Exercise 9 - Selecting the correct simulation region, X-length and Y-length

Skill Building: The X-length is the longest (one direction) distance from a stable boundary to the opposite site of the field. The Y-length is the longest (90 degrees to the X-length) distance from a stable boundary to the opposite side of the field. These are the unsheltered distances WEPS will use to calculate the erosion rate. A stable boundary is one that stops surface creep and saltation phases of wind erosion. A grass strip at least 13 ft wide and 1.5 foot high would be an example of a stable boundary. Vegetation width, height, and porosity are to be considered in declaring a stable boundary.

This exercise will assist the user in selecting the correct distance for the X-length and the Y-
length in the Simulation Region Information panel of the main WEPS interface window.

Scenario: See Figure 1.84. A 40 acre field (tan) is to be evaluated for wind erosion. The remaining land (green) is not controlled by the 40 ac owner and the management on the remaining land cannot be controlled by the owner of the 40 acre tract. The remaining land may however contribute to the wind erosion process on the 40 acre field. The green land (one section in size, 640 ac) has a road on all sides with a 50 ft band of green vegetation (grass 1.5 ft tall perimeter buffer). The non-erodible buffer is represented in dark brown. Both the 40 acres and the remaining 600 acres are farmed in a winter wheat-summer fallow, conventional tillage rotation.

Question: What is the correct X-Length and Y-Length to evaluate or enter in the Simulation Region Information panel on the WEPS interface?

A. Y=1320 ft by X=2640 ft,

B. Y=3960 ft by X=5280 ft,

C. Y=1320 ft by X=1320 ft, or

D. Y=5280 ft by X=5280 ft.

Answer: B. Y=3960 ft by X=5280 ft.

Remember that in WEPS, wind directions are simulated to vary from day to day throughout the simulation. Because the management outside the field is not controlled by the same owner for conservation planning purposes, the unsheltered distance should be the longest distance from a stable boundary through the length of the field to the down-wind edge of the field. On the X axis the longest distance (X-Length) would start at the East boundary and extend West, all the way though the 40 acre field (5280 ft.). On the Y axis the longest distance (Y-Length) would start at the South boundary and extend North all the way though the 40 acre field (3960 ft.). Therefore the dimensions (X-Length, L-Length, and acres) entered on the WEPS interface will be larger than the actual field for this scenario.