**Revised Universal Soil Loss Equation**

The Revised Universal Soil Loss Equation (RUSLE) is designed to predict the long term, average, annual soil loss from sheet and rill flow at nominated sites under specified management conditions. It is widely used throughout the world. It was developed with agriculture in mind and has been extended into the construction industry.

Quantifying and mapping potential erosion problems on a farm can be achieved by the use of the Revised Universal Soil Loss Equation. Once defined, a number of mitigating measures can be utilised to neutralise the problem.

**RUSLE SIMULATION**– Mapping computed soil loss with variable ground cover

The equation is represented by:

**A = R K LS P C**

where, A = computed soil loss (tonnes/ha/yr)

R = rainfall erosivity factor

K = soil erodibility factor

LS = slope length/gradient factor

C = ground cover and management factor

P = erosion control practice factor

While it does have great practical value, its limitations should be recognised.

· It predicts average annual soil loss and not that for a particular storm event;

· It is effective for erosion through sheet and rill flow only on short slopes (<300m) and not for concentrated flow or long slopes; and

· It does not adequately take into account soil dispersibility in assessment of the K-factor.

**Rainfall Erosivity Factor – R**

The rainfall erosivity factor, R, is a measure of the ability of rainfall to cause erosion. It is the product of two components: total energy (E) and maximum 30-minute intensity for each storm (I30). So, the total of EI for a year is equal to the R-factor.

R-factor numbers for New South Wales are available from The Blue Book (Managing Urban Stormwater: Soils and Construction Landcom NSW).

**Soil Erodibility Factor – K**

The soil erodibility factor, K, is a measure of the susceptibility of soil particles to detachment and transport by rainfall and runoff. Soil texture is the principle component affecting K, but soil structure, organic matter and profile permeability also contribute. In the RUSLE, it is a quantitative value experimentally determined.

K-factor numbers for New South Wales are available from the blue book.

**Slope Length/Gradient Factor – LS**

The slope length–gradient factor, LS, describes the combined effect of slope length and slope gradient on soil loss. It is the ratio of soil loss per unit area at any particular site to the corresponding loss from a specific experimental plot of known length and gradient.

LS-factor numbers are site specific and can be calculated from digital elevation models generated from lidar.

**Cover Factor – C**

The cover factor, C, is the ratio of soil loss from land under specified crop or mulch conditions to the corresponding loss from continuously tilled, bare soil. It is a measure of the relative effectiveness of soil and crop management systems in preventing or reducing soil loss. It is affected by:-

· crop canopy (leaves and branches of the crop, which intercept the raindrops and dissipate some of their erosive force),

· surface cover (crop residues and live vegetation on the soil surface),

· soil biomass (all vegetative matter within the soil; residue helps to improve the flow of water into the soil and the soil water-holding capacity),

· soil disturbance (profiling, sweeping ; surface roughness and compaction),

C-factor numbers are site specific and can be determined by the grower.

The erosion control practice factor, P, is the ratio of soil loss with a nominated surface condition ploughed up and down the slope. It is reduced by practices that reduce both the velocity of runoff and the tendency of runoff to flow directly downhill. Unfortunately there is little information available for P factors on undisturbed soils. For this exercise it is left as 1 (no disturbance) and is reduced by management practices that reduce water flow such as applying compost or mulch on the tree line. Major reductions can be achieved by contour banks, headwater drains and cross drains to reduce the length of the slope.

**Erosion Control Practice Factor – P**The erosion control practice factor, P, is the ratio of soil loss with a nominated surface condition ploughed up and down the slope. It is reduced by practices that reduce both the velocity of runoff and the tendency of runoff to flow directly downhill. Unfortunately there is little information available for P factors on undisturbed soils. For this exercise it is left as 1 (no disturbance) and is reduced by management practices that reduce water flow such as applying compost or mulch on the tree line. Major reductions can be achieved by contour banks, headwater drains and cross drains to reduce the length of the slope.