What is soil? Buckman and Brady (1969):Soil is a dynamic natural body on the surface of the earth in which plants grow, composed of mineral and organic materials and living forms.
What is soil erosion? Christine and Josef (2007) defined soil erosion as the wearing away of the land surface by physical forces such as rainfall, flowing water, wind, ice, temperature change, gravity or other natural or anthropogenic agents that abrade, detach and remove soil or geological material from one point on the earth’s surface to be deposited elsewhere.
Soil erosion in Ethiopia
In Ethiopia, the productivity of the agricultural sector of the economy, which supports about 85% of the workforce, is being seriously affected by soil productivity loss due to erosion and unsustainable land management practices. The average crop yield from a piece of land in Ethiopia is very low according to international standards mainly due to soil fertility decline associated with removal of topsoil by erosion (Sertsu, 2000). Hurni (1990, 1993) estimated that soil loss due to erosion of cultivated fields in Ethiopia amounts to about 42 t ha−1 year1.
According to an estimate by FAO (1986), some 50% of the highlands of Ethiopia were already “significantly eroded” in the mid-1980s and erosion was causing a decline in land productivity at the rate of 2.2% per year. The study also predicted that by the year 2010, erosion could reduce per capita incomes of the highland population by about 30%. Taddese (2001) indicated that Ethiopia loses over 1.5 billion tons of soil each year from the highlands by erosion resulting in the reduction of about 1.
5 million tons of grain from the country’s annual harvest.
Few studies have estimated soil loss rates at local, regional or national levels in Ethiopia. Studies in the northern Ethiopian highlands (Nyssen et al., 2005) and the Central Rift Valley of Ethiopia (Meshesha et al., 2014) reported larger erosive power of rainfall as compared with elsewhere in the world. Niang et al. (2014) indicate a likely increase in precipitation and extreme precipitation events throughout the 21st century, particularly in the highlands. The considerable effects of land use on soil loss were reported by Taye et al. (2013), with soil loss rates of 38.7 t ha-1 yr-1from rangeland as compared with 7.2 t ha-1 yr-1from cropland. They attributed the higher soil loss in rangeland to increased runoff resulting from intensive grazing and soil compaction, whereas soil tillage supports infiltration and causes less runoff and soil loss in croplands. There is also strong variation in soil loss rates within certain land use types located in the same drainage basin, implying the heterogeneous nature and effects of other environmental factors, for which no data are available.
Studies that aim at a better understanding of the extent, causes, and impacts of soil erosion, as well as the soil water conservation practices implemented through the various initiatives, are fragmented. In addition, comprehensive studies that evaluate past activities and draw lessons from experiences with the aim of aiding future development both at the regional and national levels are scarce. Osman and Sauerborn (2001) is the latest review study that can be found.
In the northwestern and southwestern parts of Ethiopia, where drought risks are low and the productivity of soils is relatively high (Hurni, 1988; Nyssen et al., 2004b), limited attention has been given to both soil water conservation development and research. There is clear evidence, however, of active erosion in regions other than the already highly degraded areas of northeastern Ethiopia. Furthermore, most of the research conducted to date has focused on sheet and rill erosion and gully erosion to a lesser extent. Sheet and rill erosion throughout the country, gullying in the highlands, and wind erosion in the Rift Valley and the peripheral lowlands have been identified as the most important present-day geomorphic processes in Ethiopia (Nyssen et al., 2004b). More rarely reported forms of erosion include tillage erosion (Nyssen et al., 2000b, 2007) and landslides (Abebe et al., 2010; Broothaerts et al., 2012).
Very few estimates are available about the overall soil loss rates at regional or national scale. FAO (1986) estimated of gross annual soil loss nationwide of 1.9*109 t, of which 80% originates from croplands. Hurni (1988) estimated a nationwide annual gross soil loss of 1.5*109 t extrapolating data obtained from six SCRP(soil conservation project) research stations in which the highest loss is from croplands (42 t ha-1 yr-1). Sonneveld et al. (2011) provided a tentative nationwide mean annual soil loss map combining the results of different model estimates. They stated that soil loss varies remarkably from 0 t ha-1 yr-1in the eastern and southeastern parts of Ethiopia to more than 100 t ha-1 yr-1in the northwestern part of the country, but they did not note what causes this huge spatial variation. Several interacting factors are responsible for the high erosion and land use dynamics in Ethiopia. Bard et al.(2000) studied the environmental history of north Ethiopia with particular reference to the pre-Aksumite to post-Aksumite period (500 BC–900 AD) and concluded that during this period the region underwent effects linked to demographic development, climate change, and changes in vegetation cover, all of which contributed to soil erosion.
Studies in north Ethiopia show lower gully erosion rates, and the rates have been decreasing in recent years. For example, Nyssen et al. (2006) reported an average gully erosion rate of 6.2 t ha-1 yr-1 over a 50-year time span, and this rate has decreased to 1.1 t ha-1yr-1 since 1995. Frankl et al. (2012) reported medium- to long-term (1–47 years) head-cut retreat rates to be 10 times higher than short-term ones on the basis of their analysis of archival terrestrial and aerial photographs. Frankl et al. (2011) assessed a region-wide change in gully and river channel morphology in Tigray over the period 1868–2009, using historical photographs and integrating some gully cross-section measurements from 2006 to 2009.Overall, most of the gully erosion studies conducted in Ethiopia are based on the interpretation of relatively low-resolution aerial photographs or in combination with satellite images and repeated photographs supplemented by data obtained from questionnaires.
Africa generally has a poor spatial distribution of hydrometric stations, and the few available ones are compromised by a short length of records, data discontinuities, and poor data quality (Vanmaercke et al., 2014). Similarly, data on sediment yield, defined as the total sediment outflow from a catchment measurable at a point of a reference and for a specified period of time, are rarely available for Ethiopian rivers ( Haregeweyn et al. 2015a). The few available studies rather show significant sediment yield variability and high associated impacts. An analysis of sediment discharge data for 10 medium-sized catchments (121–4590 km2 drainage basin size) in sub-basins of the Giba River, located in Tigray, revealed sediment yield values ranging from 5 to 66 t ha-1 yr-1(Vanmaercke et al., 2010).
Reservoir sediment surveys conducted for 14 micro-dam watersheds in northern Ethiopia show a large spatial variation among the catchments, ranging from 2 to 19 t1 yr1, which also ranged in drainage basin size from 0.71 to 67 km2 (Haregeweyn, 2006; Haregeweyn et al., 2012a). Such high sediment yield values have drastic consequences for the life expectancy of many reservoirs in the Ethiopian highlands (Haregeweyn et al., 2006). The poor availability and reliability of sediment yield data for Ethiopian rivers remains a major bottleneck for a better understanding of the dynamics and drivers of sediment yield variability in the region (Haregeweyn et al., 2015a). Sonneveld et al. (2011) also stressed that the application of better soil erosion models in Ethiopia is restricted by data paucity. On the other hand, sediment yield analyses made at regional scale, relying heavily on soil loss rates measured at plot scale, could lead to wrong conclusions because of the strong dependence of erosion process rates on spatial scale (de Vente and Poesen, 2005; Nyssen et al., 2008b). Moreover, despite the significant contributions of gully erosion to the overall sediment budget (Haregeweyn et al., 2013; Nyssen et al., 2004b; Poesen et al., 2003), previous studies have not accounted for this process so far.
Overall, information on the sediment budget – that is, a detailed account of the sources and deposition of sediments as it travels from its point of origin to its eventual exit from a drainage basin (Reid and Dunne, 1996) – is missing. This requires the development of long-term and representative soil loss and related databases. Haregeweyn et al. (2015a) emphasized the need for the maintenance and monitoring of existing hydrometric stations in addition to the establishment of new ones, which are needed to reflect the different eco-hydrological environments in Ethiopia.
Conclusion and recommendation
Looking at the studies which are conducted by different researchers at different time the following conclusion are given to summarize this term paper. From the above studies, we can understand that the soil loss estimation at national level is still tentative and inconsistent so long term, permanent and large scaled study should be conducted.
The application of better soil erosion models in Ethiopia is restricted by data scarcity so soil erosion research that covers on wide range of environmental condition and land use with standardized methodology and research method should be conducted. On the other hand, sediment yield analyses made at regional scale, relying heavily on soil loss rates measured at plot scale, could lead to wrong conclusions and this may lead to omitting of adoption soil and water conservation practices to reduce soil erosion and reduce productivity problems. The above studies also indicate the absence of long term, representative soil loss and related databases. Absence of long term and comprehensive soil erosion database can hinder researchers to get long term data and also enforces researchers to use models developed elsewhere due to lack of enough information to develop prediction model related to Ethiopia conditions.so to develop model for predicting erosion and control technology we have to collect long term and accurate data then this will help us to quantify and measure soil erosion and predict the possible conservation measure.
Poor spatial distribution of hydrometric stations, short length of data records, data discontinuities, and poor data qualities are also the main hindrance for conducting accurate research in Ethiopia as well as in Africa.so to get long term and high quality data continuous data should be taken for long period of time, and enough stations which help for the collection of erosion data should be distributed with standard equipment on the area which are needed. Even indigenous soil and water conservations practice where practiced in Ethiopia the institutionalized soil and water conservation practice is started after 1970s and this was also fragmented. Here continues, and on time, soil and water conservation practices should be practiced to prevent soil loss in all corners of our country.
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