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This article throws light upon the fourteen important variables used for estimating wind erosion. Some of the variables are: 1. Soil Erodibility, I 2. Knoll Erodibility, Is 3. Surface Crust Stability, Fs 4. Soil Ridge Roughness, Kr 5. Velocity of Erosive Wind, V 6. Soil Surface Moisture, M 7. Distance Across the Field, Df 8. Sheltered Distance, Db 9. Quantity of Vegetative Cover ‘R’ and Others.
Variable # 1. Soil Erodibility, I:
Soil erodibility is the potential soil loss in tonnes per acre per annum from a wide, unsheltered, isolated field with a bare, smooth, non-crusted surface. It is related to soil cloddiness and its value increases as the percentage of soil fraction greater than 0.84 mm in diameter decreases. It can be determined by standard dry sieving procedure and use of Table 4.8.
Variable # 2. Knoll Erodibility, Is:
It is a factor needed to compute erodibility for windward slopes less than about 500 ft. long. It varies with slope and is expressed in terms of per cent slope (Fig. 4.9). The erosion rate for windward slopes longer than 500 ft. is about the same as from level land; therefore, Is is taken as 100% for this situation.
Variable # 3. Surface Crust Stability, Fs:
The mechanical stability of the surface crust, if a crust is present, is of little consequence because it disintegrates readily due to abrasion after wind erosion has started and hence for big areas and large time it is disregarded. It is of significance where erodibility of a field at a given moment is considered.
Variable # 4. Soil Ridge Roughness, Kr:
Soil ridge roughness is a measure of soil surface roughness other than that caused by clods or vegetation. It is the natural or artificial roughness of the soil surface in the form of ridges or small undulations. It can be determined from a linear measure of surface roughness.
Variable # 5. Velocity of Erosive Wind, V:
It is observed that rate of soil movement varies directly as a cube of the wind velocity. Where average annual soil loss determinations are desired, the mean annual wind velocity corrected to a standard height of 30 ft is used.
Variable # 6. Soil Surface Moisture, M:
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The rate of soil movement varies approximately inversely as the square of effective surface soil moisture. Since detailed surface soil moisture is generally not available for different geographic locations, M is assumed to be proportional to the Thornthwaite potential evaporation index.
Variable # 7. Distance Across the Field, Df:
Df is the total distance across a given field measured along the prevailing wind erosion direction. Fig. 4.10 presents an alignment chart for determining the distance, Df, along the wind direction for different widths of fields.
On an unprotected, eroding field the rate of soil flow is zero on the windward edge and increases with distance to leeward side until the flow reaches a maximum that a wind of particular velocity can sustain (only on large fields). The distance required for soil flow to reach this maximum on a given soil is the same for any erosive wind.
Variable # 8. Sheltered Distance, Db:
Sheltered distance Db, is the distance along the prevailing wind erosion direction that is sheltered by a barrier, if any, adjoining the field. Db, is determined by multiplying the height of the barrier by 10.
Variable # 9. Quantity of Vegetative Cover ‘R’:
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The total amount of vegetative residue connected with the wind erosion equation are based on washed, oven dry, weighed and multiplied by 1.2 to make them comparable to the usual field measurements where samples are air dried.
Variable # 10. Kind of Vegetative Cover, S:
S is a factor denoting the total cross-sectional area of the vegetative material. The finer the material and the greater its surface area, the more it reduces the wind velocity and the wind erosion. Values of S assigned to different kinds of vegetative materials are as given in Table 4.9.
Variable # 11. Orientation of Vegetative Cover, K0:
It is in effect the vegetative surface roughness variable. The more erect the vegetative matter, the higher it stands above the ground, the more it slows the wind velocity near the ground and lower is the rate of soil erosion. K0 includes the influence of distribution and location of vegetation such as width and direction of rows, uniformity of distribution and whether the vegetation is in a furrow or on a ridge.
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K0 has been assigned a value of 1.0 for absolutely flat, small grain stubble with straw aligned parallel with wind direction on smooth ground in rows 25 cm apart at right angles to wind direction.
For other orientations and other residues, K0 varies as a power function of the amount of residue, R’, for values of R’ greater than 1000 lbs./acre. The exponent ranges from approximately 0.5 for flattened small grain or sorghum to 0.25 for standing small grain and 50 cm high sorghum.
Variable # 12. Rate of Soil Movement:
Rate of soil movement (dry soils and dune sands) by winds of greater than minimal fluid threshold velocity can be obtained from the relation.
where, q = rate of soil movement (total weight of soil material moved past a unit width normal to the direction of movement and of unlimited height per unit time)
De = equivalent diameter of the particles moved by wind
Ïa = density of fluid
V’* = drag velocity over the eroding surface
a = coefficient, whose value depends on the size distribution of erodible particles, the proportion of fine dust particles present in the mixture, the proportion and size of non-erodible fractions, position in the field, and the amount of moisture in the soil.
V* is related to VZ (wind velocity V at height z), Vt (threshold wind velocity, i.e., minimal velocity at some specified height z to initiate soil movement) as given below:
where, k’ = height above z0 where wind velocity over the eroding surface is constant at any wind velocity.
Before erosion can occur, wind must overcome the resistance r, offered by the cohesive force due to water films around soil particles and the farce of gravity of the grains. The value of r is approximately equal to 6(Me)2; where Me is the moisture equivalent. Since for smooth (z0 < 2.5 cm); V’* is equal to 
the rate of movement of wetted erodible material, utilizing Eq. (4.15), would be:
and the relative amount of wet soil removed from the limited area would be:
Variable # 13. Weight of Erodible Soil:
Weight of wind erodible soil material, X, from a given field is related to drag velocity of wind as
X = a (V’*) …(4.19)
where, a = constant
It can also be expressed in terms of soil erodibility index IW as
IW = a (V’*)5 …(4-20)
IW is numerically equal to X2/X1, where X1 is the weight of erodible soil per unit area from a small area (width less than 9 m along the wind direction), such as in a wind tunnel, containing 60 per cent of clods greater than 0.84 mm and X2 is the weight of erodible soil under the same set of conditions from soil containing any other proportion of clods greater than 0.84 mm.
Variable # 14. Dust Concentration:
Concentration of dust in air has been found to vary with height by the relation.
C2 = a / zb …(4.21)
where, C, = dust concentration (weight of dust per unit volume of air) at height z above the ground,
a, b= constants. Value of a varies with the intensity of erosion and b = 0.28.
Figure 4.17 shows a generalized relationship between total quantity of dust load in the lower atmosphere and day time visibility. In the figure, dust load in cm refers to the square mile against the earth’s surface and 1 mile high.
Relationship between variables:
Considering the relationship between soil erodibility and some of the primary variables, some variables have been disregarded, some grouped together and some converted to their equivalents as follows:
Thus, the eleven primary variables have been reduced to five equivalent variables.
The general functional relationship between the potential average annual soil loss, Xa tonnes/acre/annum, and the equivalent variables may be expressed as:
X’a = F(I’, C’, K’, L’, V)…(4.11)
The relation between X’a and V is exponential and is of the form X’a = f(eV), while that between X’a and L’ is a power function of the form X’a =f(L’- b)n. The variables L’, K’, C’ are simple product functions. A single equation expressing X’a as a function of the 5 dependant variables has not yet been derived.
A general description for assessing each variables is briefly given below:
Soil and knoll erodibility, I’, is obtained by multiplying soil erodibility, I (Table 4.8) by knoll erodibility/, (Fig. 4.9) if a knoll or hill is present.
The local wind erosion climatic factor, C”, can be calculated from the relationship:
where, V = mean annual wind velocity for a particular geographic location corrected to a standard height of 30 ft.,
P – E = Thornthwaite’s P – E ratio = 10(P/E)
= 115 (P/T-10)1.111
Recently, FAO (1977) has used a modified index for computing C’.
where, V = mean monthly wind speed at 2 m height in m/s
P = precipitation, mm
PET = potential evapotranspiration, mm
PET – P/PET refers to the number of erosive days in a month.
The soil ridge roughness factor, K’, is expressed in terms of height of standard soil ridges spaced at right angles to the wind and with a height-spacing ratio of 1 : 4. Figure 4.11 presents a curve for obtaining the equivalent soil ridge roughness factor, K’, from a measure of Kr.
The curve is based on a design velocity of 50 miles per hour at 50 ft height with wind direction at 45° to the ridges. Ridges of 4 to 10 cm are most effective in controlling erosion.
The equivalent field length, L’, is the unsheltered distance across the field along the prevailing wind direction. Thus, L’ = Df – Db. The equivalent vegetative cover variable, V, is obtained by multiplying the variables R’, S, and K0 = f(R’) together.
Values of V computed for various kinds and amounts of residue are presented in Figs. 4.12,13 and 14. Since L’ and Fare not simple functions of X’a a graphical solution as shown in Figs. 4.15 and 4.16 for this part of calculation has been suggested.
X’a can be finally expressed as:
X’a = V x K’ x C’ x f(L’) x f(V)…(4.14)
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