Environmental Engineering Reference
In-Depth Information
Figure 27.10
The main factors
affecting types of soil erosion by water.
Source: After Cooke and Doornkamp (1990).
illustrates the factors affecting the types of soil erosion by water. The volumes of soil lost
are difficult to estimate, especially for wind-borne material, but studies suggest values of
up to 300 tonnes per hectare in the Ethiopian Highlands, where rainfall erosivity is high,
compared with less than five tonnes on grazing land and less than one tonne in forested
areas. Although these figures may sound severe, they need to be balanced against the
rates of soil formation, as it is the net loss which is of greatest significance. Even then
interpretation is not straightforward, as the erosion may take place in narrow channels
which can rapidly expand, whereas soil formation will take place slowly over the whole
catchment.
Estimates of the extent of soil degradation in susceptible dry lands are shown in Table
27.2. We can see that Africa and Asia have the largest areas affected by moderate or
severe degradation. Interestingly, the relatively small area of dry land in Europe (largely
in the Mediterranean
Table 27.2 Soil degradation in susceptible dry
lands, by process and continent, excluding
degradation in the light
category (million ha)
Process
Africa
Asia
Australia
Europe
North
America
South
America
Total
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
