USLE

By now, you may have noticed that environmental scientists like maths. You can find equations for almost anything – including the rate of soil loss.

Soil erosion and degradation can be influenced by the natural environment, as well as human activities. Fortunately, all of these details are found in a simple multiplication called the Universal Soil Loss Equation.

Ready to give it a go yourself? There's nothing to lose … except soil!

USLE: Meaning

Let’s begin with a definition of the USLE.

Environmental scientists can use the USLE to estimate soil erosion rates. This can help them assess the effectiveness of Soil Conservation programmes.

History of the USLE

The USLE technology was the culmination of decades of soil erosion research. It was first published as a complete technology in 1965 in the USDA Agriculture Handbook 282, then updated in 1978.

USLE Formula

As formulas go, the USLE is relatively simple.

A = R × K × LS × C × P

Where:

• A = long-term average annual soil loss (measured in tonnes per hectare per year)

• R = rainfall and runoff factor by geographical location

• K = soil erodibility factor

• LS = slope length-gradient factor

• C = crop and management factor

• P = support practice factor

The R and K factors of an area cannot be altered, but it is possible to reduce soil loss by constructing terraces, changing crops, or modifying agricultural practices.

USLE: R Factor

We learned earlier that the R factor represents the erosion potential, based on the rainfall and runoff by geographic location.

The greater the rainfall duration and intensity, the higher the erosion potential.

Fig. 1 – Soil erosion after a period of heavy rainfall in Sudeley, Gloucestershire. Source: Wikimedia Commons

R Factor Equation

R = E x I₃₀ ÷ 100

Where:

• I₃₀ is the maximum rainfall intensity (cm/h)

• E is the total kinetic energy of the precipitation (J/m²)

USLE: K Factor

The K factor is the soil erodibility factor. It's a measure of the susceptibility of the soil particles to detachment and transport by rainfall and runoff. The K factor is primarily influenced by soil texture, but it can also be impacted by structure, organic matter, and permeability.

USLE: LS Factor

The LS factor represents soil loss dependent on slope steepness and length. The steeper and longer the slope, the greater the risk of soil erosion.

The LS factor is usually obtained from a pre-determined table.

USLE: C Factor Table

The C factor combines soil loss from a specific crop and soil loss from a specific land management technique.

The two factors are multiplied together to find C.

USLE: P Factor

The P factor represents how an agricultural support practice will affect soil loss.

Fig. 2 – Building terraces allows farmers to use strip cropping strategies. Source: Wikimedia Commons

USLE: Worked Example

Now that we've looked at the factors affecting soil loss, let's work out an example together.

Environmental scientists have been trying to conserve soil in two different locations.

• Site A is located in a hilly, wet part of western UK.

• Site B is located in a drier, flatter part of eastern UK.

Which Soil Conservation project has been more effective?

Erosion Rate in Site A = 1200 × 0.67 × 3.99 × (0.10 × 0.25) × 0.5

Erosion Rate in Site A = 1200 × 0.67 × 3.99 × 0.025 × 0.5

Erosion Rate in Site A = 40.10

Erosion Rate in Site B = 500 × 0.40 × 0.54 x (0.35 × 0.9) × 0.75

Erosion Rate in Site B = 500 × 0.40 × 0.54 × 0.315 × 0.75

Erosion Rate in Site B = 25.52

Site B has a smaller annual average soil loss, so this conservation programme has been more effective.

Site A has minimised soil loss through crop type, tillage method, and agricultural practice. Unfortunately, these changes haven't been able to compensate for the naturally high precipitation and steep slopes of the area.

I hope that this article has clarified the Universal Soil Loss Equation (USLE) for you. Remember that it's a mathematical model used to predict the long-term average annual rate of soil erosion. The equation incorporates rainfall and runoff, soil erodibility, slope, crop type, tillage method, and agricultural practice.

^{1. Agricultural Research Service, USLE History, National Soil Erosion Research Laboratory: West Lafayette, IN, 2016}

^{2. Gabor Mezosi, Estimation of the Changes in the Rainfall Erosivity in Hungary, Journal of Environmental Geography, 2016}

^{3. Qiang Dai, Estimation of rainfall erosivity based on WRF-derived raindrop size distributions, Hydrology and Earth System Science, 2020}

^{4. Robert P. Stone, Universal Soil Loss Equation (USLE), Kings Printer for Ontario, 2016}