Using Barriers for Erosion Control in WEPS

Using WEPS, we can quickly determine the field edge where the greatest amount of eroded soil is leaving the field. In most cases, a field windbreak would be most effective on the upwind side of this field.

Wind barriers in WEPS include any structure designed to reduce the wind speed on the downwind side of the barrier. Barriers trap moving soil and reduce abrasion of the downwind immobile clods, crusts, and residues along the prevailing wind erosion direction. Barriers include, but are not limited to, linear plantings of single or multiple rows of trees, shrubs, or grasses established for wind erosion control, crop protection, and snow management. Snow fences, board walls, bamboo and willow fences, earthen banks, hand-inserted straw rows, and rock walls have also been used as barriers for wind erosion control in limited situations. Barriers also reduce evapotranspiration, shelter livestock, and provide wildlife habitat. One advantage of barriers over most other types of wind erosion control is they are relatively permanent. During drought years, barriers (excepting annual types) may be the only effective and persistent control measure on crop land. Annual barriers such as small grain or corn are used primarily to provide temporary protection during the most critical wind erosion period and can be removed and replaced every year. Barriers can also be used in sand dune areas to aid the initial stabilization of the areas while grass and trees are being established.

Barriers primarily alter the effect of the wind force on the soil surface by reducing wind speed on the downwind side of the barrier but also reduce wind speed to a lesser extent upwind of the barrier (Fig. 1.14). Research has shown that barriers significantly reduce wind speed downwind, sheltering a portion of the field from erosion and, in effect, reducing the field length along the erosive wind direction. The protected zone of any barrier diminishes as porosity increases however, and is reduced significantly when barrier porosity exceeds 60 percent. Protection is also reduced as wind velocity increases, but the protected area diminishes as the wind direction deviates from the perpendicular to the barrier. Various types of barriers are used for wind erosion control in WEPS 1.0. The WEPS interface provides a method of selecting from a list of barriers to place on the field and editing the barrier properties. The user can also modify properties in the barrier database that appear in the drop-down list. Each of these properties are described here.

The length of a barrier is defined by field length along the border on which the barrier is placed.

Width	The width of a barrier is defined as the distance from one side of the barrier to the other, in the units of measure displayed on the screen (feet or meters) (Fig. 1.15). For a single-row wind barrier, the width is equal to the diameter of the tree, shrub, or grass; for artificial barriers, it is the thickness of the material (e.g. slat fence). This is illustrated as "a" in Fig. 1.15. For multiple-row barriers, the width is the distance from one side of the barrier to the other as illustrated by "b" in Fig. 1.15.

Height	The height of a barrier is the average height of individual elements (e.g., trees) in the barrier ("a" in Fig. 1.16 for single-row barriers). The units of measure for barrier height are displayed on the input screen in feet or meters. For multiple-row barriers, use the height of the tallest barrier row ("b" in Fig. 1.16).

Area	The area of the barrier is calculated from the barrier width and length (i.e., barrier width x field length). This is not an editable item, but is calculated within WEPS 1.0.

Porosity	Barrier porosity is defined as the total optical porosity of all rows in the barrier. It is the open space (i.e., absence of leaves and stems) as viewed looking perpendicular to the barrier, expressed as a percentage of the total area (ie., (1.0 - silhouette area) x 100). WEPS 1.0 does not "grow" living barriers. Barriers in WEPS do not increase or decrease porosity with leaf growth and leaf drop (senescence) throughout the year, nor do they increase in size from one year to the next. As such, the porosity of barriers in WEPS does not change with the seasons nor from year to year. Therefore the user should input the porosity of the barrier that is present when the erosion hazard is the greatest. Figure 1.17 illustrates the effect of porosity on the near-
surface wind speed relative to an open field without a barrier (see also Fig. 1.14). The "Distance in Barrier Heights" refers to the distance from the barrier at 0, measured in multiples of the barrier height.

At times, it is most efficient to estimate optical porosity for a single row, particularly for crop barriers. Then, for multiple-row barriers, the optical porosity decreases for the entire barrier as illustrated in Figure 1.18. For example, a single row of corn has an optical porosity of 0.80. Three rows of corn have an optical porosity of 0.50, and five rows of corn have an optical porosity of 0.33.