Introduction

The Universal Soil Loss Equation (USLE) is a soil erosion model initially developed by the USDA-ARS, the USDA-NRCS, and Purdue University. The model is empirical, which means it was developed based on observations from experiments. It was first published in 1965 and later updated in 1978. A computerized version of the model, known as the Revised Universal Soil Loss Equation (RUSLE), was released in 1992 and has since replaced by the RUSLE2 version of the model which operates within Microsoft Windows.

While the manual version of USLE is no longer used for developing conservation plans, that version of the model provides an excellent example of how erosion models can be used to predict the impacts of different conservation practices on rates of water erosion rates when developing a conservation plan.

The USLE is written as:

[latex]\text{A}=\text{R}\times\text{K}\times\text{LS}\times\text{C}\times\text{P}[/latex]

where A is the estimated long term average soil loss in tons per acre per year, R is the rainfall erosivity in units of foot-tons per acre per year, K is the soil erodibility factor, LS is the slope length and steepness factor, C is the cover-management factor, and P is the supporting practices factor. The USLE is a multiplicative model, so the annual erosion rate, A, is the product of each of the five factors. The average annual erosion rate can be converted from units of tons per acre to Megagrams per hectare by multiplying by a factor of 2.24.

For this laboratory you will be predicting soil erosion rates for your study site using the USLE.

Estimate Erosion Rate Under Existing Conditions

Erosivity Factor, R

The erosivity for a given location is dependent upon the long term averages of the intensity and distribution of rainfall at a specific location. Thus, the R factor for the location of your field site will be different from other sites. The erosivity, R can be determined from this web app provided by the EPA for stormwater permitting for the National Pollution Discharge Elimination System (US EPA 2022). To determine the R value for your site, enter January 1 and December 31 for the current calendar year as the beginning and end date of the “construction” project. Next, navigate to your study site on the map or enter the latitude and longitude if it is known. Then click the “Calculate R Factor” button. This will populate the page with the R value for your study site.

Soil Erodibility Factor, K

You will find the K factor using Web Soil Survey. Using your computer, navigate to the Web Soil Survey. Navigate to the location of your study site. Using the rectangular or the polygon area of interest (AOI) tools, draw the area of interest for your study site. Click on the “Soil Map” tab. Note the soil mapping unit that covers the slope of interest at the study site. Click on the “Soil Data Explorer” tab, then click on the “Soil Properties and Qualities” sub-tab. Expand “Soil Erosion Factors, and click on “K Factor, Whole Soil”. Click on “View Rating” to load that data onto the map. Scroll down, and note the “Rating” for the soil mapping unit of interest. This is the K factor you will use for the USLE.

Alternatively, you can find the K factor using SoilWeb. Open the SoilWeb web app in a computer or mobile device browser and navigate to the study site. Click on the location of the relevant hillslope on the map. This brings up a list of soil series for that map unit. Click on the appropriate soil series (this is often the soil series for which the mapping unit is named). Next, click on the “K_f Factor” button to display the K factors which have been assigned for each horizon. Use the K factor listed for the surface horizon.

Topographic Factor, LS

The topographic factor is a function of the steepness and length of a slope. Using Table 1, determine the LS factor given the length of slope and percent slope you previously observed for your field site.

Table 1. Values for the topographic factor, LS

For Terraced Slopes

If your site has been terraced, this is done individually for each of the terraces. On broad-based terraces the percent slope and length of slope is determined from the top of a terrace to the channel behind the next terrace downslope. For step-back terraces the percent slope and length of slope is determined from the bottom of a terrace to the channel of the next terrace downslope.

Cover-Management Factor, C

Use Table 2 to determine the C factor. The crop rotations are located in the far left column. Identify the appropriate row for the current management at the study site. For example, the appropriate row for a corn (Co) field being planted into soybeans (B) is Co-B. Note the crop abbreviations in the footnote. Next, determine the C factor based on the amount of residue left on the soil surface at the time of planting for the next crop for either tillage or no-till operations.

Table 2. Values for combinations of tillage and residue for the cover and management factor, C.

†Milo may be substituted for corn. All C-values are for wide row plantings.
‡Crop abbreviations are as follows: Co, Corn; B, soybeans; W, wheat; M, meadow (alfalfa, clover, grass, etc.); Fb, field beans; SB, sugar beats; FL, mechanical fallow.

Supporting Practices Factor, P

Use Table 3 for the P factor for contour farming and contour strip-cropping and/or Table 4 for the P factor for terraces with underground outlets or grassed waterways. In situations where both contour farming and terracing are used, the P factors can be multiplied together.

Table 4. Conservation practice P values for terraces with underground outlets or waterways.

Estimate the Erosion Rate Under Existing Conditions

Now that all factors have been determined, multiply them together as described in the introduction. If one factor is not applicable, then do not include that factor in the calculation, or use the value 1 (which does not change the product). For example, if no terracing, contour tillage, or contour strip-cropping are used, then the P factor would be 1 or could be omitted from the calculation for simplicity. This may also apply to the C factor if conventional tillage is used and no residue remains at the time of planting the next crop, or if the appropriate C value was listed as a dash in the table.

Estimate Erosion Rate After Conservation Practices are Implemented

Now that you have estimated the average annual erosion rate by water for the hillslope under existing management practices, you can begin to model erosion rates under different management scenarios. The R and K factors must remain the same, since they are inherent to the site. However, the LS, C, and P factors can change with different management. The LS factor can change with terracing. The C factor can change if different crops are used, or additional residue is left on the field at the time of planting as a result of reduced tillage intensity, or if no-till is used. The P factor can change by adopting contour tillage, contour strip-cropping, or installing terraces.

Explore different management scenarios and calculate the predicted water erosion rate under those scenarios. Use the USLE to model erosion rates for at least two different management scenarios that differ in some way from the existing management system. Describe those scenarios in detail and remember to state all assumptions made for each scenario. Note that sites that are permanently vegetated, such as rangeland sites, will have small average annual erosion rates, so implementation of conservation practices may result in only nominal changes to those rates.

References

Jones, A. J., D. Walters, W. G. Hance, E. C. Dickey, and J. R. Culver. 1988. Universal Soil Loss Equation: A Handbook for Nebraska Producers. Lincoln: Nebraska Cooperative Extension Service. http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=2620&context=extensionhist.

Moorberg, C., and D. Crouse. 2021. Soils Laboratory Manual: K-State Edition, Version 2.0. K-State Edition, Version 2.0. New Prairie Press. https://newprairiepress.org/ebooks/39.

USDA ARS. 2016. “USLE History.” USDA ARS. August 12, 2016. https://www.ars.usda.gov/midwest-area/west-lafayette-in/national-soil-erosion-research/docs/usle-database/usle-history/.

US EPA. 2022. Low Erosivity Waiver (LEW). https://lew.epa.gov/.