Soil Erosion in Rwanda: Types, Causes and Effects – Soils in Rwanda (Senior 4 Geography) | Ganzaa

Soils in Rwanda

Lessons

Senior 4 · Geography

Soils in Rwanda

3 lessons

Lesson 2 of 3

Soil Erosion in Rwanda: Types, Causes and Effects

25 min

Where the soil is most at risk

In 2021 the Rwanda Water Resources Board (RWB) mapped the risk of soil erosion across the country. It found that about 46% of Rwanda's area was exposed to some form of erosion risk (1,064,799 ha out of 2,297,416 ha), and that the share varied enormously between districts (REMA, State of Environment and Outlook Report 2021). Figure 3 maps those district figures.

Figure 3: drawn by Ganzaa from the Rwanda Water Resources Board survey (2021) published in REMA's State of Environment and Outlook Report 2021, Table 5-1. Boundaries: geoBoundaries, CC BY 4.0; Natural Earth.

The darkest districts, Ngororero (83%) and Muhanga (82%), followed by Karongi and Gakenke (71%), lie on the steep slopes of the Congo–Nile Ridge and the western central plateau. The lightest, Nyagatare (8%) and Gatsibo (14%), lie on the gentle eastern lowlands. The map of erosion risk is, to a large degree, a map of relief.

Definition

Soil erosion: the removal of soil from the land surface by water, wind or gravity faster than new soil forms. Run-off: rain water that flows over the surface instead of soaking in.

Example: after a storm, a brown stream carries soil washed from cultivated slopes upstream.

Types of soil erosion

Figure 4: erosion by water grows from splash to sheet, rill and gully as run-off concentrates.

Splash erosion: raindrops strike bare soil and throw particles into the air; on a slope more land downhill.
Sheet erosion: a thin, even film of run-off removes a layer of topsoil, often unnoticed until roots are exposed.
Rill erosion: run-off concentrates into small channels a few centimetres deep that ploughing can remove.
Gully erosion: rills grow into deep trenches that cut fields apart and can swallow roads and houses.
Mass movement: landslides and slumps (Unit 3), the most severe form on steep slopes.
Wind erosion: minor in Rwanda, mostly on bare, dry soil in the east in the dry season.

A national survey of 25,144 plots found that 88% suffered a low degree of erosion (splash or wind erosion), followed by moderate (diffuse overland flow) and severe soil loss (rills, gullies, mass movement and landslides) (REMA, 2021).

Causes

Natural causes	Human causes
Slopes: around 90% of Rwanda's territory lies on slopes (REMA, 2021)	Cultivating steep slopes: REMA found crops grown on slopes of 55% and more, without recommended protection (REMA, 2009)
Heavy, intense rain in the two rainy seasons, especially in the west and north	Deforestation for farmland, fuel wood and building poles
Erodible soils: deep, clay-rich, often acid regolith	Overgrazing, which leaves soil bare and trampled
Thin vegetation in the drier east during the dry season	Poor farming methods: ploughing up and down the slope, burning crop residues, no fallow
	Roads, mining and quarrying that concentrate run-off and leave bare ground
	Population pressure: land shortage pushes farming onto fragile slopes (REMA, 2009)

Worked example 1: predicting soil loss with the Universal Soil Loss Equation

Soil scientists estimate average annual soil loss $A$ (tonnes per hectare per year) as the product of five factors:

A = R \times K \times LS \times C \times P

where $R$ is rainfall erosivity, $K$ soil erodibility, $L S$ slope length and steepness, $C$ cover and management, and $P$ support practice (1 means none). Illustrative values for a bean field on a steep Rwandan hillside: $R = 500,\ K = 0.2,\ LS = 6,\ C = 0.3,\ P = 1$ .

A = 500 \times 0.2 \times 6 \times 0.3 \times 1 = 180\ \mathrm{t\,ha^{-1}\,yr^{-1}}

With well-built bench terraces, suppose $P = 0.1$ : $A = 18\ \mathrm{t\,ha^{-1}\,yr^{-1}}$ , a tenfold cut. The equation shows which factors people can change: $C$ (cover) and $P$ (practices), not $R$ .

Worked example 2: how fast is the topsoil disappearing?

Problem. A 1986 national agricultural survey estimated that 10 tonnes of soil per hectare of arable land were carried away each year (REMA, 2009). If topsoil has a bulk density of 1.3 t per m³, what depth is lost each year, and how long would a 20 cm topsoil last?

Solution. One hectare is 10,000 m².

V = \frac{10\ \text{t}}{1.3\ \mathrm{t\,m^{-3}}} = 7.69\ \text{m}^3, \qquad d = \frac{7.69\ \text{m}^3}{10\,000\ \text{m}^2} = 0.000769\ \text{m} = 0.77\ \text{mm}

t = \frac{200\ \text{mm}}{0.77\ \mathrm{mm\,yr^{-1}}} \approx 260\ \text{years}

Less than a millimetre a year sounds harmless, yet it is the most fertile layer, and on steep, unprotected fields losses are many times the national average.

Effects

On the land and people	On water and the wider environment
Loss of fertile topsoil and nutrients; lower yields. REMA estimated that soil erosion meant about 40,000 people a year could not be fed (REMA, 2009, citing Government of Rwanda, 2004)	Siltation of rivers, lakes, marshes and reservoirs: the Nyabarongo carried 51 kg/s of soil at Kigali (REMA, 2009)
Gullies cut up fields and destroy paths, roads and houses	Nutrients washed into lakes feed water weeds and algae (nutrient loading)
Farmers abandon exhausted land or need more fertiliser	Muddier water costs more to treat for drinking
Food insecurity and poverty in the worst-affected regions, such as the Congo–Nile Ridge (REMA, 2009)	Floods, as run-off reaches rivers faster

Warning

Do not confuse erosion with weathering or with leaching. Weathering breaks rock in place; leaching washes nutrients down through the soil; erosion carries the soil itself away.

North Carolina 4-H: "Rill and Gully Formation". What happens to bare soil in a rainstorm, from splash to rills and gullies. Watch where the run-off concentrates.

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Practice

Distinguish sheet, rill and gully erosion.
Using Figure 3, describe the distribution of soil erosion risk in Rwanda.
Explain two natural and three human causes of soil erosion in Rwanda.
Kayonza District has 62,291 ha exposed to erosion risk out of 135,105 ha (RWB, 2021). Calculate the percentage.
In the USLE, a field has R = 400, K = 0.25, LS = 4, C = 0.2, P = 0.5. Find A. Which factor would you change first, and how?
If 25 t of soil per hectare were lost in a year from soil of bulk density 1.25 t per m³, what depth is that?
"Soil erosion in Rwanda is a human problem more than a natural one." Discuss.

Show answersHide answers

Sheet: an even thin layer removed by a film of run-off. Rill: small channels a few cm deep. Gully: deep trenches too big to plough out.
Highest in the west and centre, on the steep Congo–Nile Ridge and central plateau (Ngororero 83%, Muhanga 82%, Karongi and Gakenke 71%); lowest in the gentle north-east and east (Nyagatare 8%, Gatsibo 14%) and in Rusizi (17%).
Natural: steep slopes; heavy, intense rain. Human: cultivating steep slopes without terraces; deforestation; overgrazing (also poor ploughing, roads and mining).
$\frac{62\,291}{135\,105} \times 100 = 46.1\%$ .
$A = 400 \times 0.25 \times 4 \times 0.2 \times 0.5 = 40\ \mathrm{t\,ha^{-1}\,yr^{-1}}$ . Change $P$ (build terraces or contour bunds) or $C$ (mulch, cover crops, agroforestry); $R$ cannot be changed.
$V = \frac{25}{1.25} = 20\ \text{m}^3$ ; $d = \frac{20}{10\,000} = 0.002\ \text{m} = 2\ \text{mm}$ .
Natural factors (slopes, rain, soils) set the risk; human actions (deforestation, cultivating steep slopes without protection, overgrazing) turn risk into actual erosion. The contrast between protected and unprotected fields on the same slope shows the human role; but the map of risk follows relief, showing the natural base. A balanced answer gives both.

Quick check

Which district had the highest share of its area exposed to erosion risk in the RWB survey (2021)?

Key points

About 46% of Rwanda's area was exposed to erosion risk in 2021, from 83% of Ngororero to 8% of Nyagatare (RWB, in REMA 2021).
Types: splash, sheet, rill, gully, mass movement; wind erosion is minor.
Natural causes (slopes, intense rain, erodible soils) are made worse by human ones (steep-slope farming, deforestation, overgrazing, roads, mining).
Effects: lost topsoil and yields, gullies, siltation, nutrient loading and floods. USLE: $A = R\,K\,LS\,C\,P$ .

Sources and credits

Sources: Rwanda State of Environment and Outlook Report 2021, REMA (RWB erosion risk survey, Table 5-1; plot survey; slopes); Rwanda State of Environment and Outlook Report, REMA, 2009 (1986 soil loss estimate, food security, siltation, steep-slope farming).

Figure 3 was drawn by Ganzaa from the RWB data (boundaries: geoBoundaries, CC BY 4.0; Natural Earth). Figure 4, the worked examples (USLE values illustrative) and the exercises were written by Ganzaa.

Photo: Mukungwa river passing under a bridge, Mrbobax, CC BY-SA 4.0, via Wikimedia Commons.

Video: "Rill and Gully Formation", North Carolina 4-H, embedded from YouTube.

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

1Where the soil is most at risk

2Types of soil erosion

3Causes

4Effects

Where the soil is most at risk

Types of soil erosion

Causes

Effects