Strip Cropping (Field Design)

Introduction

Dividing large fields into smaller fields is a design practice that often aids in controlling both wind and water erosion. The main effect achieved by reducing field size for wind erosion control is to reduce the amount of damaging abrader composed of saltating aggregates that impact immobile clod or crust surfaces, as well as to reduce the breakage of the mobile saltation and creep aggregates. Reducing abrader permits the surface to become armored with immobile material as the loose soil is removed or moved into a sheltered area on rough surfaces. But merely reducing field size with strips provides only a small amount of wind erosion reduction on fields that lack significant amounts of clods, crust, growing biomass, or residues. Reducing abrader is also important for seedling protection. The Wind Erosion Prediction System (WEPS) provides a means of evaluating the effectiveness of strip cropping in reducing wind erosion. The objectives of this guide are to provide an overview of the effects of field size on wind erosion as simulated in the WEPS model and to suggest some possible field designs to enhance wind erosion control. Examples using WEPS to evaluate strip-cropping scenarios are also provided.

One method to accomplish a reduction in field size is to divide large fields into alternating strips. This practice is referred to as strip cropping, and an example is illustrated in Figure 1.8. In low rainfall areas, one-half the strips may be fallowed on alternate years. In other areas, one-half the strips may be producing low-residue crops. Comparison of Figure 1a and Figure1b illustrates important differences between the large fallow field and the same field when farmed in strips. On the large field, both moving soil and snow are removed. In contrast, when strip-cropped, part of the moving soil and snow are typically trapped in the next strip, and this results in a gain in both moisture and soil on portions of the stripped field. The saltation/creep crossing eroding strip boundaries may be used in WEPS to estimate the soil gain on adjacent non-eroding strips.

Effects of Field Scale on Wind Erosion

The erosion processes in WEPS are simulated for an initially uniform field, and these include: emission (entrainment) of loose soil, abrasion of immobile clods/crusts, breakage of saltation/creep-size aggregates to suspension size, and trapping of saltation/creep-size aggregates in sheltered depressions between clods, ridges, and plants or residues (Fig. 1.9). If standing biomass is present, a reduction of wind drag at the soil surface, and biomass interception of mobile aggregates, are also simulated. Using these processes, the model simulates a horizontal discharge (i.e., amount removed up to a downwind distance) of mobile saltation/creep (Fig. 1.10) and suspension-size aggregates (Fig. 1.11). On long erodible fields, the saltation/creep discharge may reach transport capacity. Transport capacity is defined as the maximum horizontal discharge of saltation/creep possible for a given wind speed and surface condition. At transport capacity, the deposition of saltation/creep from the air stream per unit area equals the amount entrained into the air stream. There generally is still a net removal of saltation/creep aggregates from the surface to replace those lost by breakage to suspension size. In this case, saltation-creep can only approach transport capacity. In contrast, the suspended discharge is not limited by transport capacity, because these particles continually diffuse into the atmosphere. As a consequence, the mass of suspended material increases over the entire length of eroding fields, with the rate of increase controlled by the erosion processes.

Conservation planners often design control systems based on average soil loss (i.e., amount removed per area). Total horizontal discharge at any downwind distance divided by the upwind field length represents the average soil loss over the upwind area. Using this calculation, one can illustrate the effects of different field lengths on soil loss. Depending upon field surface conditions, the erosion processes cause differing patterns of horizontal soil discharge and, consequently, differing soil loss for the saltation/creep-size (Fig. 1.10) and the suspension-size soil (Fig. 1.11). After saltation/creep reaches transport capacity, dividing the nearly-constant transport capacity by field length to give average soil loss shows that there is a steady decrease in loss per unit area with increasing field length (i.e., area increases, whereas transport remains essentially the same). In contrast, there is generally a large net loss of suspension-size soil over the entire field length. Total soil loss for any field length is determined by adding the average suspension and saltation/creep soil losses (Fig. 1.13). In general, the maximum average soil loss occurs at a field length at which both the saltation/creep and suspension components are contributing significant net soil loss. On far downwind portions of long fields, the increases in the soil discharge comes mainly from suspended soil, so the average soil loss may decrease somewhat with field length. Thus, on long fields, suspension soil loss typically exceeds the saltation/creep loss. Of course, if the mobile soil is composed mainly of sand larger than suspension size (> 0.1 mm diameter), then saltation/creep will remain the dominant form of soil loss.

Smooth, loose fields often have a length at which there is a maximum soil loss, and the average loss then decreases beyond that field length (Fig. 1.13a). In this scenario, very short field lengths are necessary to control wind erosion. Thus, on all fields subject to wind erosion, planners should consider using other erosion controls in combination with strip cropping.

Suspended soil lost from long eroding fields can be considerable and is subject to long-range transport. Other detrimental effects also accompany this soil loss. These include an increase in sorting of the initial soil so that the removed soil is enriched in nutrients, organic matter, clay and silt fractions- the productive elements of the soil. The increased abrasion and breakage processes on long fields also increase the PM10 content (particulate matter <10 microns diameter) that is regulated as a health hazard. Thus, not only the amount, but also the quality and size distribution of the removed soil, changes as field length increases.

For simplicity, the Wind Erosion Equation (WEQ) predicts that average soil loss on long fields approaches a constant value. In WEPS, on the other hand, the average soil loss may increase or decrease on long fields, depending on whether creep/saltation or suspension is the dominant transport mode. Both the WEPS and WEQ models show that, for effective erosion control, the field length along the prevailing wind erosion direction needs to be significantly less than the distance to the point of maximum soil loss. The effect of reducing field length on erosion is not linear, but varies approximately with the logarithm of the distance to transport capacity.

Designs to Enhance Erosion Control

On fields susceptible to large amounts of erosion or fields with inclusions of highly erodible soil, conventional strip crop systems such as that shown in Figure 1.8b may not provide the needed extent of erosion control. It is then the task of those designing erosion controls to recommend additional measures. There are a number of ways to enhance a strip crop system to provide additional control, and several of these are illustrated.

Reduce Strip Width

One control option is to reduce the strip width in critical areas (Fig. 1.13). Wind erosion often begins within areas of a field that include a soil highly susceptible to wind erosion or in areas exposed to accelerated wind speeds. Saltating aggregates from these areas abrade downwind areas and break down immobile clods and crusts, thus increasing erodibility of the entire downwind area. On stripped fields there is potential to cover part of the problem area with a protective crop every year and thereby reduce the source area. In addition, the downwind area subject to abrasion may also be reduced. One may estimate the effect of strip width on these problem areas by choosing a soil representative of the problem area in the WEPS simulation.

Reduce Strip Width with Barriers.

A second option to reduce effective strip width is to employ wind barriers (Fig. 1.14). Correctly oriented barriers serve to shelter part of the erosive strip (Fig. 1.14a). They may also be used to divide the erosive strips to trap moving soil and further reduce the unsheltered lengths (Fig. 1.14b). If the barriers are short in height, relative to the unsheltered distance (e.g., herbaceous barriers), the latter design will reduce the unsheltered length by about one-
half. When barriers or cross-wind strips trap moving saltation/creep, they effectively create a new non-erodible (stable) boundary. WEPS evaluates field conditions on each individual strip or field interval between barriers. Therefore, various field designs can be easily evaluated in WEPS by changing the field strip width and adding appropriate wind barriers as model inputs.

Surface Roughness

Another option to enhance strip effectiveness is to employ surface roughness such as tillage ridges (Fig. 1.15). Adding tillage ridges provides additional trapping capacity for mobile soil (Fig. 1.15a). But it is often useful to orient ridges so they are not parallel to the long side of the strip, because this provides some erosion control when wind directions are parallel to the strip (Fig. 1.15b). As a starting point, consider orienting ridges about 30 degrees from parallel to the strip and along the direction of the least erosive winds during critical wind erosion periods. For example, in Figure 15b, we assumed that the southeast-northwest winds were less erosive than southwest-northeast winds. WEPS can be used to help optimize design of these systems by comparing various tillage directions.

Other Options

Other options are particularly useful when there are erosive winds parallel to conventional strips. These options include avoiding long, straight tillage ridges (Fig. 1.16a). Saltation along the furrows parallel to tillage ridges often undercuts the ridge crust, which reduces the ridge effectiveness and leaves an accumulation of mobile material. Periodically changing ridge direction can provide sheltered accumulation zones for saltating soil. Furrow diking is also highly effective in reducing saltation/creep parallel to ridges. Field strips may also be designed with curvature (Fig. 1.16b and Fig. 1.17). In terraced farming systems, curvature of strips is usually a necessity, but the practice can often be useful in other systems. The curvature of the strip provides trapping areas for eroding soil moving along a given wind direction. WEPS can be used to evaluate these systems by inputting the strip width and then estimating the length between zones where saltation/creep material is trapped.

In summary, strip cropping is most effective when appropriate strip width and orientation are determined and combined with other wind erosion control practices such as maintaining a rough, cloddy surface and residue cover or cover crop.

Examples of Strip Cropping Using WEPS

The WEPS model can be used to evaluate strip cropping for the reduction of soil loss by wind. Here, some example simulation runs are provided, which illustrate the procedure and effectiveness of breaking up a large field into narrow strips.

No Strip Cropping

The beginning scenario for this example (Fig. 1.18) is defined as follows:

Farm is located near Saint Francis in Cheyenne County, Kansas

Soil is silt loam

Wheat-fallow rotation as shown in the MCREW screen (Fig. 1.19)

Field size of 2640' x 2640', 160 acres

No barriers, 0.0 degree field orientation (North-South)

As can be seen in the Run Summary Report (Fig. 1.20), the simulation run for this non-strip cropped scenario resulted in an annual average soil loss of 21.2 tons /acre/year, with 26.9 tons/acre/year lost during the first year (fallow) of the rotation and 15.4 tons/acre/year lost during the second year (wheat) of the rotation. Note that because no barriers are present to affect loss across the field, the gross loss is equal to the net soil loss for the field. In addition, the amount of creep/saltation and suspension soil loss are presented. Notice that the suspension made up a majority of the field and boundary loss (Fig. 1.21) on the non-stripped field because the field was probably wide enough to allow creep/saltation to approach transport capacity and suspension material to continually diffuse into the atmosphere (Figs. 1.10 and 1.11).

The soil loss in this scenario would generally be considered an unacceptable amount, and conservation measures should be recommended. To determine if strip cropping would be effective, the user should first view the Boundary Loss summary report for the non-stripped field as shown in Figure 1.21. In this example, almost all of the eroding soil mass is crossing the south and east field boundaries, indicating northerly and westerly prevailing wind directions. Most of the mass is crossing the southern field border, so dividing the field in east-west oriented strips will shorten the field length in the direction having the most loss.

Strip Crop

In an attempt to reduce soil loss along the direction having the greatest loss, the field is divided into eight 330-foot-wide strips (20 rods each) with strips alternating the wheat and fallow part of the rotation (Fig.1.22). The strips are oriented with the long sides aligned east-
west. We simulate one strip of the field, with the wheat fallow management rotation. All other conditions remain the same as the original non-stripped field.

As can be seen in the Run Summary report (Fig. 1.23), breaking the field into strips reduced the annual average soil loss to 1.7 tons/acre/year, with 2.1 tons/acre/year lost during the first year (fallow) of the rotation and 1.4 tons/acre/year lost during the second year (wheat) of the rotation. Because loss is reported on a per acre basis, the loss for the strip represents the loss rate per acre for the entire field.

The Run Summary report also shows that suspension loss was less than the creep/saltation loss on the stripped field. Recall that suspension made up a majority of the loss on the non-
stripped field (Fig. 1.20). Because the processes of creep/saltation increase downwind, so an increasing amount of suspension is generated (Figs. 1.10 and 1.11). Limiting the field length therefore, limits these suspension-generating processes.

On stripped fields, creep/saltation-sized aggregates will be deposited within the alternating strips that have adequate wheat or residue cover to stop the movement. This deposition of creep/saltation material in adjacent strips is not modeled within WEPS, but can be estimated from creep/saltation crossing the strip boundaries and should be considered in control strategies.

The Boundary Loss summary (Fig. 1.24) shows that, although field length was unchanged in the east-west direction, a significant reduction in loss still occurred on the eastern boundary. This indicates that much of the original soil loss on the eastern boundary was the result of winds that were not directly out of the west. Therefore, some winds deviated from the westerly direction, causing soil loss across the east boundary. As a result of this deviation, stripping the field provided some reduction in field length along the wind direction, resulting in a reduction of soil loss in the eastern direction.

Although the Boundary Loss for creep/saltation generally stays on the field within the adjacent strip, suspension boundary loss can potentially leave the field area. Therefore, the suspension boundary loss along the long field border should be multiplied by the number of strips to determine total suspension boundary loss for the total 160-acre field. Note! In our example the suspension loss for the South border would be 3.2 x 8 or 25.6 tons/1000 ft and for the ease border, it would be 12.7 x 8 or 101.6 tons/ 1000 ft.

Strip Crop with Grass Barriers

Even though stripping the field significantly reduced erosion rates, additional reduction can be obtained by adding a herbaceous barrier between the strips. An example of strip cropping with herbaceous barriers is illustrated in Figure 1.25, and the WEPS main screen setup for this scenario is shown in Figure 1.26. In this example, we added a one-row grass barrier on each side of the strip (height = 3.0 ft., width = 1.6 ft., and porosity = 0.3).

As can be seen by the Run Summary (Fig. 1.27), adding a barrier on each side of the strip further reduced average gross loss for the rotation to 0.9 tons/acre/year. The loss for first year (fallow) of the rotation was reduced to 1.2 and for the second year (wheat) it was reduced to 0.6 tons/acre/year. Note in this scenario that because barriers are present, the net loss is less than the gross soil loss for the field. This difference is due to deposition that occurs just before the downwind barrier. The net creep/saltation was reduced to 0.18 tons/acre/year, and suspension was reduces to 0.28 tons/acre/year. Again, creep/saltation deposition in adjacent strips is not modeled within WEPS. Because almost no creep/saltation is available to generate suspension-size material, one can conclude that most of the suspension is from loose suspension-size material on the surface.

The boundary loss for this scenario is shown is Figure 1.28. Both creep/saltation and suspension leaving the field were significantly reduced on the south boundary, compared with strip-cropping without the barriers. Loss on the eastern boundary was only slightly reduced.

Strip Cropping on a Soil with Excessive Erosion

This set of examples contains field and management conditions that are the same as the previous examples, except that the soil is a loamy sand, which is susceptible to large amounts of erosion. The Run Summary for the non-stripped scenario (Fig. 1.29) indicates an annual average loss of 96.0 tons/acre/year, with an average of greater than150 tons/acre/year being lost in the first year (wheat) of the rotation. Note the net loss for creep/saltation is 22.53 tons/acre/year and suspension is 73.40 tons/acre/year. Again, the boundary loss (Fig. 1.30) shows that the most loss passes the southern boundary, so shortening the field in the north-
south direction by stripping is a control strategy that should be considered.

To observe the effects of strip width on this loamy sand field, we begin with a field divided into thirty-two, 82- foot-wide strips (5 rods each) with the long sides aligned in the east-west direction in an attempt to reduce soil loss. All other conditions for the field remain constant for the simulation. We continue to successively double the width of the strips (i.e., 82, 165, 330, 660, 1320, 2640 ft.) and simulate the soil loss as strips get wider. The results of these simulations are shown in Table 1.1 and indicate that average annual total loss continually increases with increases in the strip width. The net creep/saltation loss, however, shows an increase as width increases from the narrow-stripped scenario to a maximum loss at 660 ft width. As widths become wider than 660 ft., the creep/saltation loss decreases.

Table 1.1. Change in soil loss with changing strip width.

Strip Width	Average Net Loss from Field (tons/acre/year)	South Boundary Loss

(tons/1000 ft.)
feet	rods	Total	Creep/Salt.	Suspension	Creep/Salt.	Suspension
82	5	35.1	25.77	10.31	32.6	10.6
165	10	48.9	33.67	15.24	84.6	33.5
330	20	68.1	43.13	24.99	204.4	111.1
660	40	88.7	46.23	42.46	404.2	359.8
1320	80	95.1	34.52	60.59	539.4	917.8
2640

(no strip)	160	95.9	22.53	73.40	584.9	1849.8

To discuss these changes in loss with field length, it is important to review the concepts of "net loss" and "discharge". The model simulates a horizontal discharge (i.e.,amount removed up to a downwind distance) of mobile saltation/creep (Fig. 1.10) and suspension-size aggregates (Fig. 1.11). On long erodible fields, the saltation/creep discharge may reach transport capacity. Transport capacity is defined as the maximum horizontal discharge of saltation/creep possible for a given wind speed and surface condition. At transport capacity, the deposition of saltation/creep from the air stream per unit area equals the amount entrained into the air stream. There generally is still a net removal of saltation/creep aggregates from the surface to replace those lost by breakage to suspension size. In this scenario, saltation-
creep can only approach transport capacity. In contrast, the suspended discharge is not limited by transport capacity, because these particles continually diffuse into the atmosphere. As a consequence, the mass of suspended material increases over the entire length of eroding fields, with the rate of increase controlled by the erosion processes. Remember to add the suspension soil loss from all the strips to the total loss.

Conservation planners often design control systems based on average soil loss (i.e., amount removed per area). Total horizontal discharge at any downwind distance divided by the upwind field length represents the average soil loss over the upwind area. Using this calculation, one can illustrate the effects of field length on soil loss. Depending upon field surface conditions, the erosion processes cause differing patterns of horizontal soil discharge and, consequently, causing differing soil loss for the saltation/creep-size (Fig. 1.10) and the suspension size soil (Fig. 1.11). After saltation/creep reaches transport capacity, dividing the nearly-constant transport capacity by field length to give average soil loss shows that there is a steady decrease in loss per unit area with increasing field length (i.e., area increases, whereas transport remains essentially the same). In contrast, there is generally a large net loss of suspension-size soil over the entire field length. Total soil loss for any field length is determined by adding the average suspension and saltation/creep soil losses (Fig. 1.13). In general, the maximum average soil loss occurs at a field length at which both the saltation/creep and suspension components are contributing significant net soil loss. On far downwind portions of long fields, the increases in the soil discharge come mainly from suspended soil, so the average soil loss may decrease somewhat with field length. Thus, on long fields, suspension soil loss typically exceeds the saltation/creep loss. Of course, if the mobile soil is composed mainly of sand larger than suspension size (> 0.1 mm diameter), then saltation/creep will remain the dominant form of soil loss.

Smooth, loose fields often have a length at which there is a maximum soil loss, and the average loss then decreases beyond that field length (Fig. 1.13a). In this example, very short field lengths are necessary to control wind erosion. Thus, on all fields subject to wind erosion, planners should consider using other erosion controls in combination with strip cropping. Cross wind ridges, cover crops, and residue management are good examples.

Suspended soil lost from long eroding fields can be considerable and is subject to long-range transport. Other detrimental effects also accompany this soil loss. These include an increase in sorting of the initial soil so that the removed soil is enriched in nutrients, organic matter, clay and silt fractions- the productive elements of the soil. The increased abrasion and breakage processes on long fields also increase the PM10 content (particulate matter <10 microns diameter) that is regulated as a health hazard. Thus, not only the amount, but also the quality and size distribution of the removed soil, changes as field length increases.

For simplicity, the Wind Erosion Equation (WEQ) predicts that average soil loss on long fields approaches a constant value. In WEPS, on the other hand, the average soil loss may increase or decrease on long fields, depending on whether creep/saltation or suspension is the dominant transport mode. Both the WEPS and WEQ models show that, for effective erosion control, the field length along the prevailing wind erosion direction needs to be significantly less than the distance to the point of maximum soil loss. The effect of reducing field length on erosion is not linear, however, but varies approximately with the logarithm of the distance to transport capacity.

Recall that, in the current version of WEPS, individual field strips are modeled as a single field. The initial increase in creep/saltation loss, as the loamy sand field get wider (Table 1.1), is a result of large increases in discharge relative to small increases in field area (i.e., left part of curve in Figure 1.10a and 1.10b for the loose, smooth field). But at some point downwind on wide fields, the creep/saltation discharge reaches transport capacity, and loss and deposition are equal (i.e., where Figure 1.10a becomes nearly flat for the loose, smooth field). Thus, an increase in field width from that point does not result in any further net discharge of creep/saltation-size material (Fig. 1.10a) and actually shows a decrease in loss of creep/saltation material as strips get wider (Fig. 1.10b and 1.13a). This decrease in loss as strips get wider occurs because net loss is calculated by dividing the constant discharge (transport capacity) by the increasing area (tons/acre). This decrease in creep/saltation loss as wide fields get wider may, under some field conditions, even occur with the total net loss from the field. In the present example, however, even though the creep/saltation rate increased as wide strips got narrower, the suspension was continually reduced. The reduction in suspension was more than enough to compensate for the increase in creep/saltation and, thus, the total loss rate was reduced as strip width decreased.

It is very important to note that the boundary loss for the south boundary (Table 1.1) shows a significant reduction in both the creep/saltation and suspension leaving the field as width decreases. Notice that there was not an increase in boundary loss of creep/saltation with field distance on wide strips, as there was with the field loss. This is because boundary loss is divided by a constant distance (tons/1000 ft) as strips get wider. The boundary loss represents the amount leaving the field boundary and illustrates the value of dividing the field into strips to control erosion by wind, especially on fields susceptible to large amounts of erosion.

Adding a grass barrier on each side of a 330-foot strip, as in the previous example, further reduces soil loss over the similar-size strip without barriers (Fig. 1.31). Total gross loss was reduced to 21.2 tons/acre/year. Note that again the net loss was cut almost in half because of deposition just before the downwind barrier within the simulated strip. But, this erosion rate is still unacceptable, and much more aggressive control strategies should be examined. More aggressive residue retention with a stubble mulch system will be attempted next.

Finally, to further reduce erosion loss the management used on the previous example is changed to a stubble mulch system designed to retain more crop residue throughout the rotation. The modified management is shown in Figure 1.32 and was created by delaying the sweep operations and eliminating a chisel operation in the fall of the second year.

Switching to a stubble mulch in addition to a grass barrier on each side of a 330 ft strip, as in the previous example, further reduces gross soil loss to 7.0 tons/acre/year on the field with a loamy sand. The net loss, however, was reduced to 3.8 tons/acre/year (Fig. 1.33). Note that again the net loss was cut almost in half because of deposition just before the downwind barrier within the simulated strip. This erosion rate is still unacceptable, and more intensive control strategies should be examined. To reduce loss even further, more aggressive residue retention should be tried; even the retirement of the land into permanent grass should be considered.