Hydrological modeling of the Congo River basin: Asoil-water balance approach

2.5 Soils

Generally, there are two types of soils in the study area: those of the equatorial areas and those of the drier savanna (grassland) regions. The equatorial soils occur in the warm, humid lowlands of the central basin, which receive abundant rainfall throughout the year and are covered mainly with thick forests. This soil is almost fixed in place because of the lack of erosive forces in the forests. In the shore areas, however, swamp vegetation has built up a remarkably thick soil that is constantly nourished by humus, the organic material resulting from the decomposition of plant or animal matter. Although in the savanna regions the soils are constantly endangered by erosion, the river valleys contain rich and fertile alluvial soils. Special note should be made of the highlands of eastern Congo in the Great Lakes region, which are partly covered with volcanic lava that has been transformed into exceptionally rich soil.

An agronomic soil map at 10X10 spatial resolutions (Figure 1 1) is available at FAO-UNESCO database (1984). This map can be resampled and reclassified according to the objectives of the research. In Hydrological modelling, a textural Soil map is required to derive soil retention properties such as field capacity, wilting point, available water content, etc. For the southern Africa, Alemaw and Chaoka (2003) demonstrate the usefulness of the agronomic soil data in deriving soil texture classes for hydrological modelling.

2.6 Land cover/use and Population density

The land cover pattern of the Congo basin is viewed in Figure9 and summarised in Table 3. Mostly, the basin is coverd by green and dense forest. Dryer regions occupy less then 0.2 % of the basin. Figure 10 below shows the population density distribution over the basin.

Figure 8 Congo Basin Agronomic Soils Map. The polygon limit the Congo watershed
Table 3 Land cover distribution in the Congo River basin (after World river resources, 2003)

Percent Forest cover	44.0
Percent Grassland, Savanna and Shrubland	45.4
Percent Croplands	7.2
Percent Irrigated Cropland	0.0
Percent Dryland Area	0.2
Percent Urban and Industrial Area	0.2

Figure 9 Vegetation and Land cover and uses over the Congo River Watershed (after World river resources, 2003)

Figure 10 Population density distribution over the Congo River Watershed: Basin area 3,730,881 sq.Km, Average Population Density (people per sq.km): 15, Number of large cities (100,000 people).

2.7 Geology

The Geology of the Democratic Republic of Congo is characterized by two large structural units separated by discordance and/or a significant gap: The Formations of covers (Phanerozoic), not metamorphosed, generally fossiliferous, and of age ranging between the Upper carboniferous and the Holocene; and the Basement terrains (Precambrian shield): Highly metamorphosed + shielded contouring continuously the basin.

2.7.1 Basement formation

The Basement terrains are subdivided in "tectostratigraphic" units:

a) Archean shields of age equal to or higher than 2500 MA levelling to the northern Congo and Kasaï;

b) Lower and middle Precambrian Belts (2.500 to 1.300 MA) whose sediments settled in meridian mobile zones located on the Eastern and Western edges of the Craton and in the intra - cratonic valleys;

c) Upper Precambrian called Katangien whose sediments settled on the epicontinental platforms and in the «subsiding surfaces» of the craton of Congo (Katanga folded and tabular).

2.7.2 Surface formations

The surface formations are grouped into four zones as follows:

(i) A littoral zone, ranging between the Atlantic Ocean and the Mayumbe mounts (Crystal Mounts);marine formations tertiary and Cretacic age are well developed there;

(ii) The central basin where the deposits of Mezoic and Cenozoic ages spread out; vast terrains level on the circumference of the Basin;

(iii) The edge of old grounds subdivided in six unconnected areas ;

(iv) The tectonic valley of the East of Congo occupied by particular Cenozoic formations and characterized by recent volcanic activities.

The formations of each one of these 4 great zones are covered indifferently by recent formations, the ochre series of sands and the series of the polymorphic sandstones.