Last reviewed 1 Oct 2026 · 6 min read
What is soil erosion?
Soil erosion is the detachment, transport and deposition of soil particles by wind, water or gravity. It is natural (geological) erosion — slow, balanced by soil formation — and accelerated erosion, caused by human activity (deforestation, over-grazing, up-and-down-slope ploughing, mining), which is many times faster than soil formation and is the real problem.
Erosion by water has three phases:
- Detachment — by raindrop impact (the biggest source) and flowing water.
- Transport — by splash and runoff.
- Deposition — when the flow loses velocity (in depressions, tanks, reservoirs, streams).
Why raindrop impact matters
A raindrop strikes bare soil with considerable kinetic energy, breaks aggregates, seals the surface (crusting), reduces infiltration and increases runoff, and splashes particles up to about 1 m or more (more on slopes in the downslope direction). Vegetative cover and mulch absorb this energy — the central idea in erosion control.
Types of water erosion
| Type | Description |
|---|---|
| Splash erosion | Detachment and movement by raindrops |
| Sheet erosion | Uniform removal of a thin layer over a whole slope by unconcentrated flow; hard to notice, but removes the most fertile topsoil |
| Rill erosion | Small, well-defined channels (a few cm deep) formed by concentrated flow; can be erased by ordinary tillage |
| Gully erosion | Larger channels that cannot be removed by ordinary tillage (deeper than about 30 cm) |
| Stream-bank erosion | Undercutting and caving of banks by stream flow |
| Landslide (mass movement) | Slip of a mass of soil on steep, saturated slopes |
| Piping (tunnel) erosion | Subsurface flow carries soil, forming tunnels and collapses |
| Ravines | Extensive gully systems (Chambal, Yamuna, Mahi, Sabarmati) |
Factors affecting erosion
| Factor | Effect |
|---|---|
| Climate (rainfall) | Intensity, amount and duration; high-intensity storms cause the most erosion (a measure is EI₃₀) |
| Soil | Erodibility — silt and fine sand erode easily, clay and organic matter bind; structure, infiltration and permeability matter |
| Topography | Slope length and steepness — erosion rises sharply with both |
| Vegetation | Canopy and litter cut raindrop energy; roots bind the soil |
| Human practices | Tillage direction, grazing, burning, cultivation on steep land |
India's soil loss is large: a classic estimate (Dhruvanarayana and Rambabu, 1983) put it at about 5,334 million tonnes per year (≈ 16.4 t/ha/yr) — a figure repeated in textbooks; later studies use USLE and GIS and give updated numbers (check the latest source).
Gullies
A gully is a channel eroded by concentrated runoff, deeper than about 0.3 m.
Classification
- By depth: small (< 1 m)**, **medium (1–5 m)**, **large (> 5 m).
- By shape: U-shaped (resistant subsoil and topsoil, vertical walls), V-shaped (soft subsoil, resistant bottom).
- By continuity: continuous (permanent stream) and discontinuous.
- By the area drained and bottom width.
Stages of gully development
- Formation — channelling of the surface layer; headcut forms.
- Development — headward cutting and widening and deepening.
- Healing — vegetation and sediment begin to stabilise it.
- Stabilisation — banks and bed settle at a gentle slope; the headcut stops advancing.
Causes: concentrated flow from roads, cart tracks, paths and field drains, removal of cover, faulty terrace outlets. Control needs catchment treatment, diversion of runoff and structures at the headcut (see the Gully Control Structures note).
Universal Soil Loss Equation (USLE)
The USLE (Wischmeier and Smith, 1978) predicts the long-term average annual soil loss from sheet and rill erosion on a field slope:
- A — computed soil loss, t/ha/year.
- R — rainfall–runoff erosivity index (MJ·mm·ha⁻¹·h⁻¹·yr⁻¹), the annual sum of EI₃₀ (storm kinetic energy × the maximum 30-minute intensity).
- K — soil erodibility factor (t·ha·h·ha⁻¹·MJ⁻¹·mm⁻¹) — the soil loss per unit of R on the standard plot (22.13 m long, 9 % slope, continuous bare fallow, up-and-down cultivation).
- L — slope-length factor (dimensionless; 1 at the standard 22.13 m).
- S — slope-steepness factor (1 at 9 %).
- C — cover-and-management factor (0–1; 1 for bare fallow; lower for dense crops and residue).
- P — support-practice factor (0–1; 1 for up-and-down cultivation; contouring, strip cropping, terracing reduce it).
The USLE does not estimate gully, stream-bank or mass-movement erosion, nor sediment delivery out of the watershed; it is empirical and needs local calibration (R values for India are mapped by the ICAR/other studies).
LS factor
A common field form (slope in per cent, λ = slope length in metres):
with for slopes of 5 % or more, 0.4 for 3.5–4.5 %, 0.3 for 1–3 %, and 0.2 for less than 1 %.
A slope is 60 m long with a gradient of 5 %. Using :
; slope term . .
Worked example
A field has , , , (cereal–fallow), (contouring):
.
If the permissible (tolerable) loss is about 10 t/ha/yr, the practice is not safe; reducing (by mulch or a dense crop) to 0.10 gives t/ha/yr — acceptable.
Permissible soil loss
The soil-loss tolerance (T) is the maximum annual erosion that still allows productivity to be sustained — about 5–12 t/ha/yr depending on the soil depth and the rate of soil formation (shallow soils less, deep soils more; about 11.2 t/ha/yr (5 tons/acre) is the traditional figure for deep soils). Soil formation is typically only about 1 t/ha/yr or less.
Use of the USLE: to compare practices, to choose the support practice (solve for or so that ), and to target critical areas.