Tropical deserts and semi-arid environments - Fundamentals of the Physical Environment - page 674

EROSIVITY FACTORS ERODIBILITY FACTORS

RAINFALL FACTORS
drop size, velocity,
distribution, angle and
direction,
rain intensity, frequency,
duration

SOIL PROPERTIES
particle size, clod-forming
properties, cohesiveness,
aggregates, infiltration
capacity

Rainfall

low INTENSITY high

VEGETATION
ground cover, vegetation
type, degree of protection

Splash
erosion

Infiltration Crusting

TOPOGRAPHY
slope inclination and length,
surface roughness, flow
convergence or divergence

RUN-OFF FACTORS
supply rate, flow depth,
velocity, frequency,
magnitude, duration,
sediment content

LAND USE PRACTICES
e.g. contour ploughing,
gully stabilization,
rotations, cover cropping,
terracing, mulching,
organic content

Run-off

Sheet erosion
(unconcentrated flow)

Rill or gully erosion
(concentrated flow)

Figure 26.10 The main factors affecting types of soil erosion by water.
Source: After Cooke and Doornkamp (1990)

Table 26.2 Soil degradation in susceptible dry lands, by process and continent, excluding degradation in the light
category (M ha)

Process Africa Asia Australia Europe North South Total

America America

Water 90·6 107·9 2·1 41·7 28·1 21·9 292·3

Wind 81·8 72·7 0·1 37·3 35·2 8·1 235·2

Chemical 16·3 28·0 0·6 2·6 1·9 6·9 66·3

Physical 12·7 5·2 1·0 4·4 0·8 0·4 23·9

Total 201·4 213·8 3·8 86·0 66·0 37·3 617·7

Area of susceptible dry land 1286·0 1671·8 663·3 299·7 732·4 516·0 5169.2

% degraded 15·6 12·8 0·6 28·6 9·0 7·2 11·9

Source: After Thomas and Middleton (1994).

area of dry land in Europe (largely in the Mediterranean
basin) has a high proportion degraded, whilst the arid
continent of Australia with its low density of population
and generally low slopes has a very small proportion
degraded.

the soil surface; (2) soil management techniques which
improve soil structure; (3) mechanical methods which
modify surface topography to control wind and water
movement. Where properly conducted, such techniques
can prevent soil erosion from susceptible areas or can be
used to help restore damaged areas if the erosion is not
too severe. What must not be forgotten is why the problem
developed in the first place. Normally, even in dry lands,
the vegetation cover is of sufficient density to prevent wind
and soil erosion. It can become a problem when human

Remedies
There are a number of ways in which cultivated soils can
be protected from erosion. These can be subdivided into
three groups: (1) agronomic measures which protect