Universal Soil Loss Equation (USLE)

Agdex#:	572/751
Publication Date:	05/00
Order#:	00-001
Last Reviewed:	05/00
History:	Original Factsheet
Written by:	Robert P. Stone - Engineer, Soil Management/OMAFRA; Don Hilborn - Engineer, Byproduct Management /OMAFRA

Table of Contents

1. Background
2. Procedure for Using the USLE
3. Soil Loss Tolerance Rates
4. Management Strategies to Reduce Soil Losses
5. Table 1: R Factor Data
6. Table 2: K Factor Data
7. Table 3A: LS Factor Calculation
8. Equation for Calculation of LS
9. Table 3B: NN Values
10. Table 4A: Crop Type Factor
11. Table 4B: Tillage Method Factor
12. Table 5: P Factor Data
13. Table 6: Soil Loss Tolerance Rates
14. Table 7: Management Strategies to Reduce Soil Losses
15. Example: Calculation of Soil Erosion Using USLE

Background

The Universal Soil Loss Equation (USLE) predicts the long term average annual rate of erosion on a field slope based on rainfall pattern, soil type, topography, crop system and management practices. USLE only predicts the amount of soil loss that results from sheet or rill erosion on a single slope and does not account for additional soil losses that might occur from gully, wind or tillage erosion. This erosion model was created for use in selected cropping and management systems, but is also applicable to non-agricultural conditions such as construction sites. The USLE can be used to compare soil losses from a particular field with a specific crop and management system to "tolerable soil loss" rates. Alternative management and crop systems may also be evaluated to determine the adequacy of conservation measures in farm planning.

Five major factors are used to calculate the soil loss for a given site. Each factor is the numerical estimate of a specific condition that affects the severity of soil erosion at a particular location. The erosion values reflected by these factors can vary considerably due to varying weather conditions. Therefore, the values obtained from the USLE more accurately represent long-term averages.

A calculation of soil losses using the USLE may also be done in OMAFRA's Nutrient Management (NMAN 2000 or upgrade) computer program, SOF001. The soil loss value generated from the USLE equation is used to determine the "soil erosion rating value" in the calculation of the Phosphorus Index.

A =R x K x LS x C x P

R is the rainfall and runoff factor by geographic location as given in Table 1, R Factor Data. The greater the intensity and duration of the rain storm, the higher the erosion potential. Select the R factor from Table 1 based on the county and corresponding station where the calculation is to be made.

K is the soil erodibility factor. (See Table 2, K Factor Data). It is the average soil loss in tons/acre per unit area for a particular soil in cultivated, continuous fallow with an arbitrarily selected slope length of 72.6 ft. and slope steepness of 9%. K is a measure of the susceptibility of soil particles to detachment and transport by rainfall and runoff. Texture is the principal factor affecting K, but structure, organic matter and permeability also contribute.

LS is the slope length-gradient factor. The LS factor represents a ratio of soil loss under given conditions to that at a site with the "standard" slope steepness of 9% and slope length of 72.6 feet. The steeper and longer the slope, the higher is the risk for erosion. Use either Table 3A, LS Factor Calculation or the Equation for Calculating LS included in this Factsheet to obtain LS.

C is the crop/vegetation and management factor. It is used to determine the relative effectiveness of soil and crop management systems in terms of preventing soil loss. The C factor is a ratio comparing the soil loss from land under a specific crop and management system to the corresponding loss from continuously fallow and tilled land. The C Factor can be determined by selecting the crop type and tillage method (Table 4A, Crop Type Factor and Table 4B,Tillage Method Factor respectively) that corresponds to the field and then multiplying these factors together.

The C factor resulting from this calculation is a generalized C factor value for a specific crop that does not account for crop rotations or climate and annual rainfall distribution for the different agricultural regions of the country. This generalized C factor, however, provides relative numbers for the different cropping and tillage systems; thereby helping you weigh the merits of each system.

P is the support practice factor. It reflects the effects of practices that will reduce the amount and rate of the water runoff and thus reduce the amount of erosion. The P factor represents the ratio of soil loss by a support practice to that of straight-row farming up and down the slope. The most commonly used supporting cropland practices are cross slope cultivation, contour farming and strip-cropping (Table 5, P Factor Data).

Procedure for Using the USLE

1. Determine the R Factor. (Table 1)
2. Based on the soil texture determine the K value (Table 2). If there is more than one soil type in a field and the soil textures are not very different, then use the soil type that represents the majority of the field. Repeat for other soil types as necessary.
3. Divide the field into sections of uniform slope gradient and length. Assign an LS value to each section (Table 3A).
4. Choose the crop type factor and tillage method factor for the crop to be grown. Multiply these 2 factors together to obtain the C factor.
5. Select the P factor based on the support practice used (Table 5).
6. Multiply the 5 factors together to obtain the soil loss per acre.

Soil Loss Tolerance Rates

A tolerable soil loss is the maximum annual amount of soil, which can be removed before the long term natural soil productivity is adversely affected.

The impact of erosion on a given soil type, and hence the tolerance level varies, depending on the type and depth of soil. Generally, soils with deep, uniform, stone free topsoil materials and/or not previously eroded have been assumed to have a higher tolerance limit than soils which are shallow or previously eroded.

Soil loss tolerance rates are included in Table 6, Soil Loss Tolerance Rates.

The suggested tolerance level for most soils in Ontario is 3 tons/acre/year or less.

Management Strategies to Reduce Soil Losses

Having obtained an estimate of the potential annual soil loss for a field, you may want to consider ways to reduce this loss to a tolerable level. Table 7, Management Strategies to Reduce Soil Losses, outlines management strategies to help you reduce soil erosion.

Equation for Calculation of LS (if not using Table 3A above)

LS = [0.065 + 0.0456(slope) + 0.006541(slope)²] x (slope_length ÷ const)^NN

Where:

slope =  slope steepness (%)

slope length = length of slope (ft.)

constant = 72.5 Imperial or 22.1 metric

NN =  see Table 3B below

Example: Calculation of Soil Erosion Using USLE

A =R x K x LS x C x P

Rainfall and Runoff Factor (R)

The sample field is in Middlesex County. Therefore the R Factor is obtained in Table 1 from the London weather station.

R Factor = 100

Soil Erodibility Factor (K)

The sample field consists of fine sandy loam soil with an average organic matter content. The K Factor is obtained from Table 2.

K Factor = 0.18

Slope Length-Gradient Factor (LS)

The sample field is 800 feet long with a 6% slope. The LS factor can be obtained directly from Table 3A or may be calculated using the "Equation". The NN value from Table 3B to be used in the "Equation" is 0.5.

LS Factor = 1.91

Crop/Vegetation and Management Factor (C)

The sample field was plowed in the spring and grain corn was planted. The C Factor is obtained from the crop type factor (Table 4A) and the tillage method factor (Table 4B).

Crop Type Factor for grain corn = 0.4
Tillage Method Factor for spring plow = 0.9

C Factor = 0.4 x 0.9 = 0.36

Support Practice Factor (P)

Cross slope farming is used on this sample field. The P Factor was obtained from Table 5.

P Factor = 0.75

Therefore,

A = R x K x LS x C x P

= 100 x 0.18 x 1.91 x 0.36 x 0.75

= 9.28 tons/acre/year

Referring to Table 6 in this Factsheet, you will see that this soil loss rate of 9.28 tons/acre/year is in the moderate range and considerably higher than the "tolerable loss level" of 3 tons/acre/year. To reduce the soil losses for this sample field below 3 tons/acre/year we will make the following changes to the above example.

Change tillage method from "spring plow (0.9)" to "no-till (0.25)"

Therefore, C Factor (Revised) = 0.4 x .25 = 0.10

The adjusted annual soil loss value is:

A = R x K x LS x C x P

= 100 x 0.18 x 1.91 x 0.10 x 0.75

= 2.58 tons/acre/year

Thus by changing the tillage practice, the average annual predicted soil loss for this field is below the "tolerable soil loss" of 3 tons/acre/year.