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Die Wirksamkeit von Boden

Die Wirksamkeit von Boden

Study area and general

Study area and general methodology Figure 3.4 Geological map of North and South Wello zones (Tefera et al. 1996) The other formations such as quaternary sediments, Antalo, Abay, Tarmaber- Gussa and Alkali granite and syenite formations cover about 4% (Tefera et al. 1996). Ashangi formations are deeply weathered alkaline and transitional basalt within rare intercalations. Tarmaber-Megezez formations are transitional and alkaline basalt, while the Alajae formation mainly consists of aphyric flood basalts associated with rhyolites (ignimbrites) and subordinate trachytes. The Aiba basalts are pulse of fissural basalt volcanism, which are generally aphyric, compact rocks, in place showing stratification and containing rare inter-bedded basic tuffs. The Aiba basalts unconformably overlay the Ashangi formation, which is as thick as 200 to 600 m. The Amba-Aradom formation consists of sandstone, shale and marls that conformably overlies the Antalo formation (Tefera et al. 1996). Soils Wello has various soil types, the major soils being Leptosols, Cambisols, Vertisols, Andosols and Luvisols (FAO, 1984). Leptosols are the dominant soil types, which cover 26

Study area and general methodology over 35% of the study area and followed by Cambisols, Vertisols, Andosols and Luvisols (FAO, 1984). Leptosols are shallow, mostly brown or yellowish brown and clay to clay loam. Leptosols are characterized by common to abundant surface stoniness, few to common rock outcropping and many coarse fragments throughout the profiles. At the second level, the Leptosols are further classified into Eutric, Dystric and Lithic. Cambisols are further classified as Vertic and Dystric. Cambisols have wide physical, chemical and morphological characteristics and are shallow to moderately deep. Vertisols in the study area are moderately deep to very deep on flat to almost flat lands. They are characterized by dark to dark gray color, clayey texture, poor drainage and workability, and develop wide and deep cracks upon drying and swell upon wetting. The Vertisols of Wello are further classified into Euthric and Calcic. Luvisols occur in cooler areas in very small parts and on gently sloping lands. They are well drained, brownish in color, clayey textured, have high CEC (>24 cmol/kg clay) and higher base saturation, mostly >50%. Andosols cover small parts of the study area in the Rift Valley and the escarpments towards the valley. They are shallow to very deep, clayey textured and have a high pH (pH >7) (FAO, 1984). Hydrology The study area drains to three major river basins of the country and to a river that sinks within the study area. The rivers are Abay (Blue Nile), Tekeze, Awash and Golina (Figure 3.5). Tekeze River joins the Blue Nile at Atbara in Sudan. The Tekeze and Golina drain the northwest and northeast parts, respectively, while the other parts are drained by the Abay and Awash rivers. The Abay, Tekeze, Awash and Golina rivers drain 17550 km 2 , 5265 km 2 , 4385 km 2 and 2770 km 2 , respectively. The concentrated drainage network and the closely spaced watershed divides indicate dissected topographic characteristics and possibility of intensified differential erosion. The intense drainage systems and steep slopes also indicate vulnerability of the area to erosion and degradation, if there is improper anthropogenic intervention. 27

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