As discussed earlier, the assessment of the extent of land degradation and its causes in Ethiopia in general remain largely theoretical and qualitative with inadequate efforts to quantify it. In fact the measurement of land degradation is an imprecise and value-laden activity and there are competing perceptions of what degradation is. For example, a forest conservator may view the scraping away of natural forests in mining areas (Laga Dambi in Borana and Youbdo in East Wellega) as degradation, while the mining authority may view the tree as a barrier to mining operations and consider its removal necessary.
Some of the more direct and intermediate impacts of various forms of degradation were mentioned in earlier sections. The immediate impact of degradation is on soil productivity leading to impacts on people's welfare. Soil degradation through erosion, nutrient loss and other processes results in undesirable physico-chemical soil properties and thereby considerably depresses crop yields. The most important factors reducing soil productivity by soil degradation are reduced soil depth (reduced root depth) and soil water storage capacity, and loss of nutrients. The reduction in soil depth can depress crop yields by reducing the amount of water that the soil can hold. Thin soils are unable to retain as much water as thick soils, and therefore exhibit lower crop yields. The effect of soil depth on crop yield is particularly pronounced during periods of drought.
Based on the findings of the Soil Conservation Research Project sites in different parts of Ethiopia, it is estimated that about 1500 million t of soil was lost from the highlands of Ethiopia every year, about 50% of the rural population were affected to some degree, and 1–2% of the country's agricultural production was lost (Hurni 1998). Adugna et al. (1996) reported a 72% yield reduction of barley in west Hararghe as a result of soil erosion over a 15-year period. Annual productivity losses on croplands in the Ethiopia highlands due to erosion are estimated to be from 0.12–2% (Kappel 1996). Since Oromiya region constitutes a greater proportion of the Ethiopian highlands, these figures would be applicable. The incidence of crop failure is repeatedly reported in areas like Hararghe, Wellega, Arsi, Bale, East Shewa and North Shewa, which have been suffering from high rates of soil erosion. It has been found that agricultural soils around the lakes in the Rift Valley are being affected by salt and aggravated with an increasing moisture deficit, leading to reduced crop productivity (Fisseha 1998).
A study indicated that on red and volcanic soils, negative impacts on crop yield commence when soil depths are reduced to 80–95 cm and that crop failures occur when soil depths are reduced to 30–45 cm (Hurni 1985). An analysis of the projected impact of soil erosion on agricultural production in Ethiopia for the period 1985–95 indicated that the annual financial costs of grain and livestock production foregone due to top soil erosion and nutrient breaches would amount to about 3.5% of the 1985 annual GDP (Sutcliffe 1997).
Reduced grazing resources and quality and productivity of grasslands and the loss of nutritious and palatable plant and grass species due to deforestation have all contributed to reduced livestock productivity. Feed shortages are a major reason for many small-holders to not keep livestock any more. Many smallholders cannot afford to maintain draught oxen for land preparation and so they rent or borrow them. However, such means of acquisition usually takes place at less than optimal ploughing times (as owners generally prefer to prepare their land before opting to sell draught power). Consequently, it is claimed that less than optimal crop productivity is achieved by farms without oxen (Cabal 2000). In Ginchi watershed in west Shewa, feed shortages compel farmers to reduce herd size and cows are usually given up, as oxen ownership for crop production remains a priority. The result is, among other things, reduced consumption of milk and milk products in the community. Similar situations may prevail in other communities.
Land degradation not only impacts crop and livestock productivity but also water resources on which welfare of human life depends. Many activities, both good and bad, have been practised in the watersheds of lakes, reservoirs, rivers and streams due to an increasing human population. As a result of extensive deforestation, overgrazing and poor crop and soil management practices in the watersheds over long periods, large sediment loads have accumulated in river channels, lakes and reservoirs. A study conducted with the help of remote-sensing techniques and direct observation to assess degradation of vegetation and gully erosion showed that on average 10% of the soil moved by water in the highlands actually ends up in rivers and thus represents a permanent loss (Wright 1984). Siltation along river valleys has led to the disappearance of once perennial streams. Although reliable hydrological data is not available to evaluate the situation, continuous reduction of the depth and volume of water in the lakes due to siltation is observed. For example, Lange Lake in East Hararghe has dried up and Lake Haro Maya in the same zone, is almost filled up with silt. About 85% of the farmers' associations in Fincha'a watershed have reportedly experienced recent famines due to a shortage of land to cultivate and they claim that siltation-induced overflow of the Chomman Lake was the cause of land shortage (Bezuayehu Tefera, personal communication).
The sediments ending up in rivers and lakes carry excess nutrients from soils, sewage, livestock and human waste, fertilisers, industrial waste, mining etc. The release and migration of nutrients and other chemicals like pesticides is an economic loss and a threat to the quality of water and life in the region. No precise estimates of these effects are yet available.
The high incidence of flooding in the downstream areas of Awash River could be due to the effect of land degradation in the upstream catchment area. Increased runoff and reduced infiltration due to land degradation contributes to the flooding problem. Streams change their course when silted up by soil erosion or from river bed erosion. Deposition of such loads along riverbanks especially during heavy floods might have led to changes in river courses and resulted in the degradation of water resources.
Increases in overland flows due to deforestation, overgrazing, excessive cultivation etc. on steep slopes reduce ground water recharge. This could be the reason why many perennial springs in the highlands of Oromiya have dried up and the existence of others is precarious. Therefore, livestock herds are forced to concentrate at a few watering points thus leading to trampling of the soil surface and formation of rills and gullies.
