Quoted from: https://efotg.sc.egov.usda.gov/references/public/WA/The_Wind_Erosion_Equation_(WEQ).htm#:~:text=The%20WEQ%20Critical%20Period%20Procedure,Woodruff%20and%20Siddoway%20in%201965.&text=It%20is%20the%20period%20when,greatest%20potential%20for%20wind%20erosion.

Using wind tunnels and field studies, the late Dr. W. S. Chepil and co-workers set out in the mid-1950's to develop the first wind erosion prediction equation which is now used by the Natural Resources Conservation Service (NRCS) and other action agencies throughout the country.

By 1954, Chepil and his coworkers began to publish results of their research in the form of wind erosion prediction equations. In 1959, Chepil released an equation:

E = IRKFBWD

where:

Wind velocity at geographic locations was not addressed in this equation. In 1962, Chepil’s group released the equation:

E = ¦ (ACKLV)

where:

A =percentage of soil fractions greater than 0.84millimeter.

Factors C, K, L, and V were the same as in the present equation although they were not handled the same. A C-factor map for the western half of the United States was also published in 1962.

In 1963, the current form of the equation:

E =ƒ(IKCLV)

was first released. In 1965, the concept of preponderance in assessing wind erosion forces was introduced.

In 1968, monthly climatic factors were published. These are no longer used by NRCS. Instead, NRCS adopted a proposal for computing soil erosion by periods using wind energy distribution. In 1981, the Wind Erosion Research Unit provided NRCS with data on the distribution of erosive wind energy for the United States and in 1982 provided updated annual C factors.

Although the present equation has significant limitations, it is the best tool currently available for making reasonable estimates of wind erosion. Currently, research and development of improved procedures for estimating wind erosion are underway.

The present Wind Erosion Equation is expressed as:

E = ¦ (IKCLV)

where:

E =estimated average annual soil loss in tons per acre per year

ƒ =indicates relationships that are not straight-line mathematical calculations

I = soil erodibility index

K = soil surface roughness factor

C = climatic factor

L =the unsheltered distance

V =the vegetative cover factor

The I factor, expressed as the average annual soil loss in tons per acre per year from a field area, accounts for the inherent soil properties affecting erodibility. These properties include texture, organic matter, and calcium carbonate percentage. I is the potential annual wind erosion for a given soil under a given set of field conditions. The given set of field conditions for which I is referenced is that of an isolated, unsheltered, wide, bare, smooth, level, loose, and noncrusted soil surface, and at a location where the climatic factor (C) is equal to 100.

The K factor is a measure of the effect of ridges and cloddiness made by tillage and planting implements. It is expressed as a decimal from 0.1 to 1.0.

The C factor for any given locality characterizes climatic erosivity, specifically windspeed and surface soil moisture. This factor is expressed as a percentage of the C factor for Garden City, Kansas, which has a value of 100.

The L factor considers the unprotected distance along the prevailing erosive wind direction across the area to be evaluated and the preponderance of the prevailing erosive winds.

The V factor considers the kind, amount, and orientation of vegetation on the surface. The vegetative cover is expressed in pounds per acre of a flat small-grain residue equivalent.

Solving the equation involves five successive steps. Steps 1, 2 and 3 can be solved by multiplying the factor values. Determining the effects of L and V (steps 4 and 5) involves more complex functional relationships.

Step 1: E₁ = I

Step 2: E₂ = IK

Step 3: E₃ = IKC

Step 4: E₄ = IKCL

Step 5: E₅ = IKCLV

Limitations of the equation

When the unsheltered distance, L, is sufficiently long, the transport capacity of the wind for saltation and creep is reached. If the wind is moving all the soil it can carry across a given surface, the inflow into a downwind area of the field is equal to the outflow from that same area of the field, for saltation and creep. The net soil loss from this specific area of the field is then only the suspension component. This does not imply a reduced soil erosion problem because, theoretically, there is still the estimated amount of soil loss in creep, saltation, and suspension leaving the downwind edge of the field.

The equation does not account for snow cover or seasonal changes in soil erodibility. The equation does not estimate erosion from single storm events, and surface armoring by non-erodible gravel is not usually addressed in the I factor.