Interpreting Outputs

Interpreting outputs of WEPS is an important part of controlling wind erosion through conservation planning. By observing how the soil loss is affected by weather and field conditions, the management operations can be adjusted to reduce soil loss. In developing new conservation plans, the user should build or modify several different scenarios and compare outputs to determine the best management to control wind erosion. Existing or current field conditions should be evaluated to determine if an erosion problem exists. If run times are an issue, a shorter run length may be chosen to assess possible problems. WEPS needs at least fifteen rotation cycles to obtain stable results. This will allow more runs to assess relative soil loss values for comparisons. Once one or two scenarios are selected, fifteen rotation cycles are recommended for more accurate erosion loss estimates. The number of erosion cycles can be set by selecting "Configuration" then "WEPS Developers Options" menus on the main screen.

The Detail Reports screen (Figure 3.1) is a date ordered list of numerous parameters generated by the WEPS model. The Detail Reports can be accessed by clicking the Detail Report button on the Main or Run Summary screens as well as through the View Output menu. At the top of the Detail Reports window is a button bar that allows the user to close the window , print the data , or view the Run Summary , Management Summary , or Crop Summary .

Since the detailed reports contains large amounts of information a method is provided to select smaller portions of information to view. Below the menu bar is a drop down list labeled Select Report. Clicking the down arrow to the right displays a list of reports that are subsets of all data available. These allow the user to pick a smaller, easier to view report. The Output Details is the most comprehensive report and is displayed by default. Note that the Detail Reports window may be expanded to display more dates and parameters than the default window.

The following section outlines the content of the Output Details portion of the Detail Reports screen and describes how they can be used to interpret results and design management systems to control erosion by wind. The WEPS User Guide section titled "Interface Reference: Output" contains the definition of each row and column in the detailed reports.

Date

This column contains the start and end date of the period for which the row information is reported (start day-end day month rotation year). Items in each row represent values from the end of the previous period to the current date. The date column, along with soil loss, will indicate which periods have the greatest wind erosion and are thus in need of changes of management to control wind erosion.

The number of rows in the Output Details screen differ, depending on the number of cropping years in the rotation and the number of management operations in each year of the rotation. Each year of the rotation has regular output displayed for either fifteen day periods (1st of month to 14th and 15th to the end of the month) or from the date of an operation to the end of the regular period. Occasional reporting from the date of a management operation are reported since operations can change field conditions. This output allows the user to view the erosion and other output for each year of the rotation. At the end of each year in the rotation is a row that contains the average annual value for that rotation year. The last row in the output form contains the average annual values for the complete crop rotation.

Operation

This column contains the management operation that occurred on the specified date. It is the management operation or the date of operation that most users will modify to affect field conditions and thus wind erosion.

Crop

This column lists the name of the crop planted on the date shown. Crop is another choice the land manager may change to control wind erosion. Crops that produce substantial residue (e.g., corn or small grain) will tend to lower the erosion rate if the residue is managed properly.

Wind Erosion

The Wind Erosion columns provide a summary of all the wind erosion soil loss for the simulation run. The numbers in these columns are those that the user will try to affect by adjusting management dates and operations. If an erosion event occurred, but values generated by the model are too small to be displayed on the output table (i.e., < 0.001 kg/m2 or 0.00455 tons/acre), then the amount is listed as "trace". If amounts are too large to be accurately displayed then, the amount is listed simply as greater than a specified amount (i.e., > 300 kg/m2 or 1338 tons/acre). In these cases, erosion amounts are so large that they are generally unacceptable.

Number and Loss per Erosion Event

These columns give the user an indication of the frequency and severity of erosion events. Some periods will have numerous events while others may have one. One event may have severe erosion while many others may have only slight erosion totaling less erosion than another single event.

Average Total Gross Soil Loss

This column contain the gross erosion within the field, averaged across the field, as well as averaged over the number of simulation years in each rotation year (kg/m2 or tons/acre).

Net Soil Loss from Field

These columns contain net soil loss from the field averaged over the number of simulation years in each rotation year (kg/m2 or tons/acre). Some deposition within a field can occur, especially when barriers are present downwind. Net soil loss is the amount of gross loss minus deposition. Average Total is the average total net loss from the field, Average Creep and Saltation is the average creep plus saltation net loss from the field, Average Suspension is the average suspension net loss from the field, and Average PM10 is the average PM10 (particulate matter less than 10 microns) net loss from the field.

Mass Passing Indicated Field Boundary

These columns contain the mass per unit length of various-sized material that passed the field boundary for each direction (kg/m or tons/1000 ft). This information is useful in determining how much material is leaving the field in each direction. For the Creep+Saltation size, the material will most likely be deposited on the field boundary, such as a stream, fence, ditch, or road. If deposited in a ditch, subsequent rainfall may wash the material into waterways, where it can affect water quality. If deposited on a roadway, the roadway will likely need to be cleared. For Suspension and PM10 sizes, the material may travel great distances, affecting air quality. The material passing each boundary may indicate that barriers may be needed on the opposite or upwind side of the field to control wind erosion. The direction of soil loss may also indicate a needed change in the direction of tillage.

Within Field Wind Erosion Activity

The information in these columns is useful in determining how much of the field is actively eroding and how much is not, which may impact what control measures, if any, should be applied and where. This information is also useful in understanding to what extent the field is actively eroding and thus causing plant or soil damage, or how much is subject to burial. Finally, this information is useful in understanding how much of the field is contributing to overall (net) field loss. Acres indicate the size of the eroding area and the fraction the proportion of the field eroding.

Weather Information

The Weather columns provide a summary of some of the weather information for the simulation run and help the user understand which periods are erosive and why.

Average Total Precip.

This column contains the total precipitation for the period, averaged over the simulation years in each year of the crop rotation (mm or inches). This section is useful in determining how precipitation amounts may be affecting biomass production and roughness decay.

Average Wind Energy > 8m/s (18 miles/hour)

This column contains the average daily wind energy for the period for winds greater than 8 m/s (~18 mph), averaged over the simulation years in each year of the crop rotation (KJ/day). This will indicate which periods have the most erosive winds.

Snow Depth > 20 mm (0.787 in)

Fractions of the field covered with snow greater than 20 mm deep are considered non-
erodible.

Average Biomass Surface Conditions on Date

The Average Surface Biomass Conditions on Date columns provide a summary of average surface conditions, including crop biomass and soil roughness, for the simulation run.

Crop Vegetation (Live)

These columns provide information on the structural configuration of live growing biomass. By observing the canopy cover, the standing silhouette area index, and the above-ground mass, the user can determine which periods are not providing sufficient cover to control wind erosion.

Crop Residue (Dead)

These columns provide information on the structural configuration of dead biomass or residue. By observing the flat cover, the standing silhouette area index, the flat mass, and the standing mass, the user can determine which periods are not providing sufficient residue cover to control wind erosion.

Live and Dead Biomass

These columns provide information on the structural configuration of both the live growing biomass and the dead biomass or residue. By observing the flat cover, the standing silhouette area index, the flat mass, and the standing mass, the user can determine which periods are not providing sufficient cover to control wind erosion.

Average Soil Surface Conditions on Date

Roughness

For cropping systems that do not produce sufficient residue for erosion control (e.g., cotton and most vegetable crops), roughness management is often used to reduce wind friction velocity at the soil surface. This reduces the amount of soil detachment and transport and increases deposition and thus reduces soil loss.

Oriented Roughness

Oriented roughness is also know as ridge roughness. These columns refer to regularly spaced roughness elements caused by tillage implements such as ridges, furrows and dikes. Ridge orientation, width, and height may be adjusted for periods of high soil loss to determine its effect on wind erosion. The user can also follow the roughness decay over time as result of rainfall or wind erosion.

Random Roughness

This column contains soil surface random roughness, defined as the standard deviation of the elevation from a plane across a tilled area. Random roughness does not take into account oriented roughness. Random roughness is the value at the period end, averaged over the simulation years in each rotation year (inches or mm). Random roughness is primarily the result of aggregate size distribution, but is also affected by various types of tillage tools. Random roughness values for typical field operations are listed in Table 3.1. Photographs (Figs. 3.2 - 3.10) can be used as a guide to determine relative random roughness values. These photos were taken at an oblique angle to provide an image similar to that seen by an observer standing a few feet from the plot.

Aggregation

Soil aggregate size and aggregate dry stability affect erosion by wind. Soil aggregates less than 0.84 mm (0.03 inches) in diameter are generally considered to be erodible and so the higher the fraction of aggregates < 0.84mm, the more erodible the surface. Dry stability is related to abrasion resistance where harder, more stable aggregates result in a lower erodibility of the soil. The larger the dry stability value (ln(J/m2)), the more resistant the aggregates to abrasion and erosion by wind.

Crust Cover

A soil crust will resist abrasion and erosion more than a loose, finely divided soil surface. In general, the more of the surface is covered by a crust, the less erosion occurs. Crusts are transient and generally represent a degraded soil quality, and therefore, crusts should not be relied upon to control erosion by wind. But a greater crust cover may explain a lesser erosion amount that would normally be expected.

Table 3.1	Random roughness values for typical management operations, based on a silt loam soil (from USDA Agriculture Handbook 537 and National Agronomy Manual 703, Tab 5-5).

Field Operation	Random Roughness

(inches)

Field Operation	Random Roughness

(inches)

Chisel, sweeps

1.2	Fertilizer applicator,

anhydrous knife

0.6
Chisel, straight point	1.5	Harrow, spike	0.4
Chisel, twisted shovels	1.9	Harrow, tine	0.4
Cultivator, field	0.7	Lister	0.8
Cultivator, row	0.7	Manure injector	1.5
Cultivator, ridge till	0.7	Moldboard plow	1.9
Disk, 1-way	1.2	Mulch threader	0.4
Disk, heavy plowing	1.9	Planter, no-till	0.4
Disk, Tandem	0.8	Planter, row	0.4
Drill, double disk	0.4	Rodweeder	0.4
Drill, deep furrow	0.5	Rotary hoe	0.4
Drill, no-till	0.4	Vee ripper	1.2
Drill, no-till into sod	0.3