Barriers

Wind barriers in WEPS include any structure designed to reduce the wind speed on the downwind side of the barrier. They also trap moving soil. 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 that they are relatively permanent. During drought years, barriers may be the only effective and persistent control measure on crop land. 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 they also reduce wind speed to a lesser extent upwind of the barrier (Fig. 6.1).

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, and is reduced significantly when barrier porosity exceed 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 in this section.

Adding and Removing Barriers Using the Interface

The Field View panel (Fig. 6.2), located in the center of the WEPS1.0 main screen, is designed to give the user a view of the field size, shape, and orientation (green). The placement of any barriers present is displayed in red. Note that if the ratio of actual length to width of the field or barriers is too great to display to scale, this will be indicated within the panel, and an approximation of the field or barrier shape will be displayed. This panel is for viewing only and is not editable.

The Wind Barrier Information panel (Fig. 6.3) is used to add barriers to the field. Note that WEPS1.0 only allows barriers on the borders of the field. The barrier location for each field border is labeled 'N' for north, 'S' for south, 'E' for east, and 'W' for west. The barrier type can be selected from the drop-down list in the panel by clicking the down arrow to the right of the barrier type to bring up the list of available barriers, and then clicking on the appropriate barrier. Once a barrier type is selected, the barrier properties may be viewed and edited by clicking the 'Edit Selected Barrier' button at the bottom of the panel. A separate panel opens, in which the user may change the default barrier width, height, and porosity values in the appropriate fields. The modified barrier parameters are stored with the project. If a barrier other than 'None' is selected, the 'Edit Selected Barrier' button will open the properties panel when the radio button is clicked on for that barrier. To remove a barrier from the field, click the radio button to select it (notice the barrier in the View Panel will be 'highlighted' when selected), then select the barrier type 'None' to remove it.

Edit Selected Barrier

To view and edit the properties of a barrier, click the radio button for the corresponding barrier , then click the 'Edit Selected Barrier' button. A window will open displaying the properties described next. If properties are modified by the user through the interface, the barrier type will display '<mod>' in front of the barrier type name.

The length of a barrier is defined by the 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. 6.4). For a single-row wind barrier, the width is equal to the diameter of the tree, shrub, or grass, or for artificial barriers, the thickness of the material (e.g., slat fence). This is illustrated as "a" in Fig. 6.4. For multiple-row barriers, the width is the distance from one side of the barrier to the other as illustrated by "b" in Fig. 6.4.

Height	The height of a barrier is the average height of individual elements (e.g., trees) in the barrier ("a" in Fig. 6.5 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. 6.5).

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 (i.e., (1.0 - silhouette area) x 100). WEPS 1.0 does not "grow" living barriers. They do not increase or decrease porosity with leaf growth and leaf drop (senescence), 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 6.6 illustrates the effect of porosity on the near-surface wind speed, relative to an open field without a barrier (see also Figure 6.1).

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 6.7. 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.

Barrier Property Database

Default barrier properties specified in the barrier property database cannot be permanently changed through the WEPS interface. But they can be modified and stored with the current project. Barrier properties may, however, be modified in the barrier database file. Figure 6.8 shows the barrier database file, 'barrier.dat', which is located in the "WEPS1.0 Install" directory. This ASCII file may be edited (for NRCS only by designated qualified agronomists) by using a standard text editor to add new barriers or modify parameters of existing barriers. The file separates barriers into various categories (i.e., TREES, SHRUBS, HERBACEOUS, etc.). The user interface does not read nor display these barrier categories, and they only serve as a visual aid within the database. Actual database values are in rows which begin with a blank in column one, and each database parameter is separated by the pipe symbol, '|'. The parameters are listed as follows: barrier name | height (meters) | number of rows (not used) | porosity (fraction) | width (meters). Barrier height, width, and porosity were defined previously in this document. The barrier name is a character descriptor of the barrier and is the name displayed in the choice lists. The 'number of barrier rows' parameter is not currently used by WEPS nor is it displayed in the interface. Once the barrier database file has been updated, restart WEPS and the new barrier and/or modified parameter values should appear in the barrier drop-down list on the WEPS user interface.

Supplemental Barrier Information for Users Manual

The following are some of the assumptions used in building the default property values for the barriers. For the Tree and Shrub barriers, the heights used are based on the 20-year heights which is the standard height used for designing windbreaks. Since the model does not grow the barriers, it needs to be recognized that erosion rates will gradually decrease during the growth of the barrier. Because of this, alternative treatment options will need to be used during this growth period such as annual barriers, herbaceous barriers and/or changes in crop residue management.

The Herbaceous barriers were divided between perennial and annual. The perennial barriers include species such as switchgrass, tall wheatgrass, elephant grass, etc. The annual barriers were divided into three height categories: short, medium and tall. The short annual barriers could include small grains e.g, wheat, barley, or rye. These small grains provide protection to wind sensitive crops in the early growth stages and are usually sprayed with herbicide. The medium annual barriers also include the small grains as well as flax that are allowed to grow nearly to maturity before being sprayed. The tall annual barriers may include corn, sunflowers, or sorghum reaching heights of 4 to 5 feet. The default porosities assigned to these annual barriers are based on the assumption that the planting rate/acre is the same whether it is one row or two rows or three rows. The porosity of a single row could be altered by increasing the number of plants per acre.

The orchard default values include two age categories: one year old and mature height. They are also divided into three size classes: dwarf, semi-dwarf, and standard. The height of some fruit trees can be controlled by the type of root stock such as an apple tree that is dwarf may have a 7 to 8 foot mature size while a standard tree may reach 16 to 18 feet. It was assumed that the dwarf size trees were spaced about 12 feet apart, the semi-dwarf about 20 feet and the standard up to 30 feet for the larger nut trees such as walnut and pecan.

The "Forest Edge" example is trying to account for large patches of forest on the field edge. These forest patches are assumed to be "wide" i.e. several hundred feet wide. These patches do not function the same as a windbreak in modifying wind flow. The tendency is for the wind reduction profile adjacent to these wide patches to be reduced in length acting more similarly to the wind profile of a more porous barrier where the wind will return to open field velocity more quickly than a narrower windbreak.

Two artificial fences were also included. One assumes a four foot height with a 50 percent porosity similar to a slatted snow fence. The other is a solid board fence with a similar height but very low porosity.