Universal Soil Loss Equation and Spreadsheet
Calculating Soil Loss From Construction SitesThe Dane County Land and Water Resources Department has developed an Excel worksheet that calculates soil loss from construction sites. This spreadsheet uses the Universal Soil Loss Equation (USLE) to determine whether the combination of proposed erosion control practices will limit soil loss from sheet and rill erosion to 7.5 tons/acre/year or less. The USLE, its variables, and an example calculation are provided below. Note that the USLE estimates soil loss from sheet and rill erosion only. It does not predict soil loss resulting from high channel velocities, gully erosion or streambank erosion.
Universal Soil Loss Equation for Construction SitesA = (R) x (% R) x (K) x (LS) x (C) x (P)
A: Computed Soil Loss Rate (ton/acre)
R: Annual Rainfall Factor (150 for Dane County)
% R: The percentage of the annual R factor that has passed to date
K: Soil Erodibility Factor (based on soil type)
LS: Slope Length/Steepness Factor (based on slope length and percent slope)
C: Land Cover Factor (based on condition of soil cover)
P: Not used for construction site calculations. The P factor accounts for the effect of support practices such as contouring, strip cropping, or installing terraces in an agricultural setting. This factor does not apply to construction sites.
USLE SpreadsheetAn Excel spreadsheet for use in calculating soil loss can be downloaded from the LWRD Website. This spreadsheet, shown below in Figure 1, calculates soil loss from inputs entered by the user.
Figure 1: Dane County USLE spreadsheet
The Universal Soil Loss Equation for Construction Sites worksheet was developed to estimate soil loss from sheet and rill erosion. It does not predict soil loss resulting from gully erosion.
The user must enter information into the "Land Disturbing Activity", "Begin Date", "Soil Map Unit", "Slope %" and "Slope Length" cells for the spreadsheet to calculate soil loss off the site. All other cells are automatically calculated.
Variable DescriptionsLand Disturbing Activity (entered by user via pull-down menu)
The land disturbing activity relates to the type of disturbance that is occurring on the ground. The land disturbing activity inputs may be selected using the drop-down menu.
- Activity Inputs
- disturb ground - Usually the initial disturbance and occurs when the ground is left bare due to stripping vegetation, grading, or other actions that leave the soil devoid of cover.
- apply mulch - The application of a minimum of 1.5 tons/acre straw or other comparable mulch.
- seed and mulch - The application of a minimum of 1.5 tons/acre straw or other comparable mulch. Enter this activity if the seeding and mulching are done at the same time. It is not necessary to also enter seeding if this input is used. Requires 60 days of cover establishment during the growing season. Mulching is recommended on all disturbed areas that are to be seeded to control erosion and establish cover.
- seeding - The application of permanent or temporary seeding without the use of mulch. Not to be used with seed and mulch. Requires 60 days of cover establishment during the growing season.
- sod - The installation of sod for cover establishment.
- paving - The installation of a hard surface that covers the disturbed ground completely such as asphalt paving, stone base coarse, or geotextile.
Begin Date (entered by user)
The date the planned land disturbing activity begins, e.g. 5/15/2012. The activity is assumed to continue until the next activity is entered. A 60 day cover establishment period, during the growing season, is recommended for the establishment of seeding.
- Permanent seeding is recommended to be completed by September 15.
- Temporary seeding is recommended to be completed by October 15.
- Temporary seeding of oats or sudan grass are normally sown between May 15 and July 15, and rye grass or winter wheat are normally sown between July 15 and September 15. It is recommended that the seeding be incorporated into the soil prior to the permanent seeding application to minimize competition.
- When the seeding dates are later than the noted recommended dates, the end of the cover establishment should be extended to May 15 of the following spring to allow for growth.
% R to Date (automatically calculated)
The percentage of the annual R factor from January 1 to the entered date.
Period % R (automatically calculated)
The percentage of the annual R factor calculated for the period from one land disturbing activity to the next.
Annual R factor (automatically calculated)
The rainfall factor, R, is the number of erosion-index units in a normal year’s rain. The erosion index is a measure of the erosive force of a specific rainfall. In Dane County the rainfall factor is 150.
Soil Map Unit (entered by user via "Soil Types" button)
The soil-mapping unit symbol for the predominant soil type in the area of the land disturbing activity, e.g. PnB. This information is available in the published soil survey of Dane County at the Land Conservation Department office.
Soil Erodibility K Factor (automatically calculated)
A factor used to express the erosiveness of the soil layer below the topsoil for a specific soil type.
Slope % S (entered by user)
The percent slope for the representative portion of the disturbed area, regarding overland flow and not channel flow. For example, a 5 foot drop over 100 feet is expressed as 5.
Slope Length L (entered by user)
Slope length (in feet) measured along the overland flow path from the top to the bottom of the slope of the representative disturbed area. Channel lengths are not included in the slope length.
LS Factor (automatically calculated)
The spreadsheet calculates LS factor value based on the ratio between the percent slope and length of slope of the representative disturbed area.
Land Cover C Factor (automatically calculated)
The cover and management factor is the ratio of soil loss from an area with a specified cover and management practice to that from a unit plot of bare land. The input for the Land Disturbing Activity corresponds to this factor.
Soil Loss (automatically calculated)
The predicted value of soil loss (tons per acre) that corresponds to the time period of each land disturbing activity entered. This value is calculated using the equation: A=(%R)x(R)x(K)x(LS)x(C).
Percent Reduction Required to Meet Ordinance (automatically calculated)
The percentage value in the total’s row corresponds to the reduction in soil loss necessary to comply with Dane County’s Erosion Control Ordinance. It is required that the cumulative soil loss rate not exceed 7.5 tons per acre for all sites.
Compliance with the ordinance standards can be achieved by:
- Adjusting the management of the disturbed area, ie. tightening schedule or installing erosion control measures.
- Installing a sediment basin or other sediment control measures below the disturbed area.
- Obtaining cooperative efforts of adjoining landowners.
Detention RequirementsClicking on the Detention Requirements button will cause a pop up box to appear that returns the size of the soil particle that would need to be trapped to obtain compliance with the Dane County Erosion Control Ordinance soil loss standard of 7.5 tons/acre. The soil particle size distribution used for the calculation is a Plano silt loam, a typical and common distribution for Dane County. If the percent reduction is greater than what can theoretically be trapped by a single sediment control structure, a message stating “Trap efficiencies greater than 80% are not possible with detention alone.” is returned.
If no reduction is necessary to comply with the ordinance, clicking the Detention Requirements button will return “Detention is not necessary. The soil loss rate is below standards.”
Soil TypesClicking on the Soil Types button will activate a combo box listing the soil mapping units and soil descriptions for Dane County soil types. Once highlighted and OK has been clicked, another pop-up box will appear asking if the user would like the soil mapping unit to be automatically entered into the spreadsheet.
Print SheetOnce the spreadsheet is completed, clicking the Print Sheet button will print a copy using the proper layout and print range.
Help PageClicking the Help Page button will activate a help page. Use one of the “Back to USLE” buttons on the help page to return to the spreadsheet.
Example Soil Loss Calculation
- Construction of a 25,000 sq. ft. mini-warehouse
- Plan to break ground on April 10, 2018
- Site stabilization is planned to be completed by seed and mulch by August 12, 2018
- The soil type was found in the Dane County Soil Survey to be Kidder Loam, 2 to 6% slopes (soil map unit = KdB)
- The representative slope on the site is 4.5% and has a length of 50 feet
- Select the initial land-disturbing activity. When breaking ground on a construction site the land-disturbing activity will be disturb ground, which can be selected from the pull-down menu. This represents the fact that soil will be exposed from this date on.
- Enter the date that you expect to break ground, 04/10/18, in the next column.
- Select the “soil types” button and select "KdB" from the drop down menu and click OK. When prompted to enter the soil type into the spreadsheet, select yes.
- Under the column titled Slope (%), enter the representative slope percentage, 4.5.
- Enter the slope length in feet, 50, in the next column.
- Go back to the Land Disturbing Activity column and just below the disturb ground row select seed and mulch from the pull-down menu.
- In the next column enter the date that you expect to seed and mulch the site, 08/12/18. The rest of the columns in this row will fill in automatically.
Figure 2: Example soil loss spreadsheet
ResultsAs can be seen from the spreadsheet output shown above in Figure 2, a reduction of 40% is still necessary to comply with the ordinance. This can be accomplished in a number of ways including reducing the time that the ground is left exposed, changing the time of year that construction takes place and/or designing erosion control practices to reduce the amount of soil that leaves the site.
Additional NotesIf the slope or schedule that is required for the calculation is not representative of the whole site, more than one calculation may be submitted. The area with the higher expected soil loss would then need to receive different erosion control treatment than the other areas on the site.
- When a slope is dramatically changed by grading during the disturb ground period and it is desired to take credit for this change, select another disturb ground row under Land Disturbing Activity and enter its corresponding date. Then enter a new slope and/or slope length in the appropriate columns.
- If the site is to be graded, but the entire site is to be stabilized by methods other than vegetation establishment (i.e., by paving or graveling), instead of seed and mulch or sod activities and select paving .
- When sites are to be temporarily seeded and mulched, enter land disturbing activities of disturb ground and seed and mulch. Under these rows select disturb ground for when grading is expected to resume and seed and mulch for final stabilization date.