation of erosion using the WEE if the integral method is more correct. Estimate of I Based on Wind Erosion Groups The TORI estimates of wind erosion used tables showing values of I for the various subsets of the soil texture domain wind erosion groups (WEG)
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From page 135... ...
For disturbed sand-textured soils, the mean and standard deviation of threshold friction velocity for seven soils was 31.6 + 8.2 cm/s. For disturbed gravelly soils having sand textures, threshold friction velocity was 61 + 20 cm/s.
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From page 136... ...
The great variability of I values will not be explained solely by a relationship with soil texture. This unexplained variability will probably be greater for soil textures other than sand and loamy sand, although the latter may have considerable variability if gravel is a significant constituent of the surface soil layer.
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From page 137... ...
C Term -- Erodibility Corrected for Climate Effect of Mean Wind Speed Difference In correcting for areas having different wind speed and rainfall climates (using Garden City, Kansas, as the reference area) , the WEE uses a correction based on mean quantities of wind speed and rainfall evaporation.
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From page 138... ...
go ~ o 200 180 160 140 120 1 80 00 40 20 o 200 180 160 140 o 100 80 60 40 20 o ~2 0~2 0~ d (mm)
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From page 139... ...
Indeed, a significant overestimation of wind erosion would be expected using C for mean wind speeds that were lower than those at Garden City, Kansas. Effect of Soil Moisture (7)
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From page 140... ...
, is an increase with downwind distance of the horizontal flux of soil mass in wind erosion. Actually, avalanching is a misnomer because the increase of soil horizontal flux is not related to conversion of the potential energy of erodible soil particles into kinetic energy.
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From page 141... ...
set his wind tunnel 20 mm below the peak of the ridges. Eryrear's method may have simulated field conditions better because it simulates the action of a boundary layer that has its point of zero mean wind speed not too far from the tops of the soil ridges.
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From page 142... ...
i_ \ \ ,, _ _~ 1 1 1 1 0 50 100 150 RIDGE ROUGHNESS Kr (mm) 200 250 FIGURE 6 Ratio of soil loss from ridged surface to soil loss from a flat surface with the same soil versus ridge roughness.
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From page 143... ...
Again, any change of the vegetative cover should be represented in the new wind erosion equation by a change of the threshold velocity and drag coefficient. Knoll Erodibility Increased flux of soil particles on an upslope may be explained by considering the physics of saltation.
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From page 144... ...
, and the erodible soil would affect the threshold friction velocity. Threshold wind speed would also be affected by soil aggregate structure, crusting, and soil moisture.
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From page 145... ...
, which will require much research to implement, an incomplete "provisional wind erosion equation" is suggested here for estimating wind erosion from the 1982 NRI data. The proposed provisional equation corrects some of the shortcomings of the original WEE but will not correct for many effects that will require much more research.
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From page 146... ...
Since the last three factors of this new provisional WEE are either equal to or evaluated similarly to those in the original WEE, and field length is already measured in the 1982 NRI, only the variables U and Ut and their ratio Ut/U must be determined to use this provisional WEE. Values for U are given in Table 3 for selected locations in the United States.
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From page 147... ...
147 TABLE 3 Mean Wind Speed (U) for Selected U
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From page 148... ...
LONG-1) ISTANCE LOSS OF SOIL MATERIAL It would be a worthy goal for estimates of soil removal to be made for that portion of the eroded soil that is carried far from the location of erosion.
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From page 149... ...
The fine-grained sediment carried from fields represents a potential offsite impact as an air pollutant, and upon settling onto surfaces it may have damaging offsite effects. The original WEE refers to total soil loss to a farm field and not to the loss of the fine portion of the soil.
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From page 150... ...
. Considering, however, that the loss of total soil mass for a given width of eroding field is the horizontal mass flux at the downwind boundary of the field, and that the suspended material portion of this flux is approximately equal to the integrated vertical flux of fine material over the entire area of the eroding field, the loss of fine material clearly becomes a much larger fraction of the total loss of soil mass.
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From page 151... ...
The vertical suspension mass flux is probably strongly related to the horizontal saltation flux. This relationship seems to arise because the kinetic energy flux to the surface by saltating particles is related to the horizontal mass flux of saltating particles (Gillette and Stockton, 1985)
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From page 152... ...
4 O Soil 5 Soil 6 O Soil 7 0 Soil 8 v Soil 9 1 1 1 1 1 1 1 1 1 1 1 1 0 100 u* , cm/see FIGURE 8 Clockwise starting with triangle: sampled soils; illustration of horizontal flux q' and vertical flux Fta total soil movement versus wind frictzon velocity; vert~cal flux of particles smaller textures of than 0.02 mm versus wind friction velocity; ratio of vertical flux of particles smaller than 0.02 mm to total soil movement per unit area per time versus wind friction velocity (from Gillette, 1981)
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From page 153... ...
New evidence shows that increase of soil mass flux with field length is related to a feedback mechanism that increases aerodynamic roughness height with field length. An alternative WEE is proposed that would correct for perceived shortcomings of the original WEE.
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From page 154... ...
1982. Threshold friction velocities and rupture moduli for crusted desert soils for the input of soil particles into the air.
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From page 155... ...
1968. Wind Erosion Forces in the United States and Their Use in Predicting Soil Loss.
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From page 156... ...
exp[-nU2/4U2] (the Rayleigh density function)
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From page 157... ...
(A-4) where x equals Ut/U and y equals 0.866x is plotted in Figure A-1 (after Cowherd et al., 1984)
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From page 158... ...
~/L (A-8) where x equals Ut/U.
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From page 159... ...
In any case, values for erosion by wind in these areas slightly in excess of erosion by water do not present the picture of an all-important problem related to wind erosion, although adding tons of soil loss by wind and water may result in high assessment of the areas' overall erosion problems. Although all soil erosion processes deal in principle with the same phenomenon -- namely, the movement of soil in response to applied energy -- there are major differences between air and water as the source of energy.
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From page 160... ...
Hence, there is a strong winnowing effect of wind, at least at relatively low velocities. As discussed, the deterioration of soil quality because of the preferential loss of fine soil particles may be a more appropriate measure of the effects of erosion by wind than the total amount of soil lost.
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From page 161... ...
Low priorities for research on soil erosion by wind are even reflected in the terminology and agenda for this convocation, "Physical Dimensions of the Erosion Problem," which implies Baby water" when using the term erosion. The equation for predicting erosion by water is called the Universal Soil Loss Equation, yet the universe does not include erosion by wind.
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From page 162... ...
162 in a very general way can the relative impact of the two forms of erosion on the potential productivity of U.S. soils be assessed.
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