Geoscience Reference
In-Depth Information
Desert climate with sands or gravel (ergs, regs) that can locally
be black, ferruginous in nature and represent the residues of
cuirasses; this is the Sahara.
Dry, Sahelian climate (
P
< 500 mm) corresponding to large,
gently sloping, very slightly concave catchments:
glacis
. The soils
are arranged in toposequences an example of which will be
discussed in Chapter 6, § 6.6.
Sudanian climate with a marked dry season; we see tabular
reliefs linked to ferruginous cuirasses (in the north) or stone
lines (in the south) with kaolinite-rich saprolites below. This
corresponds to the typical soil (Fig. 5.1).
Perhumid Guinean climate: hemispherical relief with
non-indurated Ferralsols; Figure 5.8 presented later shows the
corresponding typical landscape.
The hydrological balance is as follows (Table 5.6).
Table 5.6
Hydrological balance measured in catchments; the values have been
rounded off (Tardy 1993a).
Environment
T
P
E
D
mm y
-1
(°C)
Arid (Faya Largeau)
28
20
20
0
Sub-arid (Ouagadougou)
28
800
785
15
Humid tropical (Korhogo)
26
1500
850
650
Equatorial (Guyane)
26
3750
1450
2300
P is annual precipitation (rainfall), D drainage to the outlet and E the difference, that is, actual
evapotranspiration.
The annual rainfall at Ouagadougou is close to that in Paris. But,
because of the higher temperature, the water demand of the atmosphere
(potential evapotranspiration) is very high and reaches 1900 mm y
-1
.
The soil therefore gives to the atmosphere what it can (E = 785 mm =
actual evapotranspiration), that is 40 per cent of the demand. The climate
is thus very dry. However there is slight downward drainage (15 mm,
col. D) which occurs sporadically when there is a heavy rain. This is
insufficient to lead to lessivage (leaching) of the soils. On the other hand,
large quantities of water pass through the soils of the equatorial zone,
resulting in easy removal of bases and silica from these environments.
Consequently, Ferralsols are located in the regions indicated in
Figure 5.7.
