Cameroon

Cameroon

Check the article published in Hydrological Processes summarizing the M-TROPICS activity in Cameroon here

The Nyong River basin

The studied system is the Nyong River Basin. The network integrates complementary spatial scales: (1) at the local scale, a Small Experimental Watershed (SEW) is devoted to process-based studies; (2) at the regional scale, nested watersheds from 200 to 20,000 km2 are devoted to the quantification of fluxes of dissolved and particulate matter.

Climate

The season cycle in Cameroon is essentially governed by the annual shifts of the intertropical convergence zone (ITCZ). In the Nyong basin, the climate is equatorial. For the Southern Cameroonian plateau, it is called four season-guinean climate; the short (March to May) and the long (September to November) rainy seasons are separated by a short dry season, whereas the long dry season goes from December to February. The annual rainfalls are between 1500 and 2000 mm, for a mean annual temperature of about 25°C with amplitude of 2.4°C.

Interannual isohyets (mm) over the Nyong watershed

Geology

The substratum of the Nyong basin is constituted by two main rocks: (1) In the South, the Ntem group representing the northern margin of the Congo craton, which is considered stable since about 2000 Myr; The plutonic rocks are essentially granitoids; (2) In the North, the mobile zone, strongly affected by the Pan-african orogenesis (600-500 Myr), where two facies have been distinguished (a) In the North, the Yaounde series, constituted by gneisses and garnetiferous migmatites; (b) At the contact with the craton, the Ayos-Mbalmayo-Bengbis series, mainly characterized by chlorite schists and quartzites.

Geology map of the Nyong watershed

Hydrology

Cameroon can be divided into four areas, corresponding from North to South to the basins of Tchad, of Niger, to the Atlantic basins (Sanaga, Nyong, Ntem…) and to the Congo basin. Typically, the mean discharge of the low water level month (February or March) is 7 to 9% of the discharge corresponding to the high flow month (November).

Relief

The upper Nyong basin belongs to a morphological unit called “the Southern Cameroonian Plateau”. This is a vast smoothly undulating surface, with elevation between 650 and 850 m; the characteristic landscape, exhibiting interfluves and depressions, corresponds to a more or less eroded “half-orange” original morphology. Some inselbergs and crystalline blocks (with highest point at 1295 m) dominate the surrounding plateau.

Oro-hydrography of the Nyong watershed

Soils

The Nyong basin is located in the lateritic domain. Soils are essentially ferrallitic: orthic modal soils on the South Cameroonian plateau with hydromorphic zones in the upper Nyong valley; yellow topomorphic soils in the coastal area.

Vegetal cover

The Yaoundé-Sangmélima-Abong Mbang triangle belongs to the domain of the semi-deciduous congo-guinean forest, characterized by Stertuliaceae and Ulmaceae; this area is surrounded by the grass and shrub savannah in the North, the Atlantic permanent forest in the West and South, and the Congolese forest in the East. The most characteristic species of the congo-guinean forest are Lophira alata and Pycnantus angolensis, or Gibertiodendron dewervei and Bailonella toxisperma near the Dja reserve; the swampy depressions are often colonized by Raphia cf montbuttorum. The marshy forest with Stertulia subviolacea present in the upper 80 km of the Nyong course progressively changes towards downstream into aquatic grasslands with Echinochloa pyramidalis.

Anthropization

The upper Nyong basin appears as a rather well-preserved area: (a) With a 5-10 inh/km² density (apart from Yaoundé and its suburbs), the demographic pressure is low; (b) This is not a breeding zone; (c) The studied area is not submitted to an intensive agriculture and no fertilizer and pesticide are used; (d) No important sand, gravel or mining resource exploitation is found on the area.

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