Soil erosion and the Universal Soil Loss Equation
How water and wind erode soil, the stages from splash to gully, what each USLE factor means, and how to estimate soil loss and cut it.
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Soil erosion is the detachment and removal of soil by water or wind faster than new soil forms. It takes the most fertile layer first, the topsoil rich in organic matter and nutrients, and it is effectively permanent on a human timescale: soil forms at a fraction of a millimetre a year, while an unprotected field can lose centimetres in a single season.
Global estimates put soil loss by water alone at tens of billions of tonnes a year, most of it from cropland. Erosion is a core topic in soil science, agronomy, agricultural engineering and environmental exams, and the Universal Soil Loss Equation (USLE) is the formula students are most often asked to explain or use.
How water erodes soil
Water erosion has two steps: detachment of particles, then transport by flowing water. It progresses through recognisable forms.
- Splash erosion. A raindrop striking bare soil at several metres per second shatters aggregates and throws particles up to about a metre. On a slope more lands downhill than up. Splash also seals the surface, reducing infiltration and creating more runoff.
- Sheet (interrill) erosion. A thin, fairly uniform layer is removed by overland flow. It is hard to see and easily underestimated; exposed roots, stones on pedestals and lighter-coloured patches are the signs.
- Rill erosion. Runoff concentrates into small channels, a few centimetres deep, that cultivation can smooth out.
- Gully erosion. Channels too deep to cross with machinery or remove by tillage, cutting back headward with each storm. Gullies carry enormous volumes and can make land unusable.
- Stream-bank and channel erosion, and mass movement (landslides) on steep land.
Sheet and rill erosion together usually move the most soil from farmland.
How wind erodes soil
Wind erosion dominates in dry, flat, sparsely covered land, from the Sahel and Central Asia to the North American Great Plains, where it produced the Dust Bowl of the 1930s. Particles move in three ways:
- Saltation: sand-sized grains (roughly 0.1–0.5 mm) bounce along in low arcs; usually more than half of the movement.
- Suspension: fine particles below about 0.1 mm carried high and far as dust, sometimes across oceans.
- Surface creep: larger grains rolled along by the impact of saltating ones.
Windbreaks and shelterbelts, crop residues left standing, keeping the soil moist and cloddy, and strip cropping across the wind are the main defences.
The Universal Soil Loss Equation
The USLE was developed in the United States by Wischmeier and Smith from about ten thousand plot-years of runoff measurements, and published in its standard form in 1978. It estimates long-term average annual sheet and rill erosion from a field:
The factors
- A is the average annual soil loss, in tonnes per hectare per year.
- R, rainfall erosivity: the sum, over a year's storms, of each storm's kinetic energy multiplied by its maximum 30-minute intensity (the EI30 index), averaged over many years. Intense tropical and monsoon storms give very high R values; gentle frontal rain gives low ones.
- K, soil erodibility: how easily the soil is detached and transported under standard conditions. Soils high in silt and very fine sand and low in organic matter are the most erodible; well-aggregated clays and coarse sands are less so. K can be estimated from texture, organic matter, structure and permeability using a nomograph.
- LS, topographic factor: slope length (L) and steepness (S) together, relative to the standard plot, 22.1 m long on a 9 per cent slope, where LS = 1. Loss rises faster with steepness than with length.
- C, cover-management factor: the ratio of loss under the actual crop and management to loss from continuous bare, tilled fallow (C = 1). Dense forest or permanent grass is close to 0.001–0.01; row crops with conventional tillage may be 0.3–0.5; mulch and no-till cut it sharply.
- P, support-practice factor: the ratio of loss with contouring, strip cropping or terracing to loss with up-and-down-slope farming (P = 1). Contour cultivation alone gives values around 0.5–0.9, depending on slope.
Because the equation multiplies, every factor has the same weight in the result. In practice C is usually the strongest lever: keeping the soil covered matters more than any structure.
Estimating soil loss and the effect of conservation
A maize field on a silt loam has these values (SI units): R = 3,500 MJ·mm/(ha·h·yr), K = 0.04 t·h/(MJ·mm), LS = 1.5, C = 0.3 for conventionally tilled maize, and P = 1 with rows running up and down the slope.
A = 3,500 × 0.04 × 1.5 × 0.3 × 1.0 = 63 t/ha per year.
That is roughly 4 to 5 mm of topsoil a year at a bulk density of 1.3 t/m3 (63 ⁄ 1.3 ≈ 48 m3 per hectare, or 4.8 mm), far above any tolerable rate.
Now the farmer adopts no-till with residue mulch (C = 0.1) and contour strip cropping (P = 0.5):
A = 3,500 × 0.04 × 1.5 × 0.1 × 0.5 = 10.5 t/ha per year, an 83 per cent reduction. Adding contour bunds that halve the effective slope length would reduce LS further.
Tolerable soil loss
The soil loss tolerance (T value) is the maximum average loss that still allows productivity to be sustained economically. Many soil surveys use values from about 2 to 11 t/ha per year, the higher values for deep soils with good subsoils and the lower for shallow soils. Strictly, even these rates exceed soil formation on most land, so the aim of conservation planning is to bring A below T, and lower wherever feasible.
Limits of the USLE
- It predicts long-term averages, not the loss from one storm or one year.
- It estimates sheet and rill erosion only: not gully, stream-bank or wind erosion.
- It estimates soil moved within a slope, not the sediment that reaches a river; much is deposited at the foot of slopes. A sediment delivery ratio is needed for that.
- Its factor values come from experiments, mostly in the United States; R, K and C must be calibrated for other climates and soils.
Successors keep the same structure. RUSLE and RUSLE2 revised the factors and added time-varying cover; process-based models such as WEPP simulate infiltration, runoff and sediment storm by storm. In global and regional assessments, RUSLE is now commonly applied with satellite-derived cover and gridded rainfall data.
Controlling erosion
The methods follow the factors.
Agronomic and biological measures (lower C, and partly K):
- Keep the soil covered: crop residues, mulch, cover crops, intercrops and denser stands.
- Conservation agriculture: minimum or no tillage, permanent cover and rotations.
- Build organic matter to strengthen aggregates and raise infiltration.
- Grass strips, hedgerows and agroforestry.
Supporting and mechanical measures (lower P and LS):
- Contour cultivation and contour ridges.
- Strip cropping: alternating erosion-resisting and erosion-permitting crops across the slope.
- Terraces: bench terraces on steep land; broad-based or graded terraces on gentler slopes; contour and graded bunds.
- Grassed waterways to carry runoff safely; check dams and gully plugs to stabilise gullies.
Mechanical structures are expensive and need maintenance; they work best combined with cover, which tackles erosion at its source.
Key points
- Water erosion proceeds from splash to sheet, rill and gully erosion; sheet and rill move the most soil from farmland.
- Wind moves soil by saltation (mostly), suspension and surface creep.
- USLE: A = R × K × LS × C × P, giving long-term average sheet and rill loss in t/ha per year.
- The standard plot is 22.1 m long on a 9% slope, continuously bare (C = 1) and tilled up and down the slope (P = 1).
- Silt and very fine sand soils low in organic matter have the highest K.
- Cover (C) is usually the most effective factor to manage; the USLE does not cover gully or wind erosion.
What the exams ask
- School-leaving and entrance papers: the agents and types of erosion, splash erosion, and the meaning of each USLE factor.
- Degree and research-fellowship entrances: the standard plot, the EI30 erosivity index, which soils are most erodible, tolerable soil loss, and limitations of the USLE.
- Licensing and professional examinations for agricultural engineers: computing A from given factors, the effect of changing slope length or cover, and designing terraces, bunds and waterways.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why is silt more erodible than clay or sand?
Clay particles stick together into stable aggregates, and sand grains are too heavy to carry easily. Silt and very fine sand are light enough to be transported but do not bind well, so they detach and move most readily.
Is no-till always enough to stop erosion?
It greatly reduces erosion when residues stay on the surface, but on long, steep slopes or under extreme storms, runoff can still concentrate. Combining cover with contour practices and safe waterways is more reliable.
Can the USLE be used outside the United States?
Yes, and it widely is, but its factors must be calibrated locally: rainfall erosivity from local storm records, erodibility for local soils, and cover factors for local crops and seasons.
Sources
- Wischmeier, W. H. and Smith, D. D. Predicting Rainfall Erosion Losses: A Guide to Conservation Planning. USDA Agriculture Handbook 537, 1978.
- Renard, K. G. et al. Predicting Soil Erosion by Water: A Guide to Conservation Planning with the Revised Universal Soil Loss Equation (RUSLE). USDA Agriculture Handbook 703, 1997.
- Borrelli, P. et al. "An assessment of the global impact of 21st century land use change on soil erosion." Nature Communications 8, 2013 (2017).
- Morgan, R. P. C. Soil Erosion and Conservation, 3rd edition. Blackwell, 2005.
- FAO and ITPS. Status of the World's Soil Resources. FAO, 2015.
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