Although many of the principles of wind erosion were known before the 1930's, the foundations of modern wind erosion prediction technology largely began with the publication in 1941 of Ralph Bagnold's classic book titled "The Physics of Blown Sand and Desert Dunes". Further research was needed for application to agricultural fields, which are generally more complicated than sand dunes. The complications include properties that change over time such as soil, aggregate size and stability, crusts, random and oriented roughness, field size, and vegatative cover.
The Wind Erosion Equation (WEQ)
Using wind tunnels and field studies, the late Dr. W.W. Chepil and co-workers set out in the mid-1950's to develop the first wind erosion prediction equation which was used by the Natural Resources Conservation Service (NRCS) and other action agencies throughout the country until recently. It has since been replaced by the user friendly, processed based Wind Erosion Prediction System (WEPS).
WEPS represents a significant science improvement to predict wind erosion on site and quantify the offsite movement of soil to include PM-10.
The WEQ equation expressed in function form is: E = f (I, K, C, L, V)
where E is the potential average annual soil loss, I is the soil erodibility index, K is the soil ridge roughness factor, C is the climate factor, L is unsheltered distance across a field, and V is the equivalent vegetative cover.
Because field erodibility varies with field conditions, a procedure to solve WEQ for periods of less than one year was devised. In this procedure, a series of factor values are selected to describe successive management periods in which both management factors and vegetative covers are nearly constant. Erosive wind energy distribution is used to derive a weighted soil loss for each period. Soil loss for the management periods over a year are added to estimate annual erosion. Soil loss from the periods also can be added for a multi-year rotation, and the loss divided by the number of years to obtain an average, annual estimate.
WEQ was a widely used method (Excel Spreadsheet) for assessing average annual soil loss by wind from agricultural fields. The primary user of WEQ was the Natural Resources Conservation Service (NRCS). When WEQ was developed approximately 40 years ago, it was necessary to make it a simple mathematical expression, readily solvable with the computational tools available. However, WEQ had fundamental weaknesses because of its equation structures and its empirical representation of erosion processes. Since its inception, there have been a number of efforts to improve the accuracy, ease of application, and range of WEQ. Despite efforts to make such improvements, the structure of WEQ precluded adaptation to many problems.
A power point slide presentation regarding the issues with WEQ can be viewed by clicking here.