Soil

Lessons

Senior 3 · Agriculture

Soil

4 lessons

Lesson 2 of 4

Causes of Soil Erosion and the Factors That Control Its Rate

25 min

Let's begin

Two neighbouring farms get the same storm. On one, water runs off brown and fast; on the other, it soaks in and the runoff stays almost clear. Same rain, very different erosion. The difference comes from a small set of factors: the rain itself, the slope, the soil, the plant cover and the way the land is managed. Once you can name the factors, you can predict which fields will erode worst and explain why each conservation measure in Lesson 4 works.

Definition

Cause of erosion: a human activity that exposes or loosens the soil, or concentrates runoff, and so starts accelerated erosion. Example: digging a field just before the heavy rains.

Factor of erosion: a condition that decides how fast erosion happens once it starts. Example: slope steepness.

Rainfall intensity: the amount of rain that falls in a unit of time, in millimetres per hour ( $\mathrm{mm\,h^{-1}}$ ).

Infiltration rate: how fast water soaks into the soil, also in $\mathrm{mm\,h^{-1}}$ . When intensity exceeds infiltration, the extra water becomes runoff.

Erodibility: how easily a soil's particles are detached and carried; a property of the soil itself.

Slope gradient: the steepness of a slope, given as a percentage or as an angle in degrees.

An experiment you can do at school

Question: does plant cover change how much soil the rain removes?

You need: two shallow wooden or plastic trays with a notch cut in one end, the same soil for both, a piece of grass turf (or a thick layer of dry grass mulch), two clear jars, two wooden blocks of the same height, and a watering can.

Fill both trays with the same soil and press it level. Cover tray B with turf or mulch; leave tray A bare.
Rest the upper end of each tray on a block so both have the same slope, with the notch over a jar.
From the same height, pour the same volume of water (for example 2 litres) over each tray.
Compare the volume and colour of the water in the two jars. Let them stand overnight and compare the layer of soil that settles.

Figure 2. The runoff experiment. Only one thing differs between the trays: the cover. That makes it a fair test.

Result and explanation: jar A collects more water, and it is brown with soil. On bare soil the drops strike the surface directly (splash), seal it, and the water runs off carrying particles. In tray B the leaves absorb the impact of the drops, the roots and mulch hold the soil, and the rough cover slows the water so more of it soaks in. Keep this result in mind: cover is the factor a farmer can change most cheaply.

Causes: what people do that starts erosion

Cultivating steep slopes, especially with rows running up and down the slope so that each furrow becomes a channel.
Leaving soil bare and freshly dug when the heavy rains begin at the start of Season A and Season B.
Cutting trees and clearing bush for fields, firewood and charcoal (deforestation).
Overgrazing and trampling, which remove grass cover and compact the soil so water cannot soak in.
Burning crop residues and bush, which leaves the surface naked and destroys organic matter.
Continuous cropping without manure, which breaks down soil structure (Lesson 3).
Roads, footpaths, roofs and paved yards that send concentrated runoff onto fields without a safe drain.
Mining and quarrying that strip vegetation and dump loose soil near streams.

Factors: what controls the rate

Factor	Erosion is worse when…	Reason
Rainfall	storms are intense, long or frequent, or fall on bare soil	big, fast drops detach more soil, and intensity above the infiltration rate produces runoff
Slope gradient	the slope is steep	runoff flows faster, and a faster flow carries far more soil
Slope length	the slope is long and unbroken	runoff builds up in volume and speed on its way down
Soil properties	the soil is silty or fine sandy, low in humus, poorly structured, shallow or compacted	weak crumbs break easily and water cannot soak in; clay and humus bind particles
Vegetation cover	cover is thin or absent	leaves break the fall of drops, roots bind soil, litter slows runoff
Land management	rows run downhill and no conservation structures exist	furrows and paths become channels that concentrate runoff

Wind erosion has its own extra factors: wind speed, and how dry and loose the surface soil is.

Worked example 1: measuring slope gradient

Problem. On a hillside field, the top of the field is 12 m higher than the bottom, and the horizontal distance between them is 40 m. Give the gradient as a percentage and as an angle.

Solution.

$\text{gradient} = \frac{\text{vertical rise}}{\text{horizontal distance}} \times 100\% = \frac{12}{40} \times 100\% = 30\%$
The angle $\theta$ satisfies $\tan\theta = \frac{12}{40} = 0.3$ , so $\theta = \tan^{-1}(0.3) \approx 16.7^\circ$ .

Answer: 30%, or about $17^\circ$ . Note that a 100% slope is $45^\circ$ , not vertical.

Worked example 2: when rain becomes runoff

Problem. A storm falls at $40\,\mathrm{mm\,h^{-1}}$ for 30 minutes on a crusted bare soil whose infiltration rate is $15\,\mathrm{mm\,h^{-1}}$ . How much runoff leaves one hectare? (Assume the soil surface is already wet, so all the excess runs off.)

Solution.

Excess rate $= 40 - 15 = 25\,\mathrm{mm\,h^{-1}}$ .
In half an hour: $25 \times 0.5 = 12.5\,\mathrm{mm}$ of runoff, which is $0.0125\,\mathrm{m}$ .
Volume from one hectare: $0.0125\,\mathrm{m} \times 10\,000\,\mathrm{m^2} = 125\,\mathrm{m^3}$ , that is $125\,000$ litres.

Answer: $125\,\mathrm{m^3}$ . If mulch raised the infiltration rate to $35\,\mathrm{mm\,h^{-1}}$ , the runoff would fall to $(40-35) \times 0.5 = 2.5\,\mathrm{mm}$ , only $25\,\mathrm{m^3}$ . (Illustrative values.)

Rwanda: where the factors combine

The National Institute of Statistics of Rwanda (NISR) records the degree of erosion seen on sampled plots in its Seasonal Agricultural Survey. The table compares five districts in 2024 Season A. "Severe" means rills, gullies or landslides; "moderate" means overland-flow (sheet) erosion.

District	Province and relief	Plots with severe erosion (%)	Plots with moderate erosion (%)
Karongi	Western; steep hills above Lake Kivu	8.5	27.5
Ngororero	Western; steep, deeply cut hills	7.2	22.7
Rulindo	Northern; steep hills	5.1	27.2
Kirehe	Eastern; gentler, lower land	0.2	8.2
Ngoma	Eastern; gentler, lower land	0.1	3.4
Rwanda		1.9	13.3

Source: NISR, Seasonal Agricultural Survey 2024 Annual Report, Table 73.

The steep western and northern districts show far more severe erosion than the gentler east, even though all are farmed intensively. A 2022 national mapping by the Rwanda Water Resources Board and IUCN reached the same conclusion: about 85% of Ngororero District's land was at high risk of erosion, and about 45% of the whole country's land was at some risk.

Warning

Common mistakes. Do not write "rain" as a factor and stop there: say which property of the rain (intensity, amount, duration) and why it matters. And do not confuse the slope's percentage with its angle: 30% is about $17^\circ$ , not $30^\circ$ .

Tip

Memory hook: "R-S-S-V-M": Rain, Slope, Soil, Vegetation, Management. In a "factors" question, give each factor a cause-and-effect sentence: "On a steeper slope runoff flows faster, so it can carry more soil."

USDA Natural Resources Conservation Service: "The Science of Soil Health: Simulating Rainfall on Pastures". Watch the trays of soil from differently managed fields under the same artificial rain, and compare the runoff and the water that soaks in, as in Figure 2.

✦

Practice

Distinguish between a cause of erosion and a factor of erosion, with one example of each.
List four human activities that cause soil erosion on Rwandan farms.
In the tray experiment (Figure 2), state the one variable that was changed and two variables that were kept the same.
A field rises 9 m over a horizontal distance of 60 m. Calculate its gradient as a percentage and as an angle to the nearest degree.
Rain falls at $30\,\mathrm{mm\,h^{-1}}$ for 1 hour on a soil with an infiltration rate of $10\,\mathrm{mm\,h^{-1}}$ . Calculate the runoff from $0.5\,\mathrm{ha}$ in cubic metres.
Explain why a silty soil low in humus erodes more easily than a well-structured loam rich in humus.
Using the NISR table, compare erosion in Karongi and Ngoma, and suggest two factors that explain the difference.

Show answersHide answers

A cause is a human activity that starts accelerated erosion, such as overgrazing. A factor decides how fast erosion proceeds, such as rainfall intensity or slope.
Any four: farming steep slopes with rows downhill; leaving soil bare at planting; deforestation; overgrazing; burning; continuous cropping without manure; uncontrolled runoff from roads and roofs; mining.
Changed: the cover (bare or grass). Kept the same: the soil, the slope, the volume of water, the height of pouring, the tray size (any two).
$\frac{9}{60} \times 100\% = 15\%$ ; $\tan^{-1}(0.15) \approx 8.5^\circ$ , about $9^\circ$ .
Excess $= 20\,\mathrm{mm} = 0.02\,\mathrm{m}$ ; area $= 5000\,\mathrm{m^2}$ ; runoff $= 0.02 \times 5000 = 100\,\mathrm{m^3}$ .
Silt particles are fine but not sticky, so its crumbs break apart easily and seal the surface; humus binds particles into stable crumbs and keeps pores open so water soaks in.
Karongi has far more plots with severe (8.5% against 0.1%) and moderate (27.5% against 3.4%) erosion. Factors: Karongi's slopes are much steeper and longer, and its rainfall is generally higher than in the lower, gentler east.

Key points

Causes are human activities that expose soil or concentrate runoff; factors control the rate.
The factors are rainfall, slope (gradient and length), soil properties, vegetation cover and land management.
Runoff forms when rainfall intensity exceeds the infiltration rate; cover raises infiltration and cuts runoff.
Rwanda's steep western and northern districts erode most (NISR, 2024; RWB and IUCN, 2022).

Sources and credits

Seasonal Agricultural Survey, Annual Report 2024, National Institute of Statistics of Rwanda (NISR), Table 73. Figures cited.
The State of Soil Erosion Control in Rwanda (summary), Rwanda Water Resources Board and IUCN, 2022. Figures cited.
Photo Rwanda - dense agriculture, by "not not phil", via Wikimedia Commons, licensed under CC BY-SA 2.0.
Video: The Science of Soil Health: Simulating Rainfall on Pastures, USDA NRCS, embedded from YouTube.

Written by Ganzaa: the text, the experiment and Figure 2, the tables and the worked examples. Rainfall and infiltration values in the worked examples are illustrative.

This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1An experiment you can do at school

2Causes: what people do that starts erosion

3Factors: what controls the rate

4Rwanda: where the factors combine

An experiment you can do at school

Causes: what people do that starts erosion

Factors: what controls the rate

Rwanda: where the factors combine