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 scenariosand compare outputs to determine the best management to control wind erosion. Because of runtime issues, it is recommended that, for early comparisons, no more than five rotations cycles be used for a simulation. This will allow relative soil loss values for comparisons. Once one or two scenarios are selected, more 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 following section outlines the content of the "Output Details" screen.

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 rows in the Output Details screen vary 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 output displayed for the first two weeks and the 15th to the last day of each month as well as for each management operation date. 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 which 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 which occurred on the specified date. It is the management operation or the date of operation which most users will modify to affect field conditions and thus wind erosion.

Crop
This column lists any crops planted on the date shown. Crop is obviously another choice which the land manager may change to control wind erosion.

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 (e.g. < 0.001 k/ m2), then the amount is listed as "trace". If amounts are to large to be accurately displayed then the amount is listed simply as greater than (i.e., >) a specified amount. In these cases erosion amounts are so large that they are generally unacceptable.

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. Total is the average total net loss from the field; Creep/Sal is the average creep plus saltation net loss from the field; Susp is the average suspension net loss from the field; and PM10 is the average PM10 net loss from the field.

Mass Passing Field Boundary
These columns contain the mass per unit length of various sized material which 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 way wash the material in to 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 direction of tillage.

Within Field 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 how much of the field is actively eroding and thus may be 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.

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

Average Snow Cover
If the field is covered with snow, it will be 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), 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 soil loss.

Oriented 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.

Random Roughness
This column contains the standard deviation of the soil surface random roughness. This 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 management operations are listed in Table 1. Photographs (Figs. 1 - 9) can be used as a guide to determine relative random roughness values. Aggregation Soil aggregate size and aggregate dry stability affect erosion by wind. Soil aggregates greater than 0.84 mm in diameter are generally considered to be non-erodible. Dry stability is related to abrasion resistance where harder, more stable aggregates result in a lower erodibility of the soil.

Crust Cover A soil crust will resist abrasion and erosion more than a loose finely divides soil surface. Generally, the more of the surface is covered by a crust, the lower the erosion that occurs. Crust are transient and generally represent a degraded soil quality and therefore should not be relied upon to control erosion by wind. However a high crust cover may explain a lower erosion amount that would normally be expected.

Table 1. Random roughness values for typical management operations based on a silt loam soil (Ag. Handbook 537).


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 treader 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


Figure 1. Random roughness of 0.25 inches(6 mm).


Figure 2. Random roughness of 0.40 inches (10 mm).


Figure 3. Random roughness of 0.65 inches (17 mm).


Figure 4. Random rughness of 0.75 inches (19 mm).


Figure 5. Random roughness of 0.85 inches (22 mm).


Figure 6. Random roughness of 1.05 inches (27 mm).


Figure 7. Random roughness of 1.60 inches (41 mm).


Figure 8. Random roughness of 1.70 inches (43 mm).


Figure 9. Random roughness of 2.15 inches (55 mm).