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Die Wirksamkeit von Boden

Die Wirksamkeit von Boden

Literature review 2.7

Literature review 2.7 Soil erosion and its consequences Soil erosion is both natural (geological) and through human induced processes caused by two agents, water and wind. Geological erosion occurs under natural conditions, where the soil loss is positively balanced by soil formation that indicates net gain (El- Swaify 1997; Kaihura et al.1999). However, soil loss due to accelerated erosion is very much higher than the soil gain through parent material weathering. Water erosion is a common phenomenon in humid environments but is increasingly an issue in semi-arid regions due to the increased incidence of intense storms. Wind erosion predominantly occurs in arid and semi-arid areas (Dregne 1990; Nana-Sinkam 1995). Erosion is a major challenge particularly in SSA countries (Nana-Sinkam 1995). Ethiopia is one of the SSA countries most severely affected by the problem, and water erosion is prominent. Water erosion mainly occurs in the highlands, which have erratic rainfall generating erosive runoff (Hurni 1993). Various studies provided empirical evidence of the severity of the problem. For example, the Ethiopian highland reclamation study (EHRS) estimated 1.9 billion tons annual topsoil loss from the highlands due to water erosion, which is equivalent to 8 mm soil depth or 130 t ha -1 annual losses. The study also indicated that out of the total highlands, 50% was significantly eroded, 25% was seriously eroded, and 4% had reached a point of no economic use (FAO, 1986). Hurni (1993) also reported as much as 300 t ha -1 annual soil loss from croplands with average rates of 42 t ha -1 . Similarly, Herweg and Ludi (1999) estimated a higher than 110 t ha -1 annual soil loss from farmlands without terraces. On the other hand, the annual soil formation of the Ethiopia highlands is estimated to be between 2 and 22 t ha -1 and varies with geologic and climatic conditions, topographical setup and agricultural practices (Hurni 1983). Soil erosion varies with soil types (erodibility) and erosive factors like slope of the land (length and steepness), rainfall characteristics (volume, intensity and duration), soil cover and land management (Prasannakumar et al. 2012). Among the soil types, Luvisols and Nitosols were found to be most vulnerable to water erosion, while Vertisols and Phaeozems were less vulnerable (Herweg and Ludi 1999). The same study indicated that rainfall erosivity and very high erosion rates were observed in high rainfall areas. This is in line with the estimation by Prasannakumar et al. (2012) using a universal soil loss model. This indicates that in the Ethiopian highlands, soil formation is much lower than the 16

Literature review erosion rate. Due to erosion, farmlands in many parts of the highlands have shallow soil depths and poor fertility (Shiferaw and Holden 1999; Ciampalini et al. 2008). The traditional agricultural practices and inappropriate land use have aggravated the erosion processes (Tamene et al. 2006; Ciampalini et al. 2008; Nyssen et al. 2009). The impact of soil erosion is complex leading to reduction in soil depth and moisture storage capacity together with soil-nutrient losses, and ultimately results in reduced agricultural production and productivity (Vancampenhout et al. 2006). Soil erosion is a threat not only to agriculture but also to the economy, as the country´s economy depends on agriculture. 2.8 Practices and implications of soil and water conservation in Ethiopia People were already aware of the negative consequences of soil erosion on agricultural production and the environment centuries ago. As a result, soil and water conservation practices exist as indigenous knowledge in some areas of Ethiopia (Nyssen et al. 2007; Watson and Currey 2009). For instance, the Konso people in southern Ethiopia are known for traditionally well developed terraces, where the terrace practices are registered by the United Nations Educational, Scientific and Cultural Organization (UNESCO) as a world heritage. The Konso terraces are estimated to be older than 400 years. Some rudimentary and poorly established terraces and lynchets depicted on older aerial photographs and physical remnants can also be observed in different parts of the northern highlands. For example, Nyssen et al. (2007) reported old lynchets in the Tigray region (northern Ethiopia). This is an indication of indigenous knowledge on SWC practices, and terracing is not only limited to the Konso area but is also found in other parts of the country. However, the SWC in Ethiopia covered very few areas and most of them, except those in Konso, have limitations in layout and construction quality (Nyssen et al. 2007; Watson and Currey 2009). As the government realized the problem of land degradation, it took policy actions. In this regard, a forest and wildlife conservation and development policy was declared in 1980 (Anonymous 1980). Following this policy, the government initiated various studies and capacity-building programs and massive SWC interventions (Herweg and Ludi 1999; Shiferaw and Holden 1999; Tekle 1999). The capacitybuilding programs involved training of professionals at the national level and farmers 17

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